Lipid Metabolism - TAG & Ketone Metabolism PDF

Lipid Metabolism Fatty Acid & Ketone Body Metabolism  Fatty Acid Significance Fatty acids play several important roles: Building blocks for phospholipids and glycolipids binds proteins to membranes Fatty acid derivatives are used as hormones and...

Lipid Metabolism Fatty Acid & Ketone Body Metabolism  Fatty Acid Significance Fatty acids play several important roles: Building blocks for phospholipids and glycolipids binds proteins to membranes Fatty acid derivatives are used as hormones and intracellular messengers High energy fuel source Along with glycerol forms storage molecule triacylglycerols  Triglycerides (Triacylglycerols - TAGs) Made of 3 fatty acids ester bonded to glycerol Highly concentrated store of energy 9kcal/g vs 4kcal/g glycogen Glycogen is highly hydrated – 2g H2O/g glycogen TAGs are stored anhydrously Body has a near infinite capacity to store TAGs Utilization of TAGs as Fuel 3 stages of processing 1. Triacylglycerol degradation Occurs in adipose tissue TAGs degraded to fatty acid and glycerol Lipase converts triacylglycerol to di and then mono and then free glycerol. Free fatty acids are exported out of adipocytes and bind albumin in blood where transported to tissues that need it. 2. Fatty acid activation Occurs in cytosol before entering mitochondria 3. Fatty acid degradation Occurs in mitochondrial matrix Pathway – b-oxidation  Glucagon Degradation of Epinephrine TAGs is Controlled by Hormones Norepinephrine ACTH Fate of Glycerol Free glycerol cannot be phosphorylated in adipocytes as lack glycerol kinase. It is transported to liver, where it is phosphorylated and then reduced to DHAP which can enter glycolysis or gluconeogenesis. NB. ATP is used and NADH is synthesized. Fatty Acid Activation & Transport Fatty acids are activated in the cytosol by addition of CoA to form (fatty) acyl CoA Enzyme - acyl CoA synthetase Uses ATP Note ATP to AMP = 2 ATP Once activated it crosses the mitochondrial membranes into the matrix with the aid of carnitine What’s Going On Here? Is this pathway anabolic or catabolic? Why? What type of pathway is it? Linear, cyclical or spiral? What is the starting metabolite? What is being produced? What is this pathway doing? What is the function of this pathway? What are the key regulatory steps/enzymes in this pathway? How do you recognize them? Fatty Acid Degradation: b- Oxidation Recall – fatty acids are long chained hydrocarbons, typically 12C or more b-oxidation degrades FAs by removing 2C at a time and converting it to Acetyl CoA Acetyl CoA produced further metabolized in TCA and ETC to generate ATP It is a strictly aerobic process Occurs in the mitochondrial matrix b-Oxidation of Saturated Fatty Acids b-oxidation occurs in 4 steps Dehydrogenation – produces FADH2 Hydration – addition of water Dehydrogenation – produces NADH + H+ Thiolysis and addition of CoA – forms Acetyl CoA and an activated fatty acid (Acyl CoA), 2 C shorter. The resulting acyl CoA enters consecutive rounds of b- oxidation until it is completely degraded (2C at a time) to Acetyl CoA Fatty Acid Synthesis Glucose is the major source of carbon skeletons for FA synthesis In the fed state, excessive amino acids can be used to make fatty acids via conversion to glucogenic intermediates or directly to acetyl CoA FA synthesis occurs in the cytosol It requires the synthesis of a primer, malonyl CoA (MCoA), from acetyl CoA (AcCoA) Fatty Acid Synthesis is Energetically Expensive FA synthesis only occurs when there is high levels of citrate and ATP in the cytosol ATP is required for the synthesis of the primer, malonyl CoA And NADPH is required for the reduction of the carbon skeleton Sources of NADPH Pentose Phosphate Pathway Conversion of oxaloacetate to pyruvate which exchanges NADH for NADPH Pentose Phosphate Pathway Sources of NADPH for Fatty Acid Synthesis  Fatty Acid Synthesis Acetyl CoA is synthesized in mitochondrial matrix from pyruvate and amino acids There is no membrane protein to transport acetyl CoA it into the cytosol This acetyl CoA is converted to citrate via the TCA then shuttled out of the mitochondria  Synthesis of Malonyl CoA – The Rate Limiting Step Formation of malonyl CoA