Lipid Metabolism 1: Triacylglycerols and Fatty Acids PDF

8.LIPID METABOLISM 1: Triacylglyerols and Fatty Acids Degradation ( -oxidation) Synthesis Regulation Credits: Unless indicated otherwise, all figures from Berg Biochemistry. 6th ed. Others from DL...

8.LIPID METABOLISM 1: Triacylglyerols and Fatty Acids Degradation ( -oxidation) Synthesis Regulation Credits: Unless indicated otherwise, all figures from Berg Biochemistry. 6th ed. Others from DL Nelson & MM Cox, Lehninger Principles of Biochemistry, 3rd Edition, Worth, NY, 2000 Fatty Acids: Structures and Nomenclature Triacylglycerols: Fatty Acids: Saturated and Unsaturated Palmitate (saturated) Palmitoleate (unsaturated) Nomenclature for Unsaturated Fatty Acids Palmitoleate = cis - 9 - hexadecenoate or -7 -hexadecenoate The energy content of triacylglyerols is higher than carbohydrates and proteins because they are highly reduced (low oxygen content) and are hydrophobic (exclude water). This makes fat a very efficient form of stored energy. The energy content of anhydrous CHO or protein = 4 kcal/gm vs. 9 kcal/gm for fat. If the high water content of glycogen is also considered, fat has 5x the energy content/gm of glycogen. The energy stored in 14 kg body fat (ave total) = 70 kg CHO.  Absorption of Fats from the Diet Lehninger, p600-601 Epinephrine/Glucagon Much of the control of fatty acid degradation and synthesis is achieved by localizing pathways to specific tissues and subcellular compartments. Most fatty acids are synthesized in liver cytosol and degraded in the mitochondrial matrix. Fatty acids are stored as triacylglycerols in adipose tissue and mobilized to specific tissues bound to serum albumin. Steps in Fatty Acid Degradation: Mobilization from adipose tissue by hormone-dependent lipase: Regulation: Activated by cAMP-dependent protein kinase (glucagon, ephinephrine) FA:Albumen to liver Glycerol is the only portion of fat that is gluconeogenic GLUCONEOGENESIS Activation of fatty acids in liver by conjugation with CoASH. 2 high energy bonds are used to activate. ATP-> AMP Reactions: 1) enter mitochondria, 2) oxidation Transfer to carnitine for transport 2. Oxidation at ß carbon ->double across the inner mitochondrial bond Reduction of FAD -> FADH2 membrane (key regulatory step) Inner mitochondrial membrane  Hydration of double bond w/ OH @ 4. Oxidation of ß-carbon -> keto ß carbon Reduction of NAD -> NADH + H+  5. Cleavage of 3-ketoacyl CoA by CoASH Result: Acetyl CoA + acyl CoA (- 2 C) The shortened acyl CoA undergoes repeated cycles of ß- oxidation. NOTE: -oxidation pathway in the inner mitochondrial membrane is similar to the last 3 steps of the citric acid cycle (succinate > fumarate > malate > oxaloacetate)  Fatty acids are degraded 2 carbons at a time. Total energy yield in the complete oxidation of palmitate (C16) is 106 ATP Palmitoyl CoA + 7 FAD + 7 NAD + 7 CoASH+ 7 H2O 8 Acetyl CoA + 7 FADH + 7NADH + 7H+ 8 Acetyl CoA Citric Acid Cycle 3 NADH = 7.5 ATP 1 FADH2 = 1.5 ATP 1 GTP = 1 ATP 10 ATP x 8 = 80 ATP 7 FADH2 10.5 7NADH 17.5 108.0 ATP consumed in Acyl-CoA formation -2.0 ATP total 106 Degradation of Odd carbon number FA requires vitamin B12 O C - SCoA B1 2 Lack of vitamin B12 leads to accumulation of methylmalonyl CoA and nerve degeneration. A secondary anemia is also induced due to interference with folic acid metabolism (also called pernicious anemia). Ketone bodies form in the mitochondrial matrix when citric acid cycle intermediates in liver are limiting due to high rates of gluconeogenesis and lipolysis/ß-oxidation (in starvation, diabetes). NOTE: Animals cannot convert Fatty Acids to Glucose. ketone bodies During starvation, ketone bodies can be used as an alternate energy source by brain, heart muscle and renal cortex by conversion back to acetylCoA. Fatty Acid Synthesis Differences between fatty acid synthesis and degradation: * Synthesis- cytosol Degradation-mitochondrial matrix * Intermediates in FA synthesis linked to acyl carrier protein (ACP) Degradation uses Acyl-SCoA * Fatty acid synthetase- a dimer of a single polypeptide chain containing all required enzymatic activities Fatty acid degradation- several separate proteins on inner mitochondrial membrane. * In synthesis: Activated donor of 2 carbon units is malonyl CoA In degradation: 2 C units are released as acetyl CoA * In synthesis- reducing power from NADPH In degradation- oxidation of fatty acid forms reduced NADH + FADH Steps in Fatty Acid Synthesis: Acetyl CoA Carboxylase Reaction: formation of malonyl CoA (rate limiting regulated step in Fatty Acid synthesis) * Biotin is vitamin cofactor * Regulation: Allosteric activation by citrate (high in fed state). Inhibition by palmitoyl CoA (product). Inhibition by glucagon and catecholamines Phosphorylated form of enzyme shows smaller stimulation by citrate. Regulation of acetyl CoA carboxylase by