Post-Absorption of Lipids MCT PDF

Royal College of Surgeons in Ireland – Medical University of Bahrain Post-absorption Processing of Lipids Module : Gastrointestinal & Hepatology Code: MED 201 Class : MedYear 2 semester 1 Lecturer : Dr Jeevan K Shetty Date : 19th Sep 2024  Learning outcomes Describe...

Royal College of Surgeons in Ireland – Medical University of Bahrain Post-absorption Processing of Lipids Module : Gastrointestinal & Hepatology Code: MED 201 Class : MedYear 2 semester 1 Lecturer : Dr Jeevan K Shetty Date : 19th Sep 2024  Learning outcomes Describe the transport of lipids in chylomicrons and other lipoproteins Describe the utilisation and storage of triglyceride in the body Describe the extraction of energy from fatty-acids by the process of beta-oxidation Describe the synthesis and use of ketone bodies Discuss in outline the functions of brown adipose tissue Describe a pathophysiological example: Medium-chain Acyl-CoA Dehydrogenase Deficiency Dietary Fat - There are three major dietary fats in food: - Saturated fats (meat, butter, cheese, chocolate) - Transfats (meat and dairy) - Unsaturated (oils from fish and plants, avocados, nuts) - Most dietary fats have a similar chemical structure - three fatty acids attached to a glycerol backbone known as triglyceride or triacylglycerol - Digested by lipase enzymes and absorbed in the small intestine - Functions: - Energy reserve - Temperature control - Protection of internal organs - Absorption of fat-soluble nutrients (vitamins)  Fast Fed cycle (state) Fed state FAs Diet 1. Absorption/transport Nutrient rich build up stores 2. Triglyceride synthesis Triglycerides (adipose tissue) Fasting Ketone Energy bodies 3. Lipolysis Nutrient poor use stores 5. Ketogenesis (liver only) Energy FAs 4. -oxidation Lipid Digestion and absorption Triacyl glycerides (TAG) +3X - made up of fatty-acids (FAs) and glycerol - Stored in adipocytes - the principal storage form of energy in the body - contain 2X energy per gram as carbohydrates FAs ~ 95% of their energy content Glycerol ~ 5% of their energy content  Triglyceride Transport Triglycerides are large & hydrophobic molecules - insoluble in aqueous environments - transported by incorporating into lipoprotein particles Lipoproteins: Biochemical assemblies with the primary function to transport lipid molecules in water Consist of TAG and cholesterol core, surrounded by an amphiphilic outer shell of phospholipids - hydrophilic portions-oriented outwards and lipophilic tails oriented inward. Apolipoprotein (B48) is embedded in the outer shell to stabilise the complex. Apolipoproteins have 3 major functions - 1) a structural role in guiding the formation of lipoproteins 2) act as ligands for lipoprotein receptors 3) regulate enzymes involved in lipoprotein metabolism.  Lipoproteins There are several classes of lipoprotein - classified based on size, lipid composition, and apolipoproteins - Size decreases and density increases as TAGs are removed  Post-absorption Transport of Triglyceride - Following digestion, fat is absorbed principally as: -2-monoacylglycerol and free FAs - Triglycerides are then resynthesised in the endoplasmic reticulum - TGs are then incorporated into chylomicrons in the golgi apparatus - cholesterol esters & fat soluble vitamins are also incorporated - Important Apolipoproteins include Apo B Apo C Apo E  Post-absorption Transport of Triglycerides Chylomicrons consist of - Phospholipids - Apolipoproteins (ApoB, ApoC, ApoE) - Triglycerides (90%) - Cholesterol Esters - Fat Soluble Vitamins - After synthesis chylomicrons are - secreted across the basolateral membrane - enter the lymphatic circulation - enter the blood via the thoracic duct to the left subclavian vein  Lipoprotein Lipase & Lipogenesis - Chylomicrons circulate in the blood until they come to organs expressing the enzyme, lipoprotein lipase (LPL). - LPL is primarily synthesised in adipose, muscle and heart tissue and then secreted and attached to the endothelium of adjacent capillaries. - (not expressed in the liver or brain) - LPL is activated when it binds to ApoC II on chylomicrons Free Fatty Acids - Primary function is to hydrolyse TGs carried in + monoacylglycerol chylomicrons to FAs and monoacylglycerol, which then diffuse into adipocyte and are re-assembed into TGs - LPL expression is upregulated by insulin in adipocytes, but it is constitutively active in muscle cells Re-assembled to - In fed state, TGs are taken up by adipocytes TGs and stored - In the fasting state TGs are preferentially taken up by muscle cells