L12b - L13a Metabolism and Bioenergetics PDF

Metabolism & Bioenergetics Carbohydrate metabolism Glucose metabolism Citric acid cycle Oxidative phosphorylation VPP3021 Veterinary Biochemistry  Carbohydr...

Metabolism & Bioenergetics Carbohydrate metabolism Glucose metabolism Citric acid cycle Oxidative phosphorylation VPP3021 Veterinary Biochemistry  Carbohydrate metabolism Mokrish Ajat Dept. of Veterinary Preclinical Sciences Faculty of Veterinary Medicine UPM [email protected] VPP3021 Veterinary Biochemistry  Learning outcome To differentiate various carbohydrate metabolism pathways VPP3021 Veterinary Biochemistry  To be discussed Introduction to metabolism Digestion, Absorption and Transport of carbohydrates - types of digestive enzymes - types of transport system in absorption - types of defects and abnormality Glycolysis Gluconeogenesis Pentose Phosphate Pathway VPP3021 Veterinary Biochemistry VPP3021 Veterinary Biochemistry VPP3021 Veterinary Biochemistry  Introduction More than 60% of our foods are carbohydrates. Starch, glycogen, sucrose, lactose and cellulose are the chief carbohydrates in our food. Before intestinal absorption, they are hydrolysed to hexose sugars (glucose, galactose and fructose). A family of a glycosidases that degrade carbohydrate into their monohexose components catalyzes hydrolysis of glycocidic bonds. These enzymes are usually specific to the type of bond to be broken. VPP3021 Veterinary Biochemistry  Digestion of carbohydrate by salivary α – amylase in the mouth This enzyme is produced by salivary glands. Its optimum pH is 6.7. It is activated by chloride ions (Cl-). It acts on cooked starch and glycogen breaking α 1-4 bonds, converting them into maltose [a disaccharide containing two glucose molecules attached by α 1-4 linkage]. This bond is not attacked by –amylase. VPP3021 Veterinary Biochemistry  ln the stomach: carbohydrate digestion stops temporarily due to the high acidity which inactivates the salivary - amylase. Digestion of carbohydrate by the pancreatic - amylase small intestine in the small intestine. A. α-amylase enzyme is produced by pancreas and acts in small intestine. Its optimum pH is 7.1. B. It is also activated by chloride ions. C. It acts on cooked and uncooked starch, hydrolysing them into maltose and isomaltose. Final carbohydrate digestion by intestinal enzymes: The final digestive processes occur at the small intestine and include the action of several disaccharidases. These enzymes are secreted through and remain associated with the brush border of the intestinal mucosal cells. VPP3021 Veterinary Biochemistry  The disaccharidases include: 1. Lactase (β-galactosidase) which hydrolyses lactose into two molecules of glucose and galactose: Lactase Lactose Glucose + Galactose 2. Maltase ( α-glucosidase), which hydrolyses maltose into two molecules of glucose: Maltase Maltose Glucose + Glucose 3. Sucrose (α-fructofuranosidase), which hydrolyses sucrose into two molecules of glucose and fructose: Sucrose Sucrose Glucose + Fructose 4. α - dextrinase (oligo-1,6 glucosidase) which hydrolyze (1 ,6) linkage of isomaltose. Dextrinase Isomaltose Glucose + Glucose VPP3021 Veterinary Biochemistry  Digestion of cellulose Cellulose contains β(1-4) bonds between glucose molecules. In humans, there is no β (1-4) glucosidase that can digest such bonds. So cellulose passes as such in stool. Cellulose helps water retention during the passage of food along the intestine ® producing larger and softer feces ® preventing constipation. In ruminants and monogastric (horses, rabbits etc), cellulose can be digested by cellulase which is produce by the microbes (cellulatic bacterias) VPP3021 Veterinary Biochemistry  Absorptions - Introduction The end products of carbohydrate digestion are monosaccharides: glucose, galactose and fructose. They are absorbed from the jejunum to portal veins to the liver, where fructose and galactose are transformed into glucose. Two mechanisms are responsible for absorption of monosaccharides: active transport (against concentration gradient i.e. from low to high concentration) and passive transport (by facilitated diffusion). Watch video on Putrablast or my youtube playlist Carbohydarate Metabolism. B. Transport Proteins Transport proteins = Integral membrane proteins that transport specific molecules or ions across biological membranes. Protein VPP3021 Veterinary Biochemistry  Mechanisms of absorption A. Active transport: 1. Mechanism of active transport: a) In the cell membrane of the intestinal cells, there is a mobile carrier protein called sodium dependant glucose transporter (SGL T-1) It transports glucose to inside the cell using energy. The energy is derived from sodium-potassium pump. The transporter has 2 separate sites, one for sodium and the other for glucose. It transports them from the intestinal lumen across cell membrane to the cytoplasm. Then both glucose and sodium