phosphorylation/dephosphorylation Epinephrine/Glucagon, Low ATP Insulin (+) Fatty Acid Synthase Reactions: Formation of Acetyl-ACP and Malonyl-ACP Condensation of Acetyl -ACP with malonyl ACP with loss of CO2 Reduction of keto group by NADPH Dehydration forming a double bond  Reduction of double bond with NADPH This series of reactions is repeated until C16-Acyl-ACP is generated. Thioesterase then cleaves palmitate from the synthase complex. Free fatty acids are transported from liver to adipose tissue bound to very low density lipoproteins (VLDLs). Stoichiometry of Fatty Acid Synthesis: Acetyl CoA + 7 malonyl CoA + 14 NADPH + 7 H+ Palmitate +7 CO2 + 14 NADP +CoASH + 6 H2O (= 7 Acetyl CoA + 7 ATP + 7 CO2) 8 Acetyl CoA + 7 ATP + 14 NADPH (= 35 ATP) Palmitate + 14 NADP + 8 CoASH + 7 ADP + 7 Pi * 42 ATP required to make palmitate which yields 106 ATP when oxidized Acetyl CoA for fatty acid synthesis is transferred across the inner mitochondrial membrane to the cytoplasm as citrate. Sources of NADPH for FA Synthesis. * One NADPH is generated for each Acetyl CoA transferred from the mitochondria to cytosol. * Additional NADPH comes from the pentose phosphate pathway. Extension of chain length beyond C16 and introduction of double bonds: * Enzyme system on endoplasmic reticulum (ER) * C-C units added from malonyl CoA to carboxyl end of free fatty acid. * ER oxidase introduces double bonds up to C9 of fatty acid chain. Regulation of Fatty Acid Synthesis and Degradation 1. Subcellular compartmentalization/allosteric regulation: Malonyl CoA (high during FA synthesis) inhibits FA-carnitine formation and mitochondrial entry of F.A. to prevent FA oxidation. Cytoplasmic citrate (high after a meal when fuel molecules are abundant) stimulates acetyl CoA carboxylase to increase FA synthesis. 2. Tissue specialization: Most fatty acid synthesis occurs in liver (less in adipose), but most storage occurs in adipose tissue. FA mobilized from adipose are distributed throughout body bound to serum albumen. They are a major fuel for resting muscle. During starvation, large amounts of FA are mobilized from adipose for liver -oxidation. Acetyl-CoA produced is used for synthesis of ketone bodies which are used as an energy source by brain, heart and kidney. 3. Hormonal Regulation: A.Catecholamine, glucagon. cAMP-dependent protein kinase stimulates phosphorylation of key enzymes to: I. stimulate lipase of adipose tissue for fatty acid mobilization 2. Inhibit acetyl CoA carboxylase to block fatty acid synthesis (liver and adipose). B. Insulin. Activation of protein phosphatases to: 1. block lipase activation to prevent fatty acid mobilization from adipose. 2. stimulate pyruvate dehydrogenase to increase acetyl CoA production for fatty acid synthesis.  Role of Subcellular Compartmentalization in FA Synthesis/Oxidation MITOCHONDRIA CYTOSOL Insulin Stimulation (-) Epineph./glucagon Glucose (-) Inhibition Acetyl-CoA Pyruvate Pyruvate (-) X OAA Citrate Acetyl-CoA MalonylCoA OAA NADPH NADH Malate Fatty Acids Pyruvate NADPH Pentose-P Pathway FADH, NADH, AcetylCoA carnitine (-) Etc. FA-CoA FA-carnitine FA-CoA FA (from Adipose) CoA  Storage and Metabolism of Fatty Acids Takes Place in Different Tissues LIVER BLOOD ADIPOSE FA synthesis (major) FA Synthesis (minor) VLDL FA:VLDL FA Glycerol-3-P T.A.G.s Lipase (storage) Glucose -oxidation FA:Albumen FA Glycerol NADH, FADH + Gluconeogen. AcetylCoA Ketone Bodies Key Points-Lipid Metabolism 1: The main storage form for fats are triacylglycerols; these are the most efficient way to store energy relative to carbohydrates & proteins. Triacylglyerols (TAGs) are composed of 3 fatty acids esterified with glycerol. Fatty acids can either be saturated or unsaturated. All dietary fat is broken down into fatty acids (FA), absorbed by the intestine and repackaged as TAG in chylomicrons for entry into the circulation for distribution to liver, adipose and other tissues. FA are mobilized by hormone-dependent lipase in response to glucagon and catecholamines. FA are degraded in the mitochondrial matrix by β oxidation and are synthesized in the cytosol. Entry of fatty acids into the mitochondria is a critical regulatory step for degradation and transfer of acetate into the cytosol is critical for FA synthesis. FA are degraded 2C at a time; odd chain FA are metabolized via conversion to succinyl-CoA by a Vitamin B12-mediated process. Excess acetyl-CoA generated by FA oxidation during starvation is converted to ketone bodies in the liver for use as an energy source by other tissues (esp. brain, heart and kidney cortex) FA are synthesized in the cytoplasm from acetyl-CoA by conversion to malonyl-CoA (regulated step-acetyl-CoA carboxylase), which is the substrate for FA synthase. FA synthesis and degradation are regulated by subcellular compartmentalization, tissue specialization and hormones (phosphorylation/dephosphorylation).