BBS2041 Course Summary PDF

BBS2041 – Summary Chiara Thömmes Course Summary BBS2041 – Human Intermediary Metabolism Content: Pages: 1. Case I – Digestive System...

BBS2041 – Summary Chiara Thömmes Course Summary BBS2041 – Human Intermediary Metabolism Content: Pages: 1. Case I – Digestive System 3 – 14 a. Anatomy & Histology of the Digestive System 3–7 b. Functions & Secretions 7 – 10 c. Digestion & Absorption of Carbohydrates 10 – 11 d. Digestion & Absorption of Proteins 11 – 12 e. Digestion & Absorption of Fats 12 – 14 2. Practical 01 – VM Liver, Gallbladder & Pancreas 15 – 21 a. Liver 15 – 17 b. Gallbladder 18 – 19 c. Pancreas 19 – 21 3. Case II – Energy Production 22 – 33 a. Carbohydrates 22 – 28 b. Triglycerides 29 – 31 c. Insulin 32 – 33 4. Case III – Starvation & Fasting 34 – 45 a. Starvation – Definition, Types, Symptoms & Phases 34 – 37 b. Energy Storage within the Human Body 37 c. Metabolic Processes 38 – 43 d. Hormonal Regulation 44 – 45 5. Case IV – Endurance vs. Strength 46 – 53 a. Different Types of Muscle Fibers 46 – 47 b. Exercise-specific diets 47 – 48 c. Different Pathways of Energy Consumption 48 – 53 6. Practical 02 – VM Muscle Tissue 54 – 55 a. Recap Muscle Tissues 54 b. Skeletal Muscle – Histology 54 – 55 7. Case V – Amino Acid Metabolism 56 – 64 a. Amino Acids 56 – 58 b. Protein Degradation & Synthesis Pathways 59 – 61 c. Role of B-Vitamins 61 – 64 8. Case VI – Micronutrients 65 – 73 a. Micronutrients 65 – 68 b. Copper & Iron 69 c. Vitamin B12 70 – 73 9. Case VII – Iron Metabolism 74 – 80 a. Heme & non-heme Iron 74 – 75 b. Absorption & Transportation 75 – 77 c. Glucose Metabolism & Iron Metabolism 77 d. Iron Deficiency & Overload 78 – 79 e. Recommended intake 79 10. Case VIII + Practical 03 – Body Composition & Energy Expenditure 80 – 86 a. Energy Expenditure 80 – 81 b. Body Composition 81 – 84 c. Direct / Indirect Calorimetry & non-Calorimetry 84 – 86 1 BBS2041 – Summary Chiara Thömmes 11. Case IX – Alcohol Metabolism 87 – 95 a. Alcohol Metabolism 87 – 90 b. Health Consequences 90 – 94 c. Recommended Diet 94 d. Evidence-base study designs 94 – 95 12. Example Questions 96 2 BBS2041 – Summary Chiara Thömmes Case I – Digestive System Anatomy & Histology of the Digestive System Organs of the Digestive System & their individual Organisation The digestive system starts with the oral cavity. It contains three types of saliva glands (parotid gland, sublingual gland & submandibular gland), our teeth and our tongue. The oral cavity goes over to the pharynx and then the esophagus, a long tube which connects the oral cavity with the stomach. The stomach is subdivided into the fundus, body and pylorus. The food, now called bolus, enters the stomach via the cardia and leaves it via the pylorus valve, while both are smooth muscle sphincters. The small intestine starts at the pylorus valve and ends at the ileocecal valve. In between it is subdivided into duodenum, jejunum and ileum. The small intestine can be about 6m long. The duodenum is the start of the small intestine and connects the ducts of the liver, gallbladder, pancreas and the stomach. The bile duct and the pancreatic duct join together to form the hepatopancreatic sphincter. Following the small intestine, the chyme goes into the large intestine, also referred to as colon. It is approx. 1m long and is subdivided into cecum + appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and finally the anal canal with the excretion muscle anus. The organs of the digestive tract can be subdivided into two groups: - Accessory digestive organs: teeth, tongue, saliva glands, gallbladder, liver, pancreas - Alimentary canal / gastrointestinal tract: pharynx, esophagus, stomach, small and large intestine 3 BBS2041 – Summary Chiara Thömmes Basic Histology of the Organs From the esophagus to the anal canal, the walls of the alimentary canal have the same four basic layers: Mucosa, submucosa, muscularis externa & serosa. Mucosa The mucosal layer is the innermost. It consists of an epithelial membrane that lines the alimentary canal lumen from mouth to anus. Its properties are renewed every 5 days and its functions are to secrete mucus, digestive enzymes & hormones as well as to absorb end products of digestion into the blood or lymph system. Besides that, the mucosal layer protects the organs against infectious diseases. The mucosal layer has three compartments: 1. Epithelial cells which include endocrine and exocrine glands as well as local stem cells 2. Lamina propria 3. Muscularis mucosae Submucosa The submucosal layer lies just external to the mucosal layer. It consists of areolar connective tissue which contains a huge number of blood and lymph vessels, as well as nerve fibers. The nerve fibers are mostly found within bundles – submucosal plexus. The function of the submucosal layer includes the elastic properties of the organs. The elastic fibers within the connective tissue ensure that the organs regain their normal shape after expanding due to a meal. Muscularis Externa The muscularis externa is a layer of smooth muscle which surrounds the submucosa. It is mostly responsible for the segmentation and the motility, which causes the food to move from organ to organ. It consists of two layers: 1. An inner layer consisting of circular smooth muscles 2. An outer layer consisting of longitudinal smooth muscles – this layer does also contain the myenteric nerve plexus which is responsible for the motility Serosa The serosa is the outermost layer of the organs. It provides structure to the other layers and is also called visceral peritoneum. It consists of areolar connective tissue, which is covered by mesothelium, which describes a single layer of squamous epithelial cells. 4 BBS2041 – Summary Chiara Thömmes Basic Information about the most important Organs Stomach The stomach is a temporary storage tank where chemical breakdown of proteins begins, and food is mixed with enzymes to a paste called chyme. The conditions within the organ are highly acidic with a pH around 1. The Stomach is an organ which is subdivided into three parts with 2 smooth muscle sphincters: 1. Fundus – it represents the upper part of the stomach which lies directly underneath the diaphragm; it describes everything above the first sphincter 2. Body – it represents the middle part of the stomach which is responsible for the mechanical and partly chemical breakdown of the bolus which arrives via the cardia (first sphincter); the bolus, now called chyme, is mixed 3. Pyloric antrum – it describes the lower part of the stomach which is connected to the duodenum of the small intestine via the lower sphincter → pylorus valve The Histology of the stomach is close to the basic histology of all organs. Just the muscularis externa and the mucosa are modified with stomach-typical features. The mucosal layer is almost entirely made up of mucosal cells which form a protective coat around the organ to inhibit autodigestion. Its inner lining is dotted with a lot of tiny and deep gastric pits which contain the gastric glands. Those glands are occupied by several important cell types: a. Parietal cells – those cells are responsible for the release of hydrochloric acid; HCl is responsible for the activation of the gastric enzymes, it denatures proteins, and it kills harmful bacteria b. Chief cells – those cells release pepsinogen, which is activated to pepsin in the presence of HCl; pepsin is responsible for the chemical breakdown of proteins c. Enteroendocrine cells – those cells release regulatory hormones, like serotonin and histamine, in the presence of HCl; they act locally to trigger further cell secretion and cause the muscles to contract d. G-cells – those cells produce gastrin in the presence of HCl; it is a hormone which is involved on the stimulation of gastric activities The muscularis externa within the stomach is also altered. Additional to the circular & longitudinal smooth muscle layers, the stomach also contains a layer of muscle fibers that run obliquely. This provides extra strength and allows the stomach to not only hold the chyme but mix it. Stomach Movement: The stomach movement is an important mechanism for the functionality of the gastrointestinal tract. It can be subdivided into three phases: 1. Stomach Filling When food is ingested and swallowed, the stomach stretches to accommodate incoming food. The pressure, thereby, stays the same until 1.5l are reached, then the pressure starts to rise. Receptive relaxation = smooth muscles within the fundus & body of the stomach relaxes in response to the bolus moving through the esophagus. This is controlled by the swallowing centre of the brain stem & the vagus nerves. Gastric Accommodation = the gastric smooth muscle cells are able to stretch without greatly increasing the tension 2. Gastric Contractile Activity Peristalsis begins neat the cardia and produces a gentle rippling movement of the stomach walls. As the contractions approach the pylorus the waves become more powerful. The pylorus acts thereby as a dynamic filter that allows only liquids and small particles to pass through the valve into the duodenum. The stomach creates a retropulsion, which describes the back and forth pumping of the remaining bolus within the stomach. 