for generating energy. Triglyceride synthesis - After a meal, most of the FAs used in TG synthesis in adipose tissue come from chylomicrons - However, FAs can also be synthesised - Occurs mostly in the liver and adipose tissue. - Following a meal high in carbohydrates - glycogen stores fill up and excess glucose is used to make TGs - Glucose is metabolised by glycolysis to acetyl CoA - Acetyl CoA is diverted from the TCA cycle into lipogenesis to form FAs - (catalysed by an enzyme complex known as Fatty Acid Synthase) - FAs combine with glycerol to form TGs (occurs in the endoplasmic reticulum) - TGs are packaged into VLDLs and secreted into the blood  Lipolysis - Lipolysis = metabolic pathway through which TGs are hydrolysed into glycerol and free fatty acids - Primary function is to mobilize stored energy during fasting or exercise - In adipocytes 3 Enzymes are involved - Successively - Adipose Triglyceride Lipase (ATGL) remove FAs from - Hormone Sensitive Lipase (HSL) the glycerol - Monoacylglycerol Lipase (MGL) backbone HSL - FFAs diffuse from the cell and are distributed throughout the body in the blood bound to albumin - Glycerol travels to the liver and can enter into glycolysis or gluconeogenesis  Hormonal Regulation of Lipolysis - Key target for hormonal regulation is the activity of HSL - Hydrolyses the 1st and 2nd FAs from glycerol - HSL activity is controlled by its phosphorylation state - Phosphorylated = Active (mediated by Protein kinase A) - Dephosphorylated = Inactive (mediated by Phosphatase) Which hormone ? A  Hormonal Regulation of Lipolysis - Primary hormones that regulate lipolysis are: - Insulin - released in the fed state - Acts on a receptor tyrosine kinase - Inhibits HSL phosphorylation - Inhibits lipolysis - Catecholamines & Glucagon - released in the fasted/exercise state - Act on GsPCRs - Elevate cAMP levels and activate PKA - Promotes HSL phosphorylation - Promotes lipolysis (breakdown of fat) - Other Lipolytic Hormones act by increasing expression of HSL - enhance effects of catecholamines & glucagon (e.g., glucocorticoids, growth hormone, thyroid hormone)  Summary - Fed state (i.e.., high levels of Insulin) - TGs are transported in chylomicrons to adipose tissue - Chylomicrons bind via ApoC to LPL - LPL breakdown TGs into monoacylglycerol and free FAs - Diffuse into adipocytes and reassembled into TGs - Stored in fat droplets - Energy is stored - Fasting state/exercise (ie., low insulin, high glucagon/cathecholamines) - HSL (and other lipases) becomes phosphorylated and activated - TGs are metabolised into free fatty acids and glycerol - FAs bind albumin and travel to their target tissue for further metabolism - Glycerol travels to the liver for further metabolism (glycolysis or gluconeogenesis) - Energy is released  Extraction of energy from fatty-acids - Occurs by a process known as β-oxidation - β-oxidation = the catabolic process by which FAs are broken down to generate energy Fatty Acid - Occurs in almost all cell types - Except erythrocytes and neurons - Enzymes required for β-oxidation are found in the mitochondria Acetyl-CoA - Occurs in 3 Stages - i) Activation of FAs in the cytosol - ii) Transport into the mitochondrion - iii) β-oxidation in the mitochondrial matrix - The overall process involves the breaking down of FA chains into Acetyl-CoA, which can then enter the TCA cycle to produce energy. ETC →ATP NADH & FADH2 & ATP  Step 1: Activation of Fatty Acids - Upon entering the cytosol - 1. Fatty Acids are activated to acyl-CoA by acyl CoA synthase (thiokinase). - Short chain fatty acyl-CoA can diffuse into the mitochondrion - Long chain FAs cannot enter by diffusion and must use the carnitine shuttle  Step 2: Transport to the mitochondrion The Carnitine Shuttle i) Carnitine Palmitoyl Transferase I (CPT I) substitutes carnitine for CoA on the FA ii) Acyl-carnitine then crosses the IMM into the matix via Acyl-Carnitine Translocase iii) Carnitine Palmitoyl Transferase II (CPT II) resubstitutes CoA for carnitine to regenerate the acyl-CoA iv) Carnitine exits the matrix through the translocase into the intermembrane space Step 3: β-oxidation in the mitochondrial matrix - A cyclic 4 step reaction reaction - Initiated by Acyl-CoA dehydrogenase - 3 isoforms of Acyl-CoA dehydrogenase exist - Short chain - Medium chain - Long chain - With each cycle 2 carbons are removed from the FA as acetyl- CoA - Each cycle produces - Acetyl CoA → TCA cycle/ETC → ATP - 1 NADH → ETC→ ATP - 1 FADH2 Regulation of -oxidation Rate of -oxidation is largely regulated by: 1) Levels