Biosynthesis Of Lipids PDF

1 BIOSYNTHESIS OF LIPIDS Doç. Dr. Özlem Unay Demirel Outline   Biosynthesis of Fatty Acids Biosynthesis of Triacylglycerols What lipids are for?    Principal form of stored energy in most organisms Major constituents of cellular membranes Specialized lipids serve as pigments (retinal, carotene)...

1 BIOSYNTHESIS OF LIPIDS Doç. Dr. Özlem Unay Demirel Outline   Biosynthesis of Fatty Acids Biosynthesis of Triacylglycerols What lipids are for?    Principal form of stored energy in most organisms Major constituents of cellular membranes Specialized lipids serve as pigments (retinal, carotene), cofactors (vitamin K), detergents (bile salts), transporters (dolichols), hormones (vitamin D derivatives, sex hormones), extracellular and intracellular messengers (eicosanoids, phosphatidylinositol derivatives), and anchors for membrane proteins (covalently attached fatty acids, prenyl groups, and phosphatidylinositol) Biosynthesis of Fatty Acids     The formation of malonyl-CoA from acetyl-CoA is an irreversible process, catalyzed by acetyl-CoA carboxylase Biotin as a prosthetic group 4-step process Fatty acid synthase multienzyme complex: 7 active sites Fatty acid synthesis     Fatty Acid Synthesis Proceeds in a repeating reaction sequence With each passage through the cycle, the fatty acyl chain is extended by two carbons. When the chain length reaches 16, palmitate leaves the cycle The reducing agent in the synthetic sequence is NADPH and the activating groups are 2 different enzyme-bound -SH groups Fatty Acid synthase         Dimer of 2 identical pp monomers 1. Ketoacylsynthase 2. Malonyl/acetyltransacylase 3.Hydratase 4. Enoylreductase 5. Ketoacylreductase 6.Acyl carrier protein 7. Thioesterase Acyl Carrier Protein Fatty Acid synthesis    FA synthase reactions are repeated to form Palmitate Production of the 4C saturated fatty acyl–ACP completes one pass through the fatty acid synthase complex Small amounts of longer fatty acids such as stearate (18:0) are also formed Synthesis of Palmitate from Acetly CoA Formation of 7 malonyl-CoA molecules 7 cycles of condensation and reduction Overall Reaction Source of NADPH in Fatty Acid synthesis Fatty acid synthesis occurs in the cytosol  NADPH is the electron carrier for anabolic reactions  1. In hepatocytes, lactating mammary gland and adipocytes, cytosolic NADPH is largely generated by the pentose phosphate pathway 2. Malic enzyme (NADP malate dehydrogenase) catalyzes the reaction from malate to pyruvate  Acetyl CoA   Principal building block of fatty acids Formed from glucose via the oxidation of pyruvate within the mitochondria Shuttle for acetyl groups    From mitochondria to the cytosol Malate can be transported into the mitochondrion where it is able to reform OAA Citrate transporter in the mitochondrial membrane requires malate to exchange with citrate Regulation of Fatty Acid Biosynthesis   When a cell or organism has more than enough metabolic fuel to meet its energy needs, the excess is generally converted to fatty acids and stored as lipids such as triacylglycerols The reaction catalyzed by acetyl-CoA carboxylase is the rate-limiting step in the biosynthesis of FA Extramitochondrial NADH to NADP      ATP citrate lyase increases in activity in the well fed state Acetyl CoA is available for malonyl CoA formation and synthesis into palmitate Resulting OAA can form malate via NADH linked malate dehydrogenase followed by generation of NADPH via the malic enzyme NADPH becomes available for lipogenesis Pyruvate can be used to regenerate Acetyl CoA after transport into mitochondria Regulation of Fatty Acid Biosynthesis     Palmitoyl-CoA, the principal product of fatty acid synthesis, is a feedback inhibitor of the enzyme Citrate is an allosteric activator which plays a central role in diverting cellular metabolism from the consumption (oxidation) of metabolic fuel to the storage as FA When the concentrations of mitochondrial acetyl-CoA and ATP increase, citrate is transported out of mitochondria; it then becomes both the precursor of cytosolic acetyl-CoA and an allosteric signal for the activation of acetyl-CoA carboxylase. At the same time, citrate inhibits the activity of phosphofructokinase-1 reducing the flow of carbon through glycolysis Regulation of Acetyl CoA Carboxylase   Covalent modification: Phosphorylation, triggered by the hormones glucagon and epinephrine, inactivates the enzyme and reduces its sensitivity to activation by citrate, thereby slowing fatty acid synthesis. In its active (dephosphorylated) form, acetyl-CoA carboxylase polymerizes into long filaments, phosphorylation is accompanied by dissociation into monomeric subunits and loss of activity. Fatty Acid elongation     Long chain saturated FAs are synthesized from Palmitate Fatty acid elongation systems present in the smooth endoplasmic reticulum and in mitochondria The microsomal system Elongates saturated and unsaturated fatty acylCoAs by 2 C using malonyl CoA as the acetyl donor and NADPH as reductant Synthesis of other FA Elongation: smooth ER and mitochondria Acetyl CoA from Malonyl CoA Coenzyme A as acyl carrier Essential fatty acid Desaturation of Fatty acids    Smooth ER Palmitate and stearate serve as precursors of the 2 most common monounsaturated fatty acids of palmitoleate, 16:1(9), and oleate, 18:1(9); both of these fatty acids have a single cis double bond between C-9 and C-10 The double bond is introduced into the fatty acid chain by an oxidative reaction catalyzed by fatty acyl–CoA desaturase (a mixed-function oxidase) Desaturation of Fatty acids    Desaturases introduce double bonds at specific positions in a fatty acid chain. Hepatocytes can readily introduce double bonds at the 9 position of fatty acids but cannot introduce additional double bonds between C-10 and the methyl-terminal end. We cannot synthesize linoleate, 18:2 (9,12), or linolenate, 18:3 (9,12,15) Desaturation of fatty acids Essential Fatty acids    Linoleate and linolenate are essential fatty acids for mammals; must be obtained from dietary plant material Once ingested, linoleate may be converted to certain other polyunsaturated acids, particularly -linolenate, eicosatrienoate, and arachidonate (eicosatetraenoate), all of which can be made only from linoleate Arachidonate, 20:4(5,8,11,14), is an essential precursor of regulatory lipids, the eicosanoids. The 20-carbon fatty acids are synthesized from linoleate (and linolenate) by fatty acid elongation reactions Biosynthesis of Triacylgylcerol   Most of the fatty acids synthesized or ingested by an organism have one of two fates: incorporation into triacylglycerols for the storage of metabolic energy or incorporation into the phospholipid components of membranes Both pathways begin at the same point: the formation of fatty acyl esters of glycerol Biosynthesis of Triacylgylcerol   TAGs and glycerophospholipids such as phosphatidylethanolamine share two precursors (fatty acyl–CoA and L-glycerol 3-phosphate) The vast majority of the glycerol 3-phosphate is derived from the glycolytic intermediate dihydroxyacetone phosphate (DHAP) by the action of the cytosolic NAD-linked glycerol 3-phosphate dehydrogenase; in liver and kidney, a small amount of glycerol 3-phosphate is also formed from glycerol by the action of glycerol kinase Biosynthesis of Triacylgylcerol  The other precursors of triacylglycerols are fatty acyl–CoAs, formed from fatty acids by acyl-CoA synthetases, the same enzymes responsible for the activation of fatty acids for oxidation Biosynthesis of Triacylglycerol    First acylation of the 2 free OH groups of L-glycerol 3phosphate by 2 molecules of fatty acyl–CoA to yield diacylglycerol 3-phosphate, more commonly called phosphatidic acid or phosphatidate Phosphatidic acid is present in trace amounts in cells but is a central intermediate in lipid biosynthesis; it can be converted either to a TAG or to a glycerophospholipid. In the pathway to TAGs, phosphatidic acid is hydrolyzed by phosphatidic acid phosphatase to form a 1,2-diacylglycerol Diacylglycerols are then converted to TAGs by transesterification with a third fatty acyl–CoA. Triacylglycerol Cycle Glyceroneogenesis Shorter version of gluconeogenesis Glycerogenesis    In adipose tissue, glyceroneogenesis coupled with reesterification of FFA controls the rate of FA release to the blood. In brown adipose tissue, the same pathway may control the rate at which free fatty acids are delivered to mitochondria for use in thermogenesis. In fasting humans, glyceroneogenesis in the liver alone supports the synthesis of enough glycerol 3phosphate to account for up to 65% of fatty acids reesterified to triacylglycerol