5 BBS2041 – Summary Chiara Thömmes 3. Stomach Emptying The stomach usually empties every 4h completely, but it is dependent on the content within the duodenum. When chyme enters the duodenum, receptors within its walls respond to chemical signals and the smooth muscles stretch. This initiates the enterogastric reflex and hormonal mechanisms start to inhibit further acid and pepsin secretions. Small Intestine The small intestine is a hollow tube which can become up to 6m long. It starts at the pylorus valve and ends at the ileocecal valve, where it joins the large intestine. The large intestine has three major parts: 1. Duodenum – it is the most retroperitoneal part which contains the hepatopancreatic sphincter (=the joint ducts of the gallbladder and liver (bile duct) & the pancreas (pancreatic duct)) 2. Jejunum – the middle part of the small intestine, it lies intraperitoneal; it extends from the duodenum to the ileum 3. Ileum – the longest and last part of the small intestine; it lies intraperitoneal; its main functions are absorption of nutrients and it joins the large intestine at the ileocecal valve As well as in the stomach, the small intestine also has modifications within the basic Histology. Three of those modifications are major contributors for the functionality of the small intestine: a. Circular folds within the mucosa & submucosa – those folds force the chyme to spiral through the lumen; this slows down the movement of the chyme which allows a full digestion and absorption of the nutrients b. Villi – the mucosal layer contains finger-like projections; they are large and leaflike within the duodenum and gradually narrow and shorten along the length of the small intestine; they contain absorptive columnar epithelial cells (=called enterocytes) as well as a dense capillary & lymphatic bed (=lacteal) which are responsible for absorption c. Microvilli on top of absorptive cells – those microvilli build the so-called brush border; it contains enzymes (=brush border enzymes) which complete digestion of carbohydrates & proteins 6 BBS2041 – Summary Chiara Thömmes Large intestine The large intestine is a hollow tube which becomes up to 1m. It starts at the ileocecal valve and ends at the excretion muscle anus. It frames the small intestine. The major functions of the large intestine include absorption of water & vitamins and elimination of indigestible substances. The large intestine can be subdivided into 6 different parts: 1. Cecum – marks the part of the large intestine just after the ileocecal valve; attached it has the appendix 2. Ascending colon 3. Transverse colon – it starts with the right colic – or hepatic – flexure just underneath the liver and ends with the left colic – or splenic – flexure just underneath the spleen 4. Descending colon 5. Sigmoid colon 6. Rectum – it marks the lowest part of the large intestine and builds up the anal canal, which is controlled by the external anal sphincter As well as the other two major organs of the GI-tract the large Intestine has some histological modifications. The mucosal layer contains deep crypts but does not have the villi, as in the small intestine. The epithelial cells are absorptive columnar cells with a striated border, many goblet cells, endocrine cells and basal stem cells. There are no Paneth cells within the large intestine. This means that the large intestine does not secrete digestive enzymes but a lot of mucus which eases the passage of faeces through the organ. The mucosal layer is renewed every 6 days. The muscularis externa contains special longitudinal smoot muscles which are arranged in 3 bands called taenia coli. Besides those modifications within the histology, the large intestine contains 10 million discrete types of bacteria. This bacterial flora has 4 major functions: a. Colonizing the colon b. Synthesizing complex vitamin B and vitamin K for the liver which are involved in the formation of clotting proteins c. Metabolizing molecules like mucin, heparin, hyaluronic acid, etc. d. Fermenting indigestible carbohydrates and release acid and gas Functions & Secretions of the Digestive System The GI-Tract has 6 major functions: 1. Ingestion – this describes the process in which the food is taken into the digestive tract via mouth, pharynx and esophagus 2. Propulsion – this describes the movement of the food through the digestive organs; it includes swallowing & peristalsis 3. Mechanical Breakdown – this describes the process of the physical preparation of the food for the digestion; it includes chewing, mixing food with saliva, churning food into the stomach, segmentation & rhythmic local constrictions within the small intestine 4. Digestion – it describes the catabolic step in which enzymes are secreted; they break down the food into its molecules 5. Absorption – it describes the absorption of the end-products of digestion; they are passed from the lumen through the mucosal cells into the blood/lymph system 6. Defecation – it describes the elimination of indigestible substances which could not be digested and/or absorbed 7 BBS2041 – Summary Chiara Thömmes Functions & Secretions of the individual Organs Mouth – Oral Cavity The digestive processes start within the mouth where food is ingested. It contains several accessory digestive organs – teeth, tongue, saliva glands. With the help of those organs the mouth has four major functions: 1. Ingestion 2. Propulsion & mastication – it describes the beginning of the mechanical breakdown due to the teeth (chewing) 3. Voluntary phase of deglutition – it describes the voluntary process of swallowing due to the tongue, which initiates the propulsion towards the stomach 4. Chemical breakdown – within the mouth the chemical breakdown of carbohydrates starts by salvia; the saliva is composed of the enzyme salivary amylase and mucus; this dissolves food and starts to break it down, so the tongue can compact it into bolus and swallow it Secretions: - Salivary amylase – responsible for the chemical breakdown of carbohydrates - Lingual lipase – responsible for the emulsification of fats Pharynx & Esophagus The pharynx and the esophagus connect the oral cavity to the stomach. They are organs where nothing actively happens to the bolus, but it is moved towards the major digestive organs. Both are covered in mucus to help the food’s passageway. These organs have two major functions: 1. Involuntary phase of deglutition – the involuntary swallowing phase which is initiated via the voluntary phase within the oral cavity 2. Propulsion – peristaltic waves start to move the bolus into the stomach Stomach The bolus enters the stomach via the cardia. This is the primary place of mechanical breakdown of the ingested food. The stomach itself has a mucosal barrier which is highly acidic. To protect itself from autodigestion the stomach is covered which a thick coating of bicarbonate-rich mucus, the epithelial cells are connected only via tight junctions and the amount of stem cells is high, to immediately replace damaged cells. This organ has several major functions: 1. Mechanical breakdown & propulsion – peristaltic waves mix the bolus with gastric juice and pass it to the duodenum (food is now called chyme) 2. Secretion of hydrochloric acid, hormones & enzymes 3. Chemical breakdown of proteins – this happens via the enzyme pepsin Secretions: - Hydrochloric acid (HCl) – it is secreted by parietal cells within the gastric glands and creates a pH within the stomach around 1pH; the hydrochloric acid has three major tasks: a. Acts as bacteriostatic agent b. Denatures proteins & salivary amylase c. Triggers the secretion of several hormones, gastric lipase & pepsin - Pepsin – it is an enzyme which is released by chief cells in an inactive form called pepsinogen; with the presence of H+ ions from HCl pepsinogen is activated; its major function is the digestion of proteins by targeting collagen fibers - Gastric lipase – it is an enzyme which is co-secreted with pepsin; its major function is to target triglycerides - Gastrin – a hormone secreted by G-cells within the gastric glands; it is released in the presence of amino acids & peptides; it is secreted in a response to a neural reflex mediated by gastrin releasing peptide (GPR); its major function is to activate HCl release and histamine release 8 BBS2041 – Summary Chiara Thömmes - Histamine – it is a hormone which is secreted by enterochromaffin-like cells (ECL cells) in response to either gastrin or acetylcholine stimulation; its major function is to stimulate HCl secretion within the parietal cells via H2 receptors - Intrinsic factor – this is a protein secreted by the parietal cells; it forms complexes with Vitamin B12 to aid its absorption within the intestine - Somatostatin (SS) – it is a hypothalamic growth hormone-inhibiting hormone secreted by the D-cells within the stomach and provides a negative feedback loop; it decreases the secretions of gastrin, histamine & pepsinogen Gallbladder, Liver & Pancreas The gallbladder, liver and pancreas are known as the accessory glandular organs of the digestive system. They lie outside of the GI-tract and are connected to the duodenum via the hepatopancreatic sphincter, a smooth muscle duct. They produce a variety of secretions that help to digest the chyme entering the small intestine via the stomach. Secretions of the pancreas: - Bicarbonate – it is an alkaline substance which neutralizes the acidic chyme coming from the stomach; its production depends on the presence of carbonic anhydrase - Trypsin & chymotrypsin – both are enzymes which are secreted in their inactive forms – trypsinogen & chymotrypsinogen; trypsinogen is activated by the presence of the brush border enzyme enteropeptidase; trypsin in turn then activates chymotrypsin; they are responsible for cutting peptides into smaller peptides - Pancreatic amylase & lipase – responsible for the chemical breakdown of carbohydrates & fats - Elastase & Carboxypeptidase – those two enzymes are involved in the protein digestion Secretions of the liver & gallbladder: - Bile – it is a nonenzymatic solution secreted by hepatocytes within the liver; it is stored, concentrated and secreted by the gallbladder; Its main components are: a. Bile salt – facilitates enzymatic fat digestion; compost of steroid bile acids combined with amino acids b. Bile pigments – waste product of haemoglobin degradation; responsible for the brown colour of faeces c. Cholesterol – a waste product which needs to be secreted Small Intestine The small intestine is the major digestive organ of the GI-tract. Alkaline mucus produced by the intestinal glands as well as bicarbonate juice from the pancreas turn the acidic conditions of the stomach into alkaline conditions, which strengthens the enzymatic activities. The small intestine has three major functions: 1. Mechanical Breakdown & propulsion – segmentation, slow movement & short distance peristaltic waves continuously mix the chyme with the digestive juice 2. Digestion – enzymes and digestive juice provided by the accessory glandular organs as well as self- produced enzymes from the brush border are responsible for the digestion of all classes of food 3. Absorption – the breakdown products are absorbed via active and passive transporter within the cell lining of the small intestine Secretions: - Alkaline mucus – a substance secreted by goblet cells which protects the epithelium - Brush Border Enzymes – those enzymes are activated by parasympathetic neurons of the vagus nerve and are regulated by hormones and paracrine signalling; They are responsible for a variety of digestive mechanisms - Digestive enzymes – secreted by the intestinal epithelial - Isotonic saline (NaCl) solution – secreted by the crypt cells within the intestine; responsible for osmotic gradient to regulate osmosis 9 BBS2041 – Summary Chiara Thömmes Large Intestine The large intestine is not involved in the digesting processes of the food, but rather responsible for the absorption of remaining nutrients as well as the storage and elimination of the indigestible substances (faeces). It has four major functions: 1. Vitamin production – the colon hosts a lot of bacteria, which digest further food while producing vitamin K and some