Lipid Metabolism PDF

Lipid Metabolism Dr. Abdur Rahman METABOLISM OF TRIACYLGLYCEROL • Fatty acids from diet are incorporated either in TAG or in phospholipids • The fate depends on the need i.e. – in actively growing organisms in PL – in non-growing organisms in TAG • An average sized 70 Kg human can store up 15 Kg...

Lipid Metabolism Dr. Abdur Rahman METABOLISM OF TRIACYLGLYCEROL • Fatty acids from diet are incorporated either in TAG or in phospholipids • The fate depends on the need i.e. – in actively growing organisms in PL – in non-growing organisms in TAG • An average sized 70 Kg human can store up 15 Kg TAG, enough for up to 12 weeks. Biosynthesis of TAG and PL • The precursors are glycerol-3-PO4 and fatty acyl CoA • glycerol-3-PO4 can come from: – dihydroxy acetone PO4 (glycolysis) – glycerol (by glycerol kinase) • Two fatty acyl CoA + glycerol-PO4 form phosphatidic acid- common precursor for both TAG and PL • Biosynthesis is regulated by hormones, mostly insulin and glucagon Mobilization and degradation of TAG and PL • When energy is needed, hormones signal the mobilization and degradation of TAG in adipocytes • hormone sensitive lipase mobilize TAG • Glucagon and epinephrine trigger the enzyme • Breaks down TAG into fatty acid and glycerol • FA gets into the blood • Taken up by skeletal muscles, kidney and liver: – Used for synthesis of cholesterol, ketone bodies – Degraded for energy • Glycerol either used for TAG synthesis, becomes glucose, or used in glycolysis Degradation of PL • Most cells continuously degrade and replace membrane PL. • Unlike hormone sensitive lipase, which hydrolyze all the three ester bonds in TAG, lipases for PL are specific for specific bonds: – PLA1 (bond b/w C1 of glycerol and FA) – PLA2 (bond b/w C2 of glycerol and FA) – PLC (bond b/w C3 of glycerol and PO4) – PLD (bond b/w PO4 and polar head group) Catabolism of TG and Fatty Acids • Complete hydrolysis of TG → glycerol and three fatty acids Enzymes involved: lipoprotein lipase (vascular endothelium) and hormone sensitive Lipase (liver and adipose tissue). • Glycerol can be used for energy by converting into glycerol phosphate by Glycerokinase. -Glycerol phosphate can enter into Glycolytic pathway for energy oxidation. - or follow gluconeogenesis pathway. - or Glycerol phosphate can be used to form TAG. • Fatty acid –rich source of energy-oxidized to CO2 and HO2. • Many tissues can oxidize fatty acids: -oxidation. Activation of Fatty Acids • FFA move through cell membrane of AT, binds with albumin in the plasma-transported to tissue. • On entry into the cells , fatty acid is first activated by Cytoplasmic enzyme fatty acyl CoA synthetase. • Requires ATP Degradation of Fatty Acids • FA degradation occurs in mitochondria • Inner mitochondrial membrane is impermeable to long chain Fatty acyl CoA • FA (acyl CoA) transported into the mitochondria across the inner mitochondrial membrane by the carnitine shuttle Mitochondrial Transfer of Acyl CoA • Activated fatty acid binds with carnitine at the cytoplasmic site of the mitochondrial membrane by carnitine acyl transferase I. • Carnitine acyl transferase II in the inner phase of the inner membrane of mitochondria release fatty acyl coA into the matrix β-Oxidation pathway • Inside mitochondrial matrix FA are degraded by a stepwise manner called the β-oxidation pathway • The pathway is completed in 4 steps: – Oxidation --->hydration ----->oxidation ----->hydrolysis • By repeated β-oxidation, all the fatty acids are converted into acetyl CoA • Acetyl CoA can be utilized by the Krebs cycle for energy Steps of β-oxidation • 1. Formation of double bond (oxidation) between  and  carbon by acyl CoA dehydrogenase, use FAD to from FADH2-provide 2 ATPs through ETS 2. Unsaturated acyl CoA accept one mol of water by enol CoA hydratase. 