are released into the cytoplasm allowing the carrier to return for more transport of glucose and sodium. VPP3021 Veterinary Biochemistry  Continuation… b) The sodium is transported from low to high concentration (with concentration gradient) and at the same time causes the carrier to transport glucose against its concentration gradient. The Na+ is expelled outside the cell by sodium pump. Which needs ATP as a source of energy. The reaction is catalyzed by an enzyme called "Adenosine triphosphatase (ATPase)". Active transport is much more faster than passive transport. c) Insulin increases the number of glucose transporters in tissues containing insulin receptors e.g. muscles and adipose tissue. VPP3021 Veterinary Biochemistry  Inhibitors of active transport: a) Ouabin (cardiac glycoside): Inhibits adenosine triphosphatase (ATPase) necessary for hydrolysis of ATP that produces energy of sodium pump. b) Phlorhizin; Inhibits the binding of sodium in the carrier protein. VPP3021 Veterinary Biochemistry  B. Passive transport (facilitated diffusion): Sugars pass with concentration gradient i.e. from high to low concentration. It needs no energy. It occurs by means of a sodium independent facilitative transporter (GLUT -5). Fructose and pentoses are absorbed by this mechanism. Glucose and galactose can also use the same transporter if the concentration gradient is favorable. C. sodium – independent transporter (GLUT-2), that facilitates transport of sugars out of the cell i.e. to circulation. VPP3021 Veterinary Biochemistry  Summary of types of functions of most important glucose transporters: Function Site SGLT- Absorption of glucose Intestine and renal 1 by active transport tubules. (energy is derived from Na+- K+ pump) GLUT - Fructose transport and Intestine and sperm 5 to a lesser extent glucose and galactose. GLUT - Transport glucose out -Intestine and renal 2 of intestinal and renal tubule cells ® circulation -β cells of islets-liver VPP3021 Veterinary Biochemistry  Defects of carbohydrate digestion and absorption: A. Lactase deficiency = lactose intolerance: 1. Definition: a) This is a deficiency of lactase enzyme which digest lactose into glucose and galactose b) It may be: (i) Congenital: which occurs very soon after birth (rare). (ii) Acquired: which occurs later on in life (common). 2. Effect: The presence of lactose in intestine causes: a) Increased osmotic pressure: So water will be drawn from the tissue (causing dehydration) into the large intestine (causing diarrhea). b) Increased fermentation of lactose by bacteria: Intestinal bacteria ferment lactose with subsequent production of CO2 gas. This causes distention and abdominal cramps. c) Treatment: Treatment of this disorder is simply by removing lactose (milk) from diet. VPP3021 Veterinary Biochemistry  B. Sucrose deficiency: A rare condition, showing the signs and symptoms of lactase deficiency. It occurs early in childhood. C. Monosaccharide malabsorption: This is a congenital condition in which glucose and galactose are absorbed only slowly due to defect in the carrier mechanism. Because fructose is not absorbed by the carrier system, its absorption is normal. Fate of absorbed sugars: Monosaccharides (glucose, galactose and fructose) resulting from carbohydrate digestion are absorbed and undergo the following: A. Uptake by tissues (liver): After absorption the liver takes up sugars, where galactose and fructose are converted into glucose. B. Glucose utilization by tissues: Glucose may undergo one of the following fate: continuation… VPP3021 Veterinary Biochemistry  Fate of absorbed sugars: 1. Oxidation: through a) Major pathways (glycolysis and Krebs' cycle) for production of energy. b) Hexose monophosphate pathway: for production of ribose, deoxyribose and NADPH + H+ c) Uronic acid pathway, for production of glucuronic acid, which is used in detoxication and enters in the formation of mucopolysaccharide. 2. Storage: in the form of: a) Glycogen: glycogenesis. b) Fat: lipogenesis. 3. Conversion: to substances of biological importance: a) Ribose, deoxyribose ® RNA and DNA. b) Lactose ® milk. c) Glucosamine, galactosamine ® mucopolysaccharides. d) Glucoronic acid ® mucopolysaccharides. e) Fructose ® in semen. VPP3021 Veterinary Biochemistry VPP3020 Veterinary Biochemistry  Glycolysis (Embden Meyerhof Pathway): A. Definition: 1. Glycolysis means oxidation of glucose to give pyruvate (in the presence of oxygen) or lactate (in the absence of oxygen). B. Site: cytoplasm of all tissue cells, but it is of physiological importance in: 1. Tissues with no mitochondria: mature RBCs, cornea and lens. 2. Tissues with few mitochondria: Testis, leucocytes, medulla of the kidney, retina, skin and gastrointestinal tract. 