types of vitamin B 2. Absorption – water, electrolytes as well as the bacterial produced vitamins are absorbed within the large intestine 3. Propulsion – movement of the remaining chyme towards the anus (chyme now called faeces) 4. Defecation – elimination of the indigestible substances triggered by the rectal distension Secretions: - Copious mucus – a mucus produced by goblet cells which eases the passageway of faeces through the colon Digestion & Absorption of Carbohydrates Digestion Carbohydrates are ingested as starch or disaccharides. Their digestion starts within the oral cavity via the presence of salivary amylase produced by the salivary glands. After passing through the esophagus, the bolus enters the stomach. Due to the hydrochloric acid, the salivary amylase loses its function and carbohydrates are not further broken down until they enter the small intestine. Within the pancreatic juice there is an enzyme called pancreatic amylase. This enzyme further breaks down the carbohydrates until they are present in the form of oligosaccharides and disaccharides. The brush border of the small intestine secretes several enzymes which break the saccharide chains into galactose, glucose and fructose: a. Lactase – breaks down lactose into galactose & glucose b. Maltase – breaks down maltose into 2 molecules of glucose c. Sucrase – breaks down sucrose into fructose & glucose d. Other involved brush border enzymes are dextrinase & glucoamylase Glucose, galactose & fructose can be absorbed and represent the end-products of the digestion of carbohydrates. Absorption The absorption is restricted to monosaccharides which are the end-products of the digestion: glucose, galactose & fructose. Glucose and galactose are absorbed via the same mechanism. They are transported from the intestinal lumen to the inside of the cells via the apical Na+-glucose SGLT symporter and they leave the cells and enter the blood stream via the basolateral GLUT2 transporter. Both transporters use a form of active transport, so ATP and sodium are needed. 10 BBS2041 – Summary Chiara Thömmes Fructose on the other hand is sodium independent while entering the cell. It moves across the apical membrane by diffusion through the GLUT5 transporter. Fructose leaves the cell and enters the blood stream by using the basolateral GLUT2 transporter. Glucose travels through the blood to the destination where it is needed – e.g., muscles, brain, …. Galactose and Fructose need to travel to the liver. The liver takes them up and transforms them into glucose. Then they renter the blood circulation and travel to their final destination. Carbohydrates that are not directly metabolized by the body are stored in the liver, the muscles and the fat tissue. Digestion & Absorption of Proteins Digestion The digestion of proteins is done via two types of enzymes: 1. Endopeptidases – also called proteases – which attack peptide bonds → long peptide bonds are broken down into smaller fragments; to this enzyme group belong pepsin, elastase, trypsin & chymotrypsin 2. Exopeptidases which release single amino acids from the peptide chain → most important enzymes which belong to this enzyme group are carboxypeptidase & aminopeptidase The chemical breakdown of proteins starts within the stomach. The gastric enzyme pepsin is secreted in the presence of hydrochloric acid. It belongs to the enzymatic group of endopeptidases and is therefore responsible for the breakdown of long peptides into smaller peptides. The proteins are now present as polypeptides which are still quite long. They now enter the small intestine and are mixed with the pancreatic juice, which contains endopeptidases as well as exopeptidases. Elastase is responsible for the chemical breakdown of elastin, while trypsin & chymotrypsin are responsible for the further 11 BBS2041 – Summary Chiara Thömmes breakdown of the polypeptides. Carboxypeptidase, secreted from the pancreas, starts to cut of single amino acids which can be absorbed. As the polypeptides travel further down in the small intestine, the peptides are becoming smaller and smaller. When they reach the brush border, the exopeptidases aminopeptidase, carboxypeptidase and dipeptidase cut the remaining peptides into single amino acids, dipeptides and tripeptides. Those end products can now be absorbed. Absorption There are three different ways of proteins to be transported inside the cells from the lumen of the small intestines. They are mostly secondary active transport mechanisms because they are not directly dependent on metabolic energy, but their co-transport molecules are: 1. Co-transport with H+ - this is the preferred transport mechanism for di- and tripeptide 2. Co-transport with Na+ - this is the preferred transport mechanism for single amino acids 3. Transcytosis – this is the preferred transport mechanism for peptides which are still longer; they are transported intact through the cell via vesicles and leave the cell via the same way Inside the cell di- and tripeptides can undergo further digestion into single amino acids via cytoplasmic peptidase or they can enter the blood stream intact. There are two ways peptides can leave the cell plasma and enter the blood stream: 1. Anti-transport with Na+ with the help of sodium-potassium pump – preferred transport mechanism of single amino acids 2. Anti-transport with H+ - preferred transport mechanism of intact di- and tripeptides The transport of amino acids depends on their chemical composition. If the amino acids are water soluble, so hydrophilic, they can enter the blood stream without any problems and travel to their final destination. But if the amino acids are not water soluble, so hydrophobic, they cannot as easily enter the blood stream. They need a transport molecule within the blood. This plasma protein is called albumin. It is produced by the liver and responsible for the carriage of insoluble molecules within the blood stream. It binds to the hydrophobic amino acids so they can be transported towards their final destination. If not directly needed within the cells to produce new proteins, amino acids are transported to the liver for storage. Digestion & Absorption of Fats 12 BBS2041 – Summary Chiara Thömmes Digestion The digestion of fats starts within the oral cavity. The salivary glands secrete the enzyme lingual lipase which targets the fats and emulsifies them (emulsification= the mixture of non-compatible substances – in this case: water + fats). The fat digestion continues within the stomach where gastric lipase is secreted. It continues with the emulsification of the fats and targets triglycerides for breakdown and creates diglycerides. The main digestion of the fats nevertheless occurs within the small intestine. The bile salt within the secreted bile continues and finishes the emulsification while the pancreatic lipase starts to break down the triglycerides and diglycerides into monoglycerides and fatty acids. Phospholipids are broken down by phospholipase or they are taken up by the bile salt as well. Micelle Formation Fats are highly hydrophobic and lipophilic. Without further regulation they would clump together and build a big fatty molecule which is mostly impossible to digest. Bile salts are hydrophilic on one side and hydrophobic on the other. This means that they stick to the fat and form a hydrophilic coat around it. Combined, bile salt and fat are called micelle. As soon as the micelles are formed, pancreatic lipase can start to work. The fat molecule within the micelle can now be broken down into fatty acids and monoglycerides without connecting to other fatty molecules because of the protective bile coat. Those micelle packages containing fatty acids and monoglycerides can now be absorbed. Absorption The absorption of fat is mostly done via monoglycerides which are the major end product of digestion. Because they are lipophilic the molecules can enter the cell by simple diffusion or facilitated diffusion via the transporter CD43 or fatty acid transport protein (FATP). They move out of their micelles and diffuse across the enterocyte membrane. Some fatty acids and cholesterol need transporter to diffuse through the cell membrane. One inside the cell the monoglycerides move to the smooth ER where they are recombined into triglycerides. Those triglycerides join together with the cholesterol molecules and form large water-soluble droplets called chylomicrons. They are then packed into secretory vesicles at the Golgi apparatus and leave the cell via exocytosis. Because the chylomicrons are relatively large molecules, they cannot diffuse into the blood capillaries to enter the blood stream. Instead, they are passed to the lacteals (lymph vessels of the villi) and afterwards they pass through the lymph system and enter the blood stream at the thoracic duct. 13 BBS2041 – Summary Chiara Thömmes The other variant is for monoglycerides to be broken down into fatty acids. Fatty acids can enter the blood stream and are transported by albumin towards the liver for storage. Phospholipids are broken down by phospholipase and their acyl-group enters the chylomicrons while their choline is transported directly to the liver. The final destinations of fats are all organs besides the brain (the BBB does not let fats diffuse into the brain). The three most important fatty acid users are adipose tissue, skeletal muscle and cardiac muscle. They use up the fatty acids by oxidation or formation of ATP. The fat which is not used can be reabsorbed by the liver or be stored within adipose tissue. 