of free FAs - metabolism of FAs by tissues is proportional to levels of plasma free FAs - dependent on activity of HSL in adipose tissue - dependent on levels of insulin (inhibitory) and glucagon & catecholamines (stimulatory) 2) Levels of cytosolic Malonyl-CoA (Fatty Acid Biosynthesis) - 1st step in FA biosynthesis is the generation of Malonyl Co-A in the cytoplasm - Malonyl Co-A inhibits CPT1 – FAs can’t enter the mitochondria - Ensures that FA biosynthesis and FA breakdown are not occurring at the same time. Ketone Bodies Primary Function – to serve as a reserve fuel in the body, especially for the bra There are 3 ketone bodies - Acetoacetate - beta-Hydroxybutyric Acid (formed from acetoacetate) - Acetone (formed in small amounts by decarboxylation of acetoacetate) - Only synthesised in the liver from acetyl Co-A Ketogenesis - Synthesis only occurs in response to an unavailability of blood glucose (e.g., fasting) - blood glucose levels decrease, and glycogen is depleted. - The body switches to lipid metabolism - Acetyl CoA enters the TCA cycle to produce energy - Oxaloacetate is required - When glucose levels are low the body also switches to gluconeogenesis - Oxaloacetate is used - When oxaloacetate is depleted acetyl CoA cannot enter the TCA cycle and instead is used to form ketone bodies - = Ketogenesis - Ketogenesis also occurs in diabetes mellitus as cells cannot metabolise glucose  Ketogenesis Ketogenesis occurs when glucose is unavailable and oxaloacetate is depleted (e.g., when fasting or in diabetes) - Acetyl CoA enters into ketogenesis in the liver - Ketone bodies are released into the blood - Taken up into target tissues - Converted back into acetyl CoA which can then enter the TCA cycle Brain Heart Muscle Ketogenesis X Acetyl CoA TCA Cycle Ketoacidosis - Under conditions of poorly managed diabetes or prolonged starvation glucose is unavailable and ketones become an important energy source - If levels in the blood become too high ketoacidosis can occur (Both acetoacetate and b-hydroxybutyrate are acids) - Diabetic ketoacidosis can occur due to - missed insulin treatments/Malfunctioning pump - Illnesses that release hormones that reduce insulin secretion (e.g., pneumonia, infection) - Symptoms: thirst, increased urination, abdominal pain, nausea, vomiting, dehydration, confusion. - Can be fatal if not managed. - Treatment: Insulin, fluids and electrolytes  Brown Adipose Tissue - White adipose tissue (WAT) comprises most of the body’s fat stores - Primary function is to store and release fatty acids for generating energy - Brown adipose tissue (BAT) is distinct to WAT and is activated in response to cold temperatures. - Primary function is to produce heat to help maintain body temperature (thermogenesis) - Especially abundant in newborns and hibernating animals - BAT is characterised by - Multiple lipid droplets (WAT contains a single lipid droplet) - Densely packed with iron-containing mitochondria (gives a brown colour) - Highly vascularised - good oxygen supply for oxidative phosphorylation - enables dissipation of heat to the body - BAT also differs from other cells by expressing low levels of ATP synthase - the ETC is not coupled to ATP production - Instead, Uncoupling protein 1 (aka. thermogenin) is expressed in BAT - uncouples oxidative phosphorylation from ATP synthesis - Energy is released as heat instead Brown Adipose Tissue - Summary - ATP Synthase expression is low - Ox-Phos is uncoupled from ATP synthase - UCP1 expression is high - Heat is generated  Medium Chain Acyl-CoA Dehydrogenase Deficiency (MCADD) MCADD Disorder of b-oxidation that impairs breakdown of medium- chain FAs into acetyl-CoA - incidence is rare: 1:4000 in Germany, 1:700,000 in Taiwan - Occurs due to a genetic defect in Medium Chain Acyl Co-A Dehydrogenase - 1st step in b-oxidation of FAs in the mitochondrion Main clinical signs: - intolerance to prolonged fasting - hypoglycaemia - impaired ketogenesis - acidosis due to FA accumulation in the blood Treatment is preventive - avoid fasting and other situations where the body relies on b-oxidation to supply energy. - Use glucose supplements  RESOURCES Introduction to Fatty Acid Metabolism: https://aklectures.com/lecture/fatty-acid-metabolism-introduction/introduction-to -fatty-acid-metabolism Overview of Fatty Acid Oxidation https://www.khanacademy.org/test-prep/mcat/biomolecules/fat-and- protein-metabolism/v/overview-of-fatty-acid-oxidation Ketogenesis: https://www.youtube.com/watch?v=dbno1Aa-E_4 Ketoacidosis: https://www.youtube.com/watch?v=LwLWo6kUst4