3. The -hydroxyl group is oxidized to the ketone by -hydroxyacyl CoA dehydrogenase , use NAD+ to NADH2 –provide 3 ATPs through ETS 4. The -ketoacyl CoA is cleaved by Acyl CoA: acyltransferase to insert CoA and cleavage at the - carbon. The products are acetyl CoA and saturated CoA activated fatty acid with two fewer carbon than original fatty acid. • Then entire sequence of reactions is repeated Energy Yields in Fatty Acid Oxidation • The activation of fatty acid require 2 ATPs. • Each cleavage of a saturated carbon-carbon bond yield 5 ATPs. • Actyl CoA produced enter into Krebs cycle and oxidized to CO2 and water, yield 12 ATPs. • For palmitate (16 carbons): 7 carbon-carbon cleavage: 7x 5=35 ATPs 8 acetyl CoAs oxidized in TCA cycle: 8x 12=96 Total ATPs produced : 131 For activation of Fatty acid (2 ATP used) = 2 Net Gain of ATPs: 131-2 = 129 -Oxidation of Unsaturated Fatty Acids • Nearly of half of dietary and body fatty acids are unsaturated and provide considerable energy. • -oxidation of unsaturated fatty acid nearly identical to that saturated fatty acids, except one fatty acyl CoA dehydrogenase reaction is not needed for each double bond present. • Unsaturated fatty acid produce less ATPs than saturated fatty acids of the same length. • Because each double bond present bypass fatty acyl CoA dehydrogenase reaction, thus no production of FADH2 Oxidation of FA with number of carbon atoms • Most FAs metabolized have even number of carbon atoms, small amounts FAs have odd number of carbon atoms. • -oxidation proceeds to produce acetyl CoA until the residual propionyl CoA remains. • The oxidation requires vitamins Biotin and B12 to succinyl CoA. Ketone Bodies • When the amount of acetyl CoA exceeds the oxidative capacity of the liver, excess acetyl CoA is converted into ketone bodies. • Ketone bodies are: – acetoacetic acid – acetone and – 3-hydroxybytyric acid. • Significance of Ketogenesis: – Water-soluble – can be transported to other tissues in blood and utilized for energy – Used by skeletal and cardiac muscles and brain (some times) Ketogenesis Utilization of ketone bodies for energy Ketogenesis • Ketone bodies are produced by liver but are not used by the liver • May lead to ketoacidosis (acidemia) → ketonuria (in diabetes) Fatty acid Synthesis Fatty Acid Synthesis • Excess carbohydrate, protein and other compounds converted into FA→ stored as TG. • Synthesis mainly takes place in liver & lactating mammary gland, to a lesser extent in adipose tissue. • Fatty acid synthesis occur in cytoplasm, while acetyl CoA as such can not pass through mitochondria. • FA synthesis-involved three steps. Production of Cytosolic acetyl coA 1st step of de novo synthesis is the transfer of acetate from mitochondrial acetyl CoA to the cytosol. Mitochondrial acetyl CoA is produced from oxidation of pyruvate, catabolism of FA and Ketone bodies & certain amino acid. The CoA can not cross mitochondria-only acetate part transported to cytosol in the form of citrate by condensation of oxaloacetae and acetyl CoA with the help of citrate synthase In cytosol, citrate is cleaved by ATP-citrate lyase to cytosolic acetyl CoA and oxaloacetate. Carboxylation of Acetyl CoA to Malonyl CoA • The basic process involved sequential assembly of “starter” molecule acetyl CoA with units of malonyl CoA. • Malonyl CoA is formed from acetyl CoA and CO2. • The reaction occur in cytoplasm. • Catalyzed by acetyl CoA carboxylase, in presence of HCO3- and ATP. Coenzyme–Biotin is involved. ----incorporate carboxyl group into acetyl CoA. Carboxylation of Acetyl CoA to Malonyl CoA Allosteric activation By citrate I Allosteric inhibition by Long-chain FA Regulation by reversible phosphorylation Excess calorie intake Acetyl Co A carboxylase Fatty acid synthesis Low calorie intake Acetyl Co A carboxylase Fatty acid synthesis Fatty Acid Synthase System • The key components of FA synthase complex are the acyl carrier protein (ACP) and condensing enzyme (CE). • Both contain free –SH groups to which acetyl CoA and malonyl CoA binding takes place. • Before start of elongation of FA chain, the two -SH groups must be loaded with acetyl CoA and malonyl CoA. Enzyme steps involved in fatty acid elongation • The carbonyl carbon of acetyl group is coupled to C-2 of malonyl ACP by removing carboxyl group by 3-ketoacyl-ACP synthease. • The -ketone is reduced with NADPH catalyzed by 3-ketoacyl-ACP reductase. • The alcohol is dehydrated yield a double bond by 3-hydroxyacyl-ACP hydratase. • The double bond is reduced to butyryl-ACP with NADPH by Enoyl-ACP reductase. • The butyryl group is transferred to the CE, expose ACP-SH to bind with second malonyl CoA. • Butyryl