14 BBS2041 – Summary Chiara Thömmes Practical 01 – VM Liver, Gallbladder & Pancreas Accessory glandular digestive organ – Liver The liver is the largest internal organ within the human body (average: 1.3 – 3 kg (2% of the body weight)). The organ is located in the right upper quadrant of the abdomen and lies just underneath the diaphragm. The main digestive function of the liver is producing bile. Bile = a non-enzymatic substance consisting of bile salts, bile pigments & cholesterol. It is needed for emulsification, hydrolysis and uptake of fats within the duodenum. Besides the production of bile, the liver is the major interface between the digestive organs and the blood circulation. The liver receives all the absorbed nutrients from the intestine and processes them before they are distributed and sent to their final location within the body. Other major functions of the liver include glycogen storage, plasma protein synthesis & drug detoxification. Anatomy The liver is a soft, pinkish-brown, triangular, peritoneal organ. It lies to the right of the stomach & overlies the gallbladder. The organ is subdivided into four lobes (only 3 are visible via ultrasound): - Left hepatic lobe – major lobe - Right hepatic lobe – major lobe; 6x larger than the left lobe and separated by the main lobar fissure & the middle hepatic vein - Caudate hepatic lobe – inferior lobe - Quadrate hepatic lobe – inferior lobe; not seen via ultrasound Each lobe is further divided into lobules, which describe the functional unit of the liver. Each lobule is made up of millions of hepatic cells which are the basic metabolic cells within the organ. The whole organ is covered by a thin connective tissue membrane called ‘Glisson’s Capsule’ which thickens at the hilum / porta hepatis on the inferior side, where the portal vein and the proper hepatic artery enter the organ and the hepatic duct exits. The single lobes are covered by a thin, fibrous capsule and mesothelium of the visceral peritoneum. The blood circulation is mediated via the portal vein & the portal / hepatic artery. About 75% of blood entering the liver is nutrient-rich and oxygen-poor blood from the portal vein. The vein transports the blood from the digestive organs (stomach, intestines & spleen) towards the liver. The other 25% of blood entering the liver comes from the portal / hepatic artery that sends the blood directly from the aorta, which is oxygen-rich and supplies the organ with the needed O2. The blood (nutrient- & oxygen-poor) leaves the liver via the hepatic vein. 15 BBS2041 – Summary Chiara Thömmes Histology As already mentioned, the liver’s functional units are the hepatic lobules. They are columnar units with a roughly hexagonal shape within a transverse section. The middle of a lobule always builds the central vein, which forms a connection to the hepatic vein. On the corner (the periphery) of each lobule there are hepatic / portal triads formed from connective tissue which contain an interlobular arteriole (descends from the proper hepatic artery), an intralobular venule (descends from the portal vein) and a bile duct (called bile ductule). Besides the definition of a hepatic lobule (centre is build by the central vein), the lobules can also be seen as portal lobules. When this term is used, then the centre is not built by the central vein but rather by the portal triads. Within each lobule of the liver, hepatocytes form hundreds of irregular plates arranged radially around the small central vein. These hepatocyte plates are supported by a delicate stroma of reticulin fibers. Hepatocytes (the major cell type within the liver tissue) are large cuboidal or polyhedral epithelial cells which contain a large, round, central nuclei. The cytoplasm is eosinophilic and rich in mitochondria. Hepatocytes are frequently binucleated and around half of them contain 2 – 8x the normal chromosomal number (polyploid cells). The major function of hepatocytes is the secretion of bile components, but they are also involved in the production of several components of the blood (e.g., albumin, clotting factors, lipoproteins, glucose, urea). Several hepatocytes form a hepatocyte plate. The blood vessels descending from the portal triad branch into sinusoids which run in between the hepatocyte plates and drain into the central vein. This means that the blood flow within the lobules goes from the interlobular arteries & the interlobular venules through the sinusoids in between the cell plates into the central vein. Within the sinusoids, exchange of molecules between blood and hepatocytes occurs. This is done via the Space of Disse which is a separation of the endothelial cells of the sinusoids. The bile components produced via the hepatocytes is not drained into the sinusoids but into the bile duct of the portal triad, which means bile flows in the perpendicular direction than the blood. The hepatocytes 16 BBS2041 – Summary Chiara Thömmes are connected to minuscule bile canaliculi, which are located in between the cells and transport the bile through the bile canals into the bile duct of the portal triad. Other cell type, besides hepatocytes (H), can be found within the lobules of the liver. Kupffer cells (K) are located along the sinusoids (S) of the lobules. They are the immune cells of the liver and have phagocytotic properties. The Kupffer cells are concentrated near the periphery (portal triad), because they need to eliminate all foreign substances (e.g., bacteria), before the blood flows through all the sinusoids and then into the central vein. Another cell type, present within the liver lobules, are Ito cells / hepatic stellate cells (HS). They are found in the space of Disse (PS) and are responsible for the storage of Vitamin A and the secretion of collagen. 17 BBS2041 – Summary Chiara Thömmes Accessory glandular digestive organ – Gallbladder The gallbladder is a small, hollow pear-shaped, intraperitoneal accessory glandular digestive organ which lies beneath the liver. The bile, which is produced by the hepatocytes within the liver, is transported to the gallbladder via the bile duct and later on the hepatic duct. The gallbladder is capable of storing 30-50ml of bile which is concentrated during the storage time. This means that the main function of the gallbladder is to store, concentrate and later on secrete the bile produced by the liver. Anatomy The gallbladder is a small organ which is entirely surrounded by peritoneum and located just underneath the liver. The organ itself is typically divided into three parts: - Fundus – rounded, distal portion - Body – largest part - Neck – gallbladder descends into the cystic duct which leads into the biliary tract / tree, where the cystic duct joints together with the hepatic duct to form the common bile duct / ductus choledochus; the common bile duct then joins together with the pancreatic duct to form the hepatopancreatic ampulla which joins the duodenum of the small intestine The flow of the bile starts within the left & right bile ducts of the liver. It travels through the common bile duct and is excreted into the duodenum via the major duodenal papilla which is controlled by the sphincter of Oddi. Is this sphincter of Oddi closed, then the bile flows retrogradely into the gallbladder via the cystic duct, where it is stored and concentrated. The gallbladder receives oxygen-rich blood via a branch of the right hepatic artery called cystic artery. The venous drainage of the neck of the gallbladder happens via the cystic veins, which drain directly into the portal vein. Histology Biliary Tree The hepatic, cystic & common bile ducts are all lined with a mucosal membrane consisting of simple columnar epithelium of cholangiocytes (=same cells which line the bile duct of the liver). The lamina propria and the submucosa are relatively thin, with mucosal glands in some areas of the cystic duct. Everything is surrounded by a thin laxer of smooth muscle cells (muscularis). This muscle layer becomes thicker near the duodenum and within the duodenal papilla it will form the sphincter of Oddi. Gallbladder The wall of the gallbladder resembled the wall of the digestive system, but there are major modifications that need to be considered. The histology of the gallbladder wall consists of mucosa composed of simple columnar epithelium which is highly folded (rugae) and contains short apical microvilli, a typical lamina propria, a thin muscularis externa containing bundles of muscle fibers with different orientations, and an external adventitia where it connects to the liver and serosa (peritoneum) where it is exposed. In between the serosa and the muscle s 18 BBS2041 – Summary Chiara Thömmes connective tissue layer rich in nerves and blood vessels (venules & arterioles) as well as adipose tissue called subserosa. The muscularis externa of the gall bladder contracts and expels bile in response to cholecystokinin, which is excreted by enteroendocrine cells in the mucosa of the duodenum, as a reaction to dietary fat in the proximal duodenum. Accessory glandular digestive organ – Pancreas The pancreas is an accessory glandular digestive organ with both exocrine & endocrine properties. It produces digestive enzymes as well as hormones and is therefore part of the digestive system as well as the endocrine system within the human body. As an endocrine gland it functions mostly to regulate blood sugar levels by secreting the hormones insulin, glucagon, somatostatin and pancreatic polypeptide. As part of the digestive system, it functions as an exocrine gland by producing and secreting pancreatic juice into the duodenum through the pancreatic duct. Pancreatic juice is composed of bicarbonate (neutralizing the acidic chyme) and digestive enzymes (responsible for breakdown of carbohydrates, proteins & fats). Anatomy The pancreas is an elongated retroperitoneal, lobulated organ located behind the stomach with a large head near the duodenum and a narrower neck, body and tail region that extends to the left (towards the spleen). The pancreas stretches from the inner curvature of the duodenum, where the head surrounds two blood vessels (superior mesenteric artery & vein). This means that the pancreas is subdivided into 3 major parts: - Head / Neck – caput - Body – corpus - Tail – cauda The pancreas contains two ducts. The main pancreatic duct and a smaller accessory pancreatic duct run through the body of the pancreas and join with the common bile duct near the sphincter of Oddi which opens into the descending part of the duodenum. 19 BBS2041 – Summary Chiara Thömmes Histology The pancreas has a thin capsule of connective tissue, from which septa extend to cover the larger vessels and ducts as well as to separate the parenchyma into lobules. Histologically the pancreas needs to be separated into endocrine pancreas & exocrine pancreas. Exocrine Pancreas The exocrine part of the pancreas is made out of branched glands with acini. The acini consist of acinar cells which synthesize different (pro)enzymes – e.g., amylase, trypsinogen, lipase… Micrograph of exocrine pancreas shows the serous, enzyme-producing acinar cells arranged in small acini with very small lumens. Acini are surrounded by only small amounts of connective tissue with fibroblasts. Each acinus is drained by an intercalated duct with its initial cells, the centroacinar cells, inserted into the acinar lumen. The acinar cells are pyramidal shaped and contain a round, basal nucleus. The apical side of the cells contain various vacuoles and secretory granules. In between the acini are small capillaries. Each acinus is drained by a short, intercalated duct of simple squamous or low cuboidal epithelium. The initial cells of these small ducts extend into the lumen of the acinus as small pale-staining centroacinar cells that are unique to the pancreas. The intercalated ducts merge with intralobular ducts and larger interlobular ducts, which have increasingly columnar epithelia before joining the main pancreatic duct that runs the length of the gland. Endocrine Pancreas The endocrine part of the pancreas consists of pancreatic islets (=known as islets of Langerhans). Those islets appear as pale, isolated clusters between the acini of the exocrine glandular tissue. The pancreas contains more than 1 million of islets, but most are abundant within the tail region of the organ. A very thin reticular capsule surrounds each islet which separates it from the adjacent acinar tissue. Islets contain 4 major cell types which secrete 4 major islet hormones: - Alpha-cells – they produce & secrete the hormone glucagon (located peripherally) - Beta-cells – they produce & secrete the hormone insulin (most numerous, located centrally) - Delta-cells – they produce & secrete somatostatin (scattered & less abundant) - PP-cells – they produce & secrete pancreatic polypeptide (PP) (located within the islets of the head of the pancreas) 20 BBS2041 – Summary Chiara Thömmes The cells of the islets are polygonal or rounded, smaller and more lightly stained than the surrounding acinar cells, arranged in cords separated by fenestrated capillaries. Most cells are acidophilic or basophilic with fine cytoplasmic granules. The islets contain a lot of capillaries which transport the secreted hormones into the blood stream and therefore the circulation. 