3. Tissues undergo frequent oxygen lack: skeletal muscles especially during exercise. VPP3021 Veterinary Biochemistry  C. Steps: Stages of glycolysis 1. Stage one (the energy requiring stage): a) One molecule of glucose is converted into two molecules of glyceraldehyde-3-phosphate. b) These steps requires 2 molecules of ATP (energy loss) 2. Stage two (the energy producing stage): a) The 2 molecules of glyceraldehyde-3-phosphate are converted into pyruvate (aerobic glycolysis) or lactate (anaerobic glycolysis). b) These steps produce ATP molecules (energy production). D. Energy (ATP) production of glycolysis: ATP production = ATP produced - ATP utilized VPP3021 Veterinary Biochemistry  In the energy investment phase, ATP provides activation energy by phosphorylating glucose. – This requires 2 ATP per glucose. In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH. 2 ATP (net) and 2 NADH are produced per glucose. VPP3021 Veterinary Biochemistry  Energy Investment Phase VPP3021 Veterinary Biochemistry  Asymetric Centers Energy-Payoff Phase VPP3021 Veterinary Biochemistry  Anaerobic respiration? VPP3021 Veterinary Biochemistry VPP3021 Veterinary Biochemistry  Anaerobic respiration Respiration in the absence of oxygen Lactate fermentation, in muscle. Glucose partially broken down to form lactate. Alcohol fermentation, in yeast & bacteria. Glucose partially broken down into ethanol. VPP3021 Veterinary Biochemistry  Importance of lactate production in anerobic glycolysis: 1. In absence of oxygen, lactate is the end product of glycolysis: Glucose ® Pyruvate ® Lactate 2. In absence of oxygen, NADH + H+ is not oxidized by the respiratory chain. 3. The conversion of pyruvate to lactate is the mechanism for regeneration of NAD+. 4. This helps continuity of glycolysis, as the generated NAD+ will be used once more for oxidation of another glucose molecule. VPP3021 Veterinary Biochemistry  As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the Krebs cycle in the matrix. – A carboxyl group is removed as CO2. – A pair of electrons is transferred from the remaining two-carbon fragment to NAD+ to form NADH. – The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA. VPP3021 Veterinary Biochemistry  Differences between aerobic and anaerobic glycolysis Aerobic Anaerobic 1. End product Pyruvate Lactate 2.energy 6 or 8 ATP 2 ATP 3. Regeneration of Through respiration Through Lactate NAD+ chain in mitochondria formation 4. Availability to TCA in Available and 2 Pyruvate Not available as lactate mitochondria can oxidize to give 30 is cytoplasmic substrate ATP VPP3021 Veterinary Biochemistry Biological importance (functions) of glycolysis: 1. Energy production: a) anaerobic glycolysis gives 2 ATP. b) aerobic glycolysis gives 8 ATP. 2. Oxygenation of tissues: Through formation of 2,3 bisphosphoglycerate, which decreases the affinity of Hemoglobin to O2. 3. Provides important intermediates: a) Dihydroxyacetone phosphate: can give glycerol-3phosphate, which is used for synthesis of triacylglycerols and phospholipids (lipogenesis). b) 3 Phosphoglycerate: which can be used for synthesis of amino acid serine. c) Pyruvate: which can be used in synthesis of amino acid alanine. 4. Aerobic glycolysis provides the mitochondria with pyruvate, which gives acetyl CoA Krebs' cycle. VPP3021 Veterinary Biochemistry  Energy production of glycolysis: ATP produced ATP utilized Net energy In absence of oxygen 4 ATP 2ATP 2 ATP (anaerobic (Substrate level From glucose to glycolysis) phosphorylation) glucose -6-p. 2ATP from 1,3 DPG. From fructose -6-p to 2ATP from fructose 1,6 p. phosphoenol pyruvate In presence of 4 ATP 2ATP 6 ATP oxygen (aerobic (substrate level -From glucose to Or glycolysis) phosphorylation) glucose -6-p. 8 ATP 2ATP from 1,3 BPG. From fructose -6-p to 2ATP from fructose 1,6 p. phosphoenol pyruvate. + 4ATP or 6ATP (from oxidation of 2 NADH + H in mitochondria). VPP3021 Veterinary Biochemistry Comparison between glucokinase and hexokinase enzymes: Glucokinaase Hexokinase 1. Site Liver only All tissue cells 2. Affinity to glucose Low affinity (high km) i.e. it High affinity (low km) i.e. it acts acts only in the presence of even in the presence of low blood high blood glucose glucose concentration. concentration. 3. Substrate Glucose only Glucose, galactose and fructose 4. Effect of insulin Induces synthesis of No effect glucokinase. 5. Effect of glucose-6-p No effect Allosterically inhibits hexokinase 6. Function Acts in liver after meals. It It phosphorylates glucose inside removes glucose coming in the body cells. This makes glucose portal circulation, converting concentration more in blood than it into glucose -6-phosphate. inside the cells. This leads to continuous supply of glucose for the tissues even in the presence of low blood glucose concentration. VPP3021 Veterinary Biochemistry  What have we discussed Introduction to metabolism Digestion, Absorption and Transport of carbohydrates - types of digestive enzymes - types of transport system in absorption - types of defects and abnormality Glycolysis VPP3021 Veterinary Biochemistry