group on the CE again couples with C-2 of the Malonyl CoA, six carbon chain is then reduced and transferred to CE. The process repeats. • The completed FA chain is hydrolyzed from the ACP by Palmitoyl thioesterase. https://www.youtube.com/watch?v=sTk3DLGG3Fo Net reaction for Palmitate synthesis • • Formation of Malonyl CoA 7 Acetyl-CoA + 7 Co2 + 7 ATP 7 Malonyl-CoA + 7 ADP + 7 Pi Seven cycle of condensation and reduction & Release of palmitate – Acetyl-CoA + 7 Malonyl-CoA+ 14 NADPH+ 14 H++ H2O Palmitate + 7 Co2 + 8 CoA + 14 NADP+ + 7 H2O • Overall reaction process: 8 Acetyl-CoA + 7 ATP + 14 NADPH + 14 H+ Palmitate + 8 CoA + 7 ADP+ 7 Pi + 14 NADP+ + 6 H2O Fatty Acid Synthesis Transcriptional control Acetyl-CoA Carboxylase 1 Xylulose-5phosphate + Insulin H2O PP Phosphorylated Acetyl CoA carboxylase (Inactive) Acetyl-CoA Transcription Citrate Palmitoyl-CoA Pi + + Protein phosphatase PKA ADP + Pi + Glucagon Covalent modification Acetyl CoA carboxylase + (Inactive) AMP CO2 ATP AMPK ATP ─ Acetyl CoA carboxylase (Active) Malonyl-CoA Allosteric regulation ADP + Pi Synthesis of Unsaturated Fatty Acid • Normal product of FA Syn is palmitate (16:0), can be elongated to Stearic acid (18:0) and even longer. • Palmitic and stearic acid can be converted to ∆9 monounsaturated FA palmitoleic (16:1) and oleic acid (18:1). • Enzyme involved is mixed-function oxidases: Oxidize FA and NADPH. • Human cells can not synthesize Linoleic acid and linolenic acid as they do not have ∆12 and ∆15 desaturase. • Once linoleic acid is aquired, longer and highly unsaturated FA can be formed. Cholesterol Metabolism Biosynthesis of Cholesterol • Cholesterol exist in two forms: – Free cholesterol (in cell membrane) – Cholesteryl ester (stored in cell, part of lipoprotein) • Synthesized in any cell but mostly in liver, intestine, adrenal cortex, and reproductive tissues • Synthesis occur in the cytosol • The precursors are acetyl CoA, NADPH, and ATP Cholesteryl ester Biosynthesis of Cholesterol • The process occur in 4 steps: 1. Three acetyl coA condense to form mevalonic acid r 2. Formations of activated isoprene units from mevalonic acid 3. Condensation of 6 isoprene units to form squalene ( a linear 30-C molecule) 4. Cyclization of squalene into the 4 rings of steroid nucleus. Biosynthesis of Cholesterol • The first steps are the same as the synthesis of ketone bodie, up to the synthesis of HMG-CoA • HMG-CoA is reduced to mevalonic acid by HMG-CoA reductase • This is the rate limiting step and the major control point • Can be controlled by a variety of allosteric modulators and competitive inhibitors (statin drugs) Biosynthesis of Cholesterol The remaining steps are summarized as follows: a. Formation of 5-carbon activated isoprene units from mevalonic acid b. Condensation of two isoprenes to make 10-carbon molecule c. Addition of another isoprene to make 15-carbon skeleton (farnesyl pyrophosphate) d. Condensation of two farnesyls to make a 30-carbon linear compound, squalene e. Cyclization of squalene into the first steroid nucleus (Lanosterol) f. Modification of lanosterol into cholesterol by many reactions Catabolism of Cholesterol • Cholesterol is not oxidized to CO2 and H2O → no energy • The major rout for its elimination is the intact steroid nucleus in the form of bile acids and bile salts, which are excreted in feces • Major metabolites of cholesterol: – Bile acids/salts oh – Steroid hormones If – Vitamin D I Bile Acid/Salts • Bile acids are cholic acid and chenodeoxy cholic acid (primary bile acids) • Bile salts: bile acids conjugated with glycine or taurine ( 3:1 for glycine salts) • Bacterial action in the GI convert into secondary bile acids • Bile synthesized in liver and stored in gall bladder • Are detergent and aids in digestion of lipids Bile metabolism • Enterohepatic circulation bile: – 15-30 g bile secreted; only 0.5 g excreted in feces Mf – 95% is absorbed; taken back to liver – Agents that interfere with absorption will decrease cholesterol (cholestyramine, dietary fiber) • Regulationzof Bile synthesis: – 7--hydroxylase is the rate-limiting enzyme – Regulated by covalent modification – P-enzyme is active Synthesis of Steroid hormones From Cholesterol Synthesis of Vitamin D from Cholesterol