21 BBS2041 – Summary Chiara Thömmes Case II – Energy Production Energy Production – Carbohydrates As discussed within Case I, carbohydrates are digested into monosaccharides (glucose, fructose & galactose) and then absorbed into the blood stream. Fructose as well as galactose need to travel to the liver where they are transformed into glucose. Glucose is the energy source which results out of carbohydrates. It can be used from every organ and every cell type within our body. To produce energy, ATP needs to be produced. ATP is the chemical currency for energy within our body. It is composed out of an adenine molecule bound to a ribose sugar and three phosphate groups. The phosphate groups have a high energy bond. Energy can also be used out of the molecules GTP, TTP & CTP. Those three also contain two high energy phosphate bonds, but ATP is the most abundantly used form of cell’s energy. The four different forms can easily be transformed in one another. The process from glucose to ATP is divided into several phases within the cell: a. Glycolysis – which happens anaerobic within the cytosol and is therefore also possible in red blood cells even though they don’t contain cell organelles b. Pyruvate oxidation – happens within the mitochondria, the transporter used it MPCT (mitochondrial pyruvate cotransporter) c. Citric acid cycle – also called Krebs cycle or tricarboxylic acid (TCA) cycle; happens within the matrix of the mitochondria d. Electron transport chain (ETC) – happens within the inner mitochondrial membrane e. Chemiosmotic coupling – follows right after the ETC and the processes are coupled to one another, the combination of both is called oxidative phosphorylation; happens within the inner mitochondrial membrane Glycolysis Glycolysis is the first step within the breakdown of glucose into the body’s energy source ATP. Its major principle is to split the glucose molecule into two pyruvate molecules. It does not actively require oxygen, which makes the process anaerobic and it happens within the cytosol. Therefore, it is also possible within cell types that does not contain oxygen and/or mitochondria. Glycolysis can be divided into two phases: 1. Energy-requiring phase 2. Energy-releasing phase Energy-requiring Phase Glucose enters the cell. After entering the molecule needs to be phosphorylated and rearranged into glucose-6- phosphate, so the molecule cannot leave the cell anymore and becomes more reactive in turn. This is done by the enzyme hexokinase. Afterwards the molecule is transformed into its isomer fructose-6-phosphate and another phosphate group is added (resulting in fructose-1,6-biphosphate). This molecule is then broken down into the two isomers dihydroxyacetone phosphate (DAHP) & glyceraldehyde-3-phosphate. Only glyceraldehyde-3-phosphate is able to enter the energy-releasing phase of glycolysis, so DAHP needs to be transformed into its isomer. This phase needs two ATP molecules to phosphorylate the glucose molecule and afterwards the fructose-6- phosphate. By using two ATP molecules, two ADP molecules are produced. So, the net energy of this phase = -2ATP molecules per glucose molecule. 22 BBS2041 – Summary Chiara Thömmes Energy-releasing Phase The energy-releasing phase starts with the two three carbon molecules of glyceraldehyde-3-phosphate. This process starts by first adding another phosphate molecule to the molecules by reducing NAD+ to NADH (resulting in the molecule 1,3-biphosphoglycerate). Then the molecule is oxidized, and one phosphate group is removed from the molecule towards an ADP forming ATP (resulting in the molecule 3-phosphoglcerate). After forming the molecule into its isomer 2-phosphoglycerate, a water molecule (H2O) is split from the molecule (resulting in the molecule phosphoenolpyruvate (PEP)). The final step is the oxidation of PEP resulting in another production of an ATP molecule and the final product of pyruvate. This process is energy-releasing. Per glyceraldehyde-3-phosphate molecule two ATP and one NADH are produced. So, the net energy of this phase per glucose molecule = 4ATP molecules and 2NADH molecules. Overall: The glycolysis converts one 6C molecule of glucose into two 3C molecules of pyruvate, which results in the production of 2ATP molecules (4 are produced but 2 are used) and 2NADH molecules. The reaction equation of glycolysis is following: 1 Glucose + 2 ATP + 2 NAD+ + 4 ADP + 2 Pi → 2 Pyruvate + 4 ATP + 2 NADH + 2 H2O + 2 H+ Pyruvate Oxidation The pyruvate which is formed out of the glucose molecule via the glycolysis needs to be modified before it can enter the citric acid cycle within the matrix of the mitochondria. This step is the key connector between the respiration every cell type is capable of (glycolysis) and the respiration only cell types containing mitochondria are capable of (cellular respiration). 23 BBS2041 – Summary Chiara Thömmes Pyruvate is still a molecule which contains a lot of extractable energy. To prepare the molecule for the citric acid, it needs to be converted into acetyl- CoA. This process happens within the matrix of the mitochondria after the pyruvate has entered via the MPCT. First, the carboxyl-group of the pyruvate molecule is snipped off and released as a molecule of carbon dioxide, which results in a 2C molecule instead of a 3C molecule. This 2C molecule then undergoes an oxidation reaction where the energy of the reduction of NAD+ into NADH is used. This oxidation reaction adds an acetyl group linked with a coenzyme A to the 2C molecule. This results in acetyl-CoA. The coenzyme A is an organic carrier molecule derived from vitamin B5 and ensures that the acetyl molecule arrives at the citric acid cycle. This process produces no ATP directly, but it reduces on NAD+ molecule into NADH. So, the net products by the pyruvate oxidation per glucose molecule (2 pyruvate molecules) = 2NADH molecules and 2CO2 molecule. This means after this step we already received out of 1 glucose molecule: - 2ATP molecules - 4NADH molecules - 2H2O molecules - 2CO2 molecules - 2H+ protons The reaction equation of the pyruvate oxidation is following: 2 Pyruvate + 2 NAD+ + 2 CoA-SH → 2 Acetyl-CoA + 2 NADH + 2 CO2 Citric Acid Cycle The citric acid cycle (also known as the Krebs cycle or the TCA cycle) is the central driver of the cellular respiration. It takes place within the matrix of the cell organelle mitochondria. It is a closed loop which depends on the insertion of acetyl-CoA to finish a round of the cycle. There are 8 major steps within the cycle: 1. Acetyl-CoA enters the cycle and combines with a 4C acceptor molecule to form a 6C molecule. In this process the coenzyme A is released. The 4C carbon molecule already within the cycle is called oxaloacetate. It is the starting molecule and the waste molecule of the cycle which forms the typical closed loop of the citric acid cycle. The 6C carbon which is formed during this process is citric acid, which also gives rise to the name of the cycle. 2. Citric acid, also called citrate, is then converted into its isomer isocitrate. It is done via the removal and reattachment of a water molecule. 3. Isocitrate is then oxidized and a CO2 molecule is released with the energy released by the reduction of NAD+ to NADH. This gives rise to a 5C molecule called α-ketoglutarate. 4. α-ketoglutarate is in turn also oxidized by reducing NAD+ to NADH resulting in a CO2 molecule and a 4C molecule. This 4C molecule then combines with another coenzyme A to form succinyl-CoA. 5. The coenzyme A on the succinyl-CoA is replaced by a phosphate group which is then directly transferred to an ADP molecule which results in ATP (in some cell types: GDP to GTP). The remaining 4C molecule is called succinate 6. Succinate is then again oxidized by transferring two hydrogen molecules towards FAD+ resulting in FADH2 and a 4C molecule called fumarate 7. A water molecule is then added to fumarate which creates a 4C molecule called malate 24 BBS2041 – Summary Chiara Thömmes 8. The last step of the citric acid cycle is the oxidation of malate. It is done via the reduction of NAD+ to NADH and results in the 4C molecule oxaloacetate, which in turn can start a new cycle with the combination of acetyl-CoA. The citric acid cycle does not produce a lot of ATP by itself. Only one ATP molecule per acetyl-CoA, so two ATP molecules per glucose. The main output of the citric acid cycle is NADH and FADH2 molecules which are needed for the next step of the carbohydrate energy metabolism. In this way, the citric acid cycle produces indirectly a lot of ATP. The main output of the citric acid cycle per glucose molecule is: 2 Acetyl-CoA + 2 Oxaloacetate → 4 CO2 + 2 ATP/GTP + 2 FADH2 + 6 NADH + 2 Oxaloacetate Electron Transport Chain (ETC) The electron transport chain occurs within the inner mitochondrial membrane. It creates a protein gradient by pumping H+-ions from the matrix into the intermembranous space – against the ions’ concentration. This process depends on the impermeability of the inner mitochondrial membrane Due to the previous synthesized NADH and FADH2 molecules, the matrix has a high concentration of H+ molecules. Those H+ protons need to be pumped into the inner mitochondrial space to create a protein gradient which can be used to generate ATP. This protein gradient is created by 3 transmembrane tunnel proteins and 1 membrane protein