is the committed step in FA synthesis It is also the rate limiting step in the process The enzyme Acetyl CoA Carboxylase (ACC) catalyzes this reaction and is allosterically and covalently regulated Note: CO2 is added Biotin cofactor (Vit B) required Fatty Acid Elongation Involves 4 Steps To start an elongation cycle, Acetyl–CoA and Malonyl–CoA are each transferred to an acyl carrier protein (ACP) Fatty acid elongation happens in 4 steps that lengthens the carbon skeleton 2C at a time: The enzyme which catalyzes the reaction is fatty acid synthase which has 7 different catalytic sites 1. Condensation – 3C malonyl CoA is bonded to 2C Actyl Co to form a 4C compound with the loss of CO2 and the use of ATP 2. Reduction – NADPH is used to reduce the carbonyl group to an alcohol 3. Dehydration – removes H2O and forms double bond 4. Reduction – uses NADPH to add H and break the double bond to form the 4C fatty acid Fatty Acid Synthesis Condensation Reduction Dehydration Reduction Resulting acylCoA which is 4C long enters the cycle again where it is lengthen by 2 carbons This occurs until the fatty acid is 16C long (palmitic acid) Fatty Acid Synthesis 4C fatty acid attached to the enzyme is then repeatedly condensed with malonyl CoA and is elongated 2C at a time via the previous 4 steps When 16 C (palmitate) have been added, fatty acid is released from enzyme. Palmitate can undergo further elongation in different subcellular compartments (endoplasmic reticulum and mitochondria) Fatty Acid Synthesis Triacylglycerol (TAG) Synthesis Glycerol phosphate serves as template Two fatty acyl CoA (activated fatty acids) are added one at a time. After second is added phosphate from glycerol is removed Third fatty acyl CoA is then added  Acetyl CoA carboxylase (ACC) is regulated both short-term and long- term Regulation Short-term of Fatty Acid Citrate stimulates conversion of inactive dimer to active polymer Biosynthesis Long-chain fatty acyl CoA inhibits polymerization Glucagon/epinephrine stimulate phosphorylation of ACC enzyme and inactivates the enzyme Insulin stimulates dephosphorylation of enzyme activating it. Regulation of Fatty Acid Synthesis Reciprocal Regulation of Fatty Acid Biosynthesis Regulation of Fatty Acid Biosynthesis – Long Term Continual consumption of excess calories leads to increased synthesis of enzyme which leads to ready conversion of extra calories to fat ACC synthesis is stimulated by ChREBP (Carbohydrate-response element-binding protein) which is a transcription factor which is activated by high glucose concentration Low-calorie diet, leads to decreased synthesis of enzyme Ketone Body Biosynthesis & Use Acetyl CoA generated by b-oxidation has 2 fates: TCA cycle Ketogenesis in liver mitochondria In animals, Acetyl CoA cannot be used to synthesize glucose (plants can do it via the glyoxylate cycle) In cases of starvation, in liver tissue, oxaloacetate is used in gluconeogenesis which can lead to depletion Without oxaloacetate, the TCA is unable to degrade Acetyl CoA to produce energy In such cases, fatty acids are metabolized to produce ketone bodies  Ketogeneisis – Synthesis of Ketone Bodies Ketone bodies are water soluble and are more readily transported than fatty acids Ketogenesis produces 3 ketone bodies 1. Hydroxybutyrate 2. Acetoacetate 3. Acetone They are transported into the blood and transported to extra-hepatic tissues (brain!) Some Amino Acids can be used in Ketogenesis Recall: Some amino acids are glucogenic ie they can be used to make glucose in gluconeogenesis Others are ketogenic – can be used to synthesize ketone bodies or acetyl CoA Some are both  Ketone Body Utilization In peripheral tissues ketone bodies are reconverted to acetyl CoA which is oxidized in the TCA to produce energy During prolonged fasting /starvation oxidation of fatty acids and ketone bodies can be used in most tissues except liver to release energy Liver lacks acetoacetate CoA transferase and therefore cannot use ketone bodies The brain cannot use fatty acids and depends on ketone bodies for energy Ketone Body Utilization Summary Slide – Fatty Acid Degradation Summary Slide – Ketone Synthesis and Use Summary – Fatty Acid Synthesis Integrated Metabolism Fed State