on the matrix’s side of the membrane. The major complexes are complex I – IV. Complexes I, III & IV directly transport protons from the matrix into the intermembrane space. Complex I work with NADH which is a good electron donor while complex II pump H+ molecules and electrons from FADH2 (not a good electron donor). Both, Complex I & II pump their electrons derived from either NADH or FADH2 to complex III & IV. 25 BBS2041 – Summary Chiara Thömmes Principle of proton-electron coupled transmembrane transport The energy needed for this transport chain is used from electrons. NADH contains 2 electrons and a hydrogen molecule (proton). Complex I, III & IV contain redox centres, which can pass electrons towards the other side of the membrane. Redox centres have different affinities for electrons. Closer to the matrix they are low affinity state while closer to the intermembrane space they are high affinity state. This ensures the electron flow from matrix to intermembranous space. A small amount of energy is released each time an electron jumps from one redox centre to the next. This energy is then used to transport the protons (hydrogen molecules from NADH and FADH2) across the membrane against their concentration gradient. NADH already provides its electrons to the first complex within the membrane, while FADH2 needs a converter protein (complex II) which extracts the electrons and sends them towards complex III & IV. To ensure that the electrons are passed from complex to complex until they arrive at complex IV, there are two additional membranous proteins. On in between complex I, II & III called ubiquinone (Q) and one in between complex III & IV called cytochrome C (cyt C). complex I sends the electrons from NADH, while complex II sends the electrons from FADH2 and both are received by protein Q and send to complex III. Complex III then sends the electrons via cyt C towards complex IV. When the electrons arrive at complex IV, they are combined with oxygen which will be split from its natural form O2 into 2O. Oxygen (O2) is therefore the final acceptor of the electrons and forms together with two hydrogen molecules each the waste product water (H2O). One NADH molecule transports 4 hydrogen molecules through complex I and III, and 2 hydrogen molecules through complex IV (net = 10), while FADH2 transports only 4 hydrogen molecules though complex III and 2 hydrogen molecules through complex IV (net = 6). Chemiosmotic Coupling The chemiosmotic coupling is a process which follows the ETC. Together those two mechanisms are referred to as oxidative phosphorylation. The chemiosmotic coupling depends on the H+ gradient created by complexes I – IV, protein Q & cyt C within the electron transport chain. Additional to those membrane proteins the inner mitochondrial membrane contains another transmembrane protein complex called F0F1 ATP-synthase, which uses this gradient as energy to transform ADP into ATP by adding a phosphate ion to the molecule. ATP-synthase can be described as a water wheel within a river. The wheel uses the flow of the water to produce energy. The ATP-synthase uses the flow of the hydrogen molecules to produce energy. The two mechanism are basically the same. The net outcome of the oxidative phosphorylation per glucose molecule is as following: 1 Glucose + 6 O2 → 6 H2O + 6 CO2 + 30-38 ATP (+ heat) The number of actual produced ATP molecules depends on the cell type and on the need of energy. Without producing heat as waste, one glucose molecule can produce a net of 38 ATP molecules. But because there is always heat as waste the actual net production of ATP lies in between 30-36 ATP molecules per glucose. 26 BBS2041 – Summary Chiara Thömmes What if Glucose is not transformed into Energy? If there is too much glucose or glucose is not needed for energy production, the molecules can be stored within the body until further use. This is done by transforming the glucose molecules into glycogen. The body creates glycogen through a process called glycogenesis. Glycogen is stored within the body until the body needs it due to a lack of glucose. The storage is mostly short term, and the glycogen is transformed back into glucose by a process called glycogenolysis. Glycogenesis Glycogenesis is initiated when extent amounts of glucose is within the body. The glucose molecules are then transformed into glycogen instead of being used for cellular respiration. The whole process is similar than the process of glycolysis. The basic function of glycogenesis is to modify glucose molecules, so that they can be stored within long chains. Glucose itself cannot be stored because enzymes are prone to use glycose directly. Besides that, it is important that the body does not use all its glucose at once, to save energy for phases without food intake. So, when the cells have used all their glucose, they can start to break down the glycogen chains for further energy resources. A second advantage of glycogen is the non-polarity and the compact structure. Glucose is polar and not tightly structured which disturbs the water balance within the cells. A cell can therefore store a lot more glucose molecules transformed into glycogen chains, without disturbing their water balance. The process of glycogenesis is initiated and/or regulated by different signal pathways – e.g., epinephrine or insulin. Pathway from glucose to glycogen: 1. The glucose molecule enters the cell and interacts with glucokinase. This causes the glucose molecule to phosphorylate. The glucose molecule becomes glucose-6-phosphate. This step is energy-requiring because the phosphate group which is added to glucose is taken from ATP, thus reduces ATP to ADP. The glucose-6-phosphate can then either be stored by further process of glycogenesis or enter the energy production by undergoing glycolysis. 2. Then the glucose-6-phosphate is transformed into its isomer glucose-1-phosphate via the activity of phosphoglucomutase. 3. Another phosphorylation occurs and the glucose-1-phosphate is transformed into uracil-diphosphate glucose. This step is mediated by the enzyme UDP-glucose pyrophosphatase. Not only does the glucose molecule now contains two phosphate groups but additionally to that an uracil nucleic acid molecule. This is done by the reduction of UTP (uridine triphosphate) which results in the uracil binding to the molecule and the production of 2 HOPO32- molecules. 4. 8 of those uracil-diphosphate glucose molecules assembly together and bind to an enzyme called glycogenin. Afterwards the enzymes glycogen synthase and glycogen branching enzyme adds and creates branches within the chain, so the compact macromolecule glycogen is created. In this process the uracil-diphosphate is split from the molecules and is released, while the glucose molecules are bound together within a long chain. 27 BBS2041 – Summary Chiara Thömmes De Novo – Lipogenesis Another way of storing glucose molecules within the body is the so-called process of de novo-lipogenesis. During this process glucose molecules are transformed into triglycerides (fats), but sometimes also cholesterol, steroids or bile salts. The main storage of the body’s energy occurs via fat storage and not via carbohydrate storage. While eating carbohydrates, the body releases insulin. When the body has more glucose than it needs for energy, and the storage capacities of glycogen are exhausted, increased insulin levels prompt the liver to convert glucose molecules into triglycerides. Triglycerides are the main energy source found within our body. They are secreted back into the blood stream and enter adipose tissue (fat cells), where they are stored until further use. The formation of triglycerides is dependent on several steps and occurs within the liver (hepatocytes) or adipose tissue (fat cells). The molecule which is needed to form fatty acids is acetyl-CoA, which is a by-product of glycolysis. During this process further carbon molecules are added to the acetyl-CoA to produce an appropriate length of a fatty acid. This process is ATP dependent and therefore consumes energy. The conversion of acetyl-CoA into fatty acids occurs within the cytoplasm of the cell, so the acetyl-CoA needs to leave the mitochondria. This occurs within the form of citric acid. There are several steps that needs to be considered while looking at the de novo-lipogenesis: 1. Citric acid is able to leave the mitochondria and enter the cytoplasm (acetyl-CoA is not able to leave the mitochondria on its own) 2. Citric acid can be split into oxaloacetate and acetyl- CoA. Oxaloacetate can be transformed back into pyruvate while acetyl-CoA undergoes further modification to form fatty acids. 3. Acetyl-CoA is transformed into malonyl-CoA by the reduction of ATP to ADP and the use of HCO 3- 4. Malonyl-CoA is able to be transformed into fatty acids. Three fatty acids join together with one glycerol molecule (which can be formed via fructose) to build a triglyceride molecule. This can then be stored in fatty cells. Overview of the Process of Carbohydrate Metabolism Glucose, as a result of carbohydrate digestion and absorption, can undergo three different modifications and therefore end up as energy fuel or storage material. To produce energy the glucose needs to enter the cellular respiration in form of glycolysis. To store energy glucose has two options: either it is stored within the tissue as long glycogen chains or it is transformed into triglycerides within the liver and later stored in adipose tissue until further use. 28 BBS2041 – Summary Chiara Thömmes Energy Production – Triglycerides Fats are digested and absorbed in the form of triglycerides. They enter the cell as fatty acids and then join together with a molecule of glycerol to form the fat molecules triglycerides. As well as glycose, triglycerides can enter a metabolic pathway to produce ATP, the body’s energy fuel. The process in which triglycerides are broken down into their single structures is called lipolysis, which includes the further processing of the glycerol into glyceraldehyde and the further processing of the fatty acids, called β-oxidation. The energy cycle of triglycerides is closely linked with the energy cycle of glucose. But triglycerides are the main energy source of the body, not carbohydrates. Lipolysis Lipolysis describes the breakdown of triglycerides within the cell. It occurs within the cytoplasm and results in free fatty acids and a glycerol molecule. This process is essential for the mobilisation and storage of energy during fasting and exercise and it usually occurs within adipose tissue cells – called fat adipocytes. Within adipocytes the triglycerides are stored within lipid droplets. The force behind the hydrolysis of the molecules are lipases. Once activated via phosphorylation, the lipases are able to enter the lipid droplets and start the process of hydrolysis. There are several steps involved in this mechanism: 1. Adipose triglyceride lipase (ATGL) catalyses the triglyceride molecule into diacylglycerol by removing the first fatty acid. 2. Then, the enzyme hormone-sensitive lipase (HSL) can attack the molecule and break it down into monoacylglycerol by removing the second fatty acid. 3. Afterwards, the monoacylglycerol can be broken down into glycerol and the last fatty acid via the activity of the enzyme monoacylglycerol lipase (MGL). The main reaction of the lipolysis is as following: Triacylglycerol + Adipose Triglyceride Lipase + H2O → Diacylglycerol + 1 free Fatty Acid Diacylglycerol + Hormone-sensitive Lipase + H2O → Monoacylglycerol + 1 free Fatty Acid Monoacylglycerol + Monoacylglycerol Lipase + H 2O → Glycerol + 1 free Fatty Acid The produced glycerol then enters the glycolysis via transformation into glyceraldehyde, while the free fatty acids will undergo β-oxidation. Glycerol can be oxidized and therefore form an alcohol group, which results in an aldehyde called glyceraldehyde. By forming glyceraldehyde-3-phosphate with an additional phosphorylation (energy-dependent – ATP → ADP) the glycerol can enter the energy-releasing phase of the glycolysis. β-Oxidation Before free fatty acids can be used for the energy cycle, they need to be processed further by a process called β- oxidation. Basically, β-oxidation describes the process of breaking down the fatty acids to form acetyl-CoA chains and later on single acetyl-CoA molecules. This reaction releases the acetyl-CoA molecules which can then enter the citric acid cycle and form energy via production of NADH and FADH2. The rest of the reaction is identical to the pathway the carbohydrates undergo. β-oxidation takes place within the mitochondria. For this to happen, the fatty acids need to bind to a coenzyme A which forms a fatty acyl CoA. That step is mediated via fatty acyl-CoA synthase (FACS) which uses the energy of one ATP (forming ADP). This molecule is able to enter the mitochondria via simple diffusion (in case of a short 29 BBS2041 – Summary Chiara Thömmes chain) or via carnitine palmitoyl transferase 1 (CPT1) which transforms the molecule into acylcarnitine and can be transported into the cell via carnitine translocase (CAT). Once inside the cell CPT2 transforms the molecule back into acyl-CoA. In case the fatty acids are too long to enter the mitochondria, they first enter the peroxisomes to undergo β- oxidation until the chains are small enough to enter the mitochondria. Peroxisomes produce heat instead of energy. β-oxidation can be subdivided into 4 major steps: 1. Dehydrogenation 2. Hydration 3. Oxidation 4. Thyolisis Dehydrogenation During this step, acyl-CoA is oxidized by the enzyme acyl CoA dehydrogenase. C2 and C3 of the molecule are then bound by a double bond instead of a single bond. This results in the molecule trans-delta-2-enoyl CoA. The double bond is created by the reduction of FAD+ into FADH2 which will directly be used within the ETC. Hydration The newly formed double bond between C2 and C3 of trans-delta-2-enoyl CoA is now hydrated, which means an OH group is added to the C2 molecule of the chain. This is done via the enzyme enoyl CoA hydratase and results in the molecule L-β-hydroxyacyl CoA. During this step a water molecule is needed. Oxidation As the name indicates, the OH-group of C2 from the molecule L-β-hydroxyacyl CoA is now oxidized via the enzyme 3-hydroxyacyl CoA dehydrogenase. This step is mediated by the reduction of NAD+ to NADH, which directly enters the ETC to produce energy. The molecule which results out of the oxidation is called β-ketoacyl CoA. Thyolisis During this step an enzyme called β-ketothiolase cleaves the β-ketoacyl CoA by a thiol group (SH) of another CoA molecule (CoA-SH). The cleavage takes place in between the C2 and C3 molecules of the long carbon chain, which results in an acetyl-CoA molecule and an acyl-CoA chain which is now two carbons shorter. Those 4 steps are repeated until an even-numbered acyl-CoA chain is broken down into two acetyl-CoA molecules, or until an odd-numbered acyl-CoA is broken down into an acetyl-CoA and a propionyl-CoA, which is later transformed into the succinyl-CoA, that is needed for the citric acid cycle step 4 (see page 4). 30 BBS2041 – Summary Chiara Thömmes Each cycle of β-oxidation starts with an acyl-CoA chain (with n numbers of C atoms) and ends with acetyl-CoA molecules, FADH2 molecules, NADH molecules and water (if odd-numbered a propionyl-CoA molecule additionally). The total energy yield per one acetyl-CoA production is 17ATP molecules. Energy Yield of one cycle of β-oxidation Each β-oxidation cycle (so the production of 1 acetyl-CoA molecule) produces: - One FADH2 molecule – enters the ETC and produces 2 ATP molecules - One NADH molecule – enters the ETC and produces 3 ATP molecules - One acetyl-CoA molecule – enters the citric acid cycle and produces 12 ATP molecules →This results in a total amount of 17 ATP molecules, minus the waste product of heat the net production is between 12 – 16 ATP molecules. Example of a total cycle on one triglyceride with 3 fatty acids of 18C 1. Glycerol – enters the glycolysis and produces a total amount of 22 ATP molecules 2. One 18C fatty acid – undergoes β-oxidation and afterwards enters the citric acid cycle. There it produces 9 acetyl-CoA molecules which results in the total amount of 9 x 12 ATP molecules (108 ATP) + 8NADH & 8FADH2 molecules resulting in 40ATP. This sums up to 148 ATP molecules. Considering the phosphorylation in the beginning we need to extract 2 ATP molecules, which leaves us with a net ATP production of 146ATP per 18C fatty acid chain. 3. Glycerol + 3 18C fatty acids then results in the production of: 146 x 3 + 22 = 460ATP 31 BBS2041 – Summary Chiara Thömmes Insulin – Production & Function Insulin is a hormone that is responsible for allowing glucose in the blood to enter cells, providing them with the energy to function. A lack of effective insulin plays a key role in the development of diabetes. Insulin is a chemical messenger that allows cells to absorb glucose from the blood stream, and additionally assists in breaking down fats and proteins for energy. It is produced within the pancreas, an accessory glandular digestive organ. Clusters of cells within the endocrine pancreas called islets produce the hormone insulin and determine the amount of the secretion based on blood glucose levels within the body. Besides the production of insulin, islets are responsible for the production of amylin, glucagon, somatostatin and pancreatic polypeptides. The islets are divided into alpha, beta, D and PP/F cells. Beta cells are responsible for the production of insulin. The higher the levels of glucose, the higher the levels of insulin. If insulin levels are too low or high, excessively high or low blood sugar can start to cause symptoms and or chronic health issues like diabetes. Insulin depresses blood glucose levels in different ways including glycogen synthesis and increasing the cell consumption of glucose. It also stimulates the conversion of glucose into proteins and lipids, which as well reduces the levels of glucose within the body. Besides that, insulin also inhibits the hydrolysis of glycogen (glycogenolysis – the breakdown of glycogen chains into glucose molecules) within the liver and muscles. Insulin has a close relationship with the liver. There the hormone increases glycogenesis and lipogenesis (reducing glucose) while it inhibits lipolysis and glycogenesis and/or gluconeogenesis (keep storage of unused glucose intact). Insulin has two major pathways: 1. Within the digestive system – when carbohydrates are ingested, digested and absorbed, the glucose enters the blood stream; this signals the pancreas for the production and release of the hormone to maintain the blood glucose levels at a normal range 2. Within the circulation – when insulin is too low, then glucose gets back into the bloodstream from the cells which stored it; because of this insulin needs to be maintained constant and adequately within the blood stream, so no diseases are developed (diabetes, nervous disorders, …) Insulin function via an extracellular membrane-bound receptor on the target cells. When this receptor is activated int phosphorylates IRS which in turn activates PI3K. PI3K is an activator of AKT. AKT is an important mediator molecule within the cell that acts as a second messenger. It has several different functions: a. It inhibits GSK-3 and therefore in the reactivation of GS – this results in the glycogen synthesis b. It activates mTOR c. It causes a GLUT4 translocation – GLUT4 is a transmembrane protein which regulates the insulin mediated glucose intake of the cells 32 BBS2041 – Summary Chiara Thömmes Insulin Glucagon Insulin and glucagon keep the plasma glucose concentrations within an acceptable range. During a meal insulin levels dominate glucagon levels, and during a fasting period glucagon levels dominate insulin levels. Insulin Domination: Insulin levels dominate during a meal. There is an increase glucose intake and therefore the glucose levels within the blood rise. This leads to glucose induced energy production and the excess glucose molecules are stored as glycogen or fat. This means that high insulin levels induce glucose oxidation, glycogen synthesis, fat synthesis and protein synthesis. Glucagon Domination: Glucagon levels dominate during a fasting state. The liver uses glycogen to synthesize glucose which is then released into the blood stream for energy production. This means that stored glucose is used as fuel for energy production and not ingested glucose. This means that high glucagon levels induce glycogenesis, gluconeogenesis and ketogenesis. Insulin Resistance & Sensitivity Insulin resistance (IR) is a pathological condition in which cells fail to respond normally to the hormone insulin. In states of insulin resistance, the same amount of insulin does not have the same effect on glucose transport and blood sugar levels than normally. Risk factors for insulin resistance include obesity, sedentary lifestyle, family history of diabetes, various health conditions, and certain medications. Insulin sensitivity describes how sensitive the body is to the effects of insulin. Someone said to be insulin sensitive will require smaller amounts of insulin to lower blood glucose levels than someone who has low sensitivity. People with low insulin sensitivity, also referred to as insulin resistance, will require larger amounts of insulin either from their own pancreas or from injections in order to keep blood glucose stable. Having insulin resistance is a sign that your body is having difficulty metabolising glucose, and this can indicate wider health problems. Low insulin sensitivity can lead to a variety of health problems. The body will try to compensate for having a low sensitivity to insulin by producing more insulin. However, a high level of circulating insulin (=hyperinsulinemia) is associated with damage to blood vessels, high blood pressure, heart disease and heart failure, obesity, osteoporosis and even cancer. It is particular dangerous in the case of type I diabetes. 33 BBS2041 – Summary Chiara Thömmes Case III – Starvation & Fasting Starvation Definition Starvation (not an adequate nutrition / malnutrition) = the result of a severe / total lack of nutrients and/or energy needed for the maintenance of the human body; can result in organ damage / death Adequate nutrition is dependent on two components: 1. Necessary nutrients 2. Energy (in form of calories) Those two components need to be in a healthy balance. The human body needs to ingest enough calories containing an adequate amount of nutrients. It is possible to eat too less calories while the nutrients are adequate, and it is possible to eat enough calories while the nutrients are lacking. This balance needs to be maintained so the body contains both in an adequate amount. Types There are several different types of starvation that can be considered. They differentiate in their time-period and in the type of nutrient that is lacking: - Acute starvation - Chronic starvation - Simple starvation – which can be further divided into three stages (early, adaption & terminal) - Protein and/or Energy malnutrition – mostly seen in the starving children of the third world countries - Chronic energy deficient / marginally inadequate intake - Illnesses connected to starvation – may lead to wasting / anorexia According to the nutrition deficiencies, there are two types of starvation according to their severity: 1. Type I – when a person is suffering from type I starvation, they have a lack in all the vitamins, most trace elements and calcium 2. Type II – when a person is suffering from type II starvation, they have a lack in all kind of nutrients: a. Chemical molecules / ions – potassium (K+), sodium (Na+), magnesium (Mg2+), zinc, nitrogen, sulphur, … b. Essential amino acids – amino acids which cannot be synthesized by the body on its own c. Water d. Sources of energy – normally taken in via diet (carbohydrates, fats & proteins) Symptoms & Signs Starvation has several different signs and symptoms according to the type and the severity. Besides that, the timing of the symptoms also depends on the age, size and the overall health of a person: 1. Metabolic changes The body starts to change the metabolism. In children those symptoms can lead to a slower growth which eventually stops completely, fat and muscle tissue is reduced and therefore the height and the weight of the child is negatively influenced. 2. Mental changes The mental changes refer to a change in attention and concentration. Starved people can be noticed when they are easily distracted, irritated, fatigue and they have trouble concentrating. 34 BBS2041 – Summary Chiara Thömmes 3. Physical changes The physical changes are the most obvious signs of someone suffering starvation. The person is weak and loses muscle strength, the heart rate increases while the breathing is getting shallow, they are easily exhausted and have a great thirst. The skin of the person starts to loosen and pale while some body regions start to swell (feet, ankle or in children belly). 4. Immune system changes Starvation also influences the immune system. When we lose our energy, the immune system is getting weaker. This means that our wound healing is slowing down and the defence mechanisms against infections is decreasing and weakening. 5. Additional changes Besides the changes in the above-mentioned body systems, there are other symptoms of starvation. The person suffering malnutrition is in danger of gallstones and women suffer from an irregular / absent menstruation. Phases of Starvation The phases of starvation are divided by their time. In total there are three phases of starvation: early, adaption & terminal stage. The early stage starts after 1 day, which marks the first day as not an official part of starvation. This day is called ‘skipping meal / fasting’ stage and starts after eating and lasts 1day. During this day the body produced and uses glycogen by the liver and skeletal muscles. Early Stage The early stage of starvation begins after 24h and lasts until 72h (day 2 and 3 without eating). The main goal of this stage is to maintain the blood glucose levels as equal as possible (about 4-6 mmol/L) to keep the energy normal within the body. To do so, the human body uses 4 different mechanisms: 1. Glycogen breakdown & onset of gluconeogenesis Glycogenolysis happens first within the liver (lasts 4-5h), then it is performed within the skeletal muscle (up until 1 day). Afterwards, the hepatocytes start the process of gluconeogenesis with the help of lactic acid, pyruvic acid, glucogenic amino acids & glycerol. 2. Lipid metabolism Lipid metabolism is the main source of energy during fasting/starvation phases. The lipolysis (see case 2) is increased within the adipocytes of the adipose tissue. This means that the fatty acid & glycerol levels within the blood stream increase. Besides entering the citric acid cycle, the fatty acids can undergo ketogenesis while the glycerol undergoes gluconeogenesis. 3. Metabolism of amino acids / proteins As soon as the glycogen is used up and the fasting state exceeds, the muscles are becoming a source for energy themselves. Their proteins contain a lot of carbon molecules which can be used for gluconeogenesis. This means that the proteolysis increases, and proteins are broken down into amino acids. Most important are thereby alanine (Ala) & glutamine (Gln) which are released into the blood. In response to a higher metabolism of amino acids, the urea cycle increases its activity as well (Ureagenesis – later more). 4. Ketogenesis Ketogenesis is linked with the lipid metabolism and is an counteract for proteolysis. It is a process in which the body produces ketone bodies though the breakdown of fatty acids and ketogenic amino acids (later more). 35 BBS2041 – Summary Chiara Thömmes Adaptive Stage The adaptive stage (also known as protein sparing stage) can last up to weeks. The body is adapting to the lack of nutrition and uses fatty acids and ketone bodies as main source of energy to spare the proteins within the muscles. During this phase, the brain starts to adapt. From 80g/day glucose it goes down to 35g/day while the rest of the energy supply is mediated by ketone bodies. The leading mechanism in this phase is: gluconeogenesis. The pyruvic acid, lactic acid, amino acids and glycerol are used to maintain the glucose levels as steady as possible. Besides that, there is an increase in lipolysis & ketogenesis. Terminal Stage The terminal stage starts within week 7. This stage is characterized by an exhausted fat storage. Most triglycerides within the body are now used up and therefore cannot prevent the protein metabolism. This means that the main source of energy are not the fatty acids and ketone bodies anymore, but the proteins within the muscles. They are broken down into amino acids and the body is therefore suffering from severe muscle loss. The loss is greater within type II fibers than within type I fibers. Besides the loss in muscle mass, the immune system is suppressed, and other organ functions are decreased. The respiratory muscle is broken down which weakens the breathing mechanism and disabled the function of coughing (the risk of getting respiratory infections increases). Additionally, the smooth muscles of the gut system are influences by the muscle loss. This leads to an impaired nutrient absorption and increases the effects of starvation even further. If this stage is reached and not immediately treated, the patient will suffer from organ damage, organ failure and eventually die. 36  BBS2041 – Summary Chiara Thömmes Overview of the mechanism involved in the three stages of starvation Those are the main processes involved in the maintenance of the body during starvation. All the pathways are explained later on in more detail. Energy Storage within the Human Body The energy storage within the body is distributed to several organs and tissues. The energy is mostly stored in the form of triglycerides, but also in the form of glucose and proteins. Approx. distribution of energy sources in a healthy typical 70kg man: Available energy in kcal and gram Organ / Tissue Glucose / Glycogen Triglycerides (TAG) Mobilizable Proteins Liver 400kcal (approx. 100g) 450kcal (approx. 100g) 400kcal (approx. 100g) Muscle 1200kcal (approx. 350g) 450kcal (100-300g) 24000kcal (approx. 5000g) Adipose Tissue 80kcal (approx. 20g) 135000kcal (approx. 12kg) 40kcal (approx. 10g) Blood & Extracellular Fluid 60kcal (approx. 15g) 45kcal (approx. 10g) 0 Brain 8kcal (approx. 5g) 0 0 37 BBS2041 – Summary Chiara Thömmes Metabolic Processes within the Body Glycogenolysis & Gluconeogenesis Glycogenolysis is the first metabolic process that starts after 4h post eating. It is the first intervention of the body in response to an empty stomach. The glycogen storage of the liver & the muscle tissues is transformed back into glucose to fuel the body with energy. After the glycogen is used up, the body starts to form new glucose molecules out of different substances, called gluconeogenesis. While glycogenolysis stops after 24h, gluconeogenesis extends until the terminal stage of starvation. Glycogenolysis The glycogen breakdown within the liver is the first resource for the body to generate energy when no food is ingested. The liver glycogen storage can provide enough energy for about 4-5h. The liver breaks down the glycogen storage into glucose (either directly (10%) or via the production of glucose-6-phosphate (90%)). The glucose-6-phosphate is then modified by cutting of the phosphate group and therefore the molecule is able to leave the liver cells and enter the bloodstream. There is can travel to the destination where it is needed the most and provide energy via the glycolysis & Krebs cycle. Glycogen is not only broken down by the liver but also within the skeletal muscles. The only difference between the two mechanisms is, that skeletal muscle cells do not contain the appropriate enzyme to cut off the phosphate group of glucose-6-phosphate. Therefore, the glucose molecule itself cannot leave the cells. Instead, the glucose undergoes glycolysis, which means the molecule is oxidized to f