Synthesis of Fatty acid Dr Vivek Joshi,MD

 Main pathway - cytosol  Occurs primarily in the liver and lactating mammary gland, less so in adipose tissue  Also present in the kidney, brain, lung  Fatty acid synthesis require:  Dietary carbohydrates Acetyl CoA  NADPH  ATP Biosynthesis of Saturated Fatty Acids HIGH INSULIN/WELL FED STATE

Malic Enzyme  Source of NADPH # Pentose phosphate pathway Chief source Also occurs in the cytosol Active in tissues active in lipogenesis # Malic enzyme Biosynthesis of Saturated Fatty Acids

 Production of Cytosolic Acetyl CoA  Carboxylation of Acetyl CoA to Malonyl CoA  Fatty acid synthase complex

5 Production of Cytosolic Acetyl CoA  Acetyl CoA Main building block of fatty acids Synthesized from carbohydrates via oxidation of pyruvate within mitochondria Doesn’t diffuse easily from mitochondria to cytosol Utilization of glucose for lipogenesis is through citrate

High levels of ATP and NADH InhibitsTCA Cycle (Isocitrate Dehydrogenase) Accumulation of Citrate Fatty Acid synthesis

Malic enzyme produces ~50% NADPH needed for FA synthesis Production of Cytosolic Acetyl CoA

 Malonyl CoA is synthesized from acetyl CoA using ATP and CO 2  The reaction is catalyzed by Acetyl CoA carboxylase Carboxylation of Acetyl CoA to Malonyl CoA

 Biotin-Dependent Carboxylation of Acetyl-CoA to Malonyl-CoA by Acetyl-CoA Carboxylase (ACC)  Biotin: water soluble vitamin – functions as a CO 2 carrier for several important reactions including:  Acetyl-CoA carboxylase  Pyruvate carboxylase  Propionyl CoA carboxylase Biotin cofactor Carboxylation of Acetyl CoA to Malonyl CoA

10 Fatty Acid Synthase Complex  A dimer with identical polypeptide monomers that lie head to tail  Each polypeptide monomer contains all 7 enzyme activities and an ACP  Only the dimer is active because fatty acid synthesis requires: # Thiol of the ACP in one monomer # Thiol of the 3-ketoacyl synthase (condensing enzyme) of the other monomer.

Fatty Acid Synthase Complex 4’ Phosphopantethein SH Cyst SH Cyst 4’ Phosphopantethein Thio estrase Ketoacyl Synthase Acetyl transacylase Malonyl transacylase Ketoacyl Reductase Dehydratase Enoyl Reductase ACP KRKR ERER D MTMT ATAT KSKS KSKS ATAT MTMT D ERER KRKR Acyl Carrier Protein

The whole point of a multi-enzyme complex: 1. Coordinated activity 2. Intermediates stably bound to enzyme complex 3. Efficiency

 Acetate (from acetyl CoA) is loaded onto ACP, immediately moved to Cys-SH on condensing enzyme (CE) in Domain 1 of polypeptide 1  Malonyl (malonyl Co-A) is loaded onto ACP on Domain 2 of polypeptide 2

19 Fatty Acid Synthesis

20 Sequence repeated (7 cycles) until Saturated 16-carbon acyl radical (palmitoyl) is formed Liberated from the enzyme complex by the 7 th enzyme, thioesterase (deacylase) Palmitate Equation for overall synthesis of palmitate: 1 acetyl CoA + 7 malonyl CoA + 14NADPH + 14H + 1 palmitic acid + 7CO 2 + 6H 2 O + 8CoA + 14NADP + Synthesis of Palmitic acid (16C) Liberated from the enzyme complex by the 7 th enzyme, thioesterase (deacylase)

21 Palmitic Acid – activated to palmityl CoA before can proceed to any other pathway

22 Elongation of Fatty Acid  Occurs in the endoplasmic reticulum &mitochondria  Uses NADPH as reductant  Lengthens a fatty acid by using malonyl CoA as acetyl donor  Fatty acid lengthens by 2 carbons  Brain-Elongation capibilities-Very long chain fatty acids(24 C) – Synthesis of Brain lipids

mtsPinlacSUSOM 23 FATTY ACID ELONGATION

24  The fatty acid desaturase system-Electron transport system in the ER that involves: # Cytochrome b5 # Desaturase # NADH-cytochrome b5 reductase FATTY ACID DESATURATION

25 Synthesis of Polyunsaturated Fatty Acids  Involves the desaturase and the elongase enzyme systems  Additional double bonds into existing monounsaturated FA are always separated from each other by a methylene group  Additional double bonds are all introduced between the existing double bond and the carboxyl group. -

Palmitate, 16:0 elongation desaturation Palmitoleate, 16:1(  9 ) Stearate, 18:0 elongation desaturation longer saturated fatty acids Oleate, 18:1(  9 ) desaturation in plants only desaturation desaturation in plants only.  -Linolenate, 18:3(  6,9,12 )  - Linolenate, 18:3(  9,12,15 ) Eicosatrienoate, 20:3(  8,12,14 ) desaturation Other polyunsaturated Arachidonate fatty acids 20:4(  5,8,11,14 ) Linoleate, 18:2(  9,12 ) elongation Synthesis of polyunsaturated Fatty acids

27 Synthesis of Unsaturated Fatty Acids  Human have 9,6,5 and 4 Desaturase-Can introduce double bonds at Δ 4, Δ 5, Δ 6 and Δ 9, but never beyond Δ 9 (Carbon 10 till the  - end)  Linoleic and Linolenic acid-Essential Fatty acids  First double bond introduced into a saturated FA - nearly always in the Δ9 position by the Δ9 desaturase in the endoplasmic reticulum -

28 Essential Fatty Acids  Cannot be synthesized in the body  Supplied in the diet-Vegetable oils  Can be synthesized by plants – can introduce double bonds in the  12 and  15 position  Found in structural lipids-Concerned with structural integrity of membranes -.

-Fatty acid Metabolism-Stringently controlled -Synthesis &degradation are highly responsive to physiological needs -Acetyl CoA Carboxylase # Catalyses the rate-limiting step in the Biosynthesis # Catalyses the rate-limiting step in the Biosynthesis of Fatty Acids of Fatty Acids # # Short Term Regulation -Allosteric regulation -Covalent modification # Long Term Regulation: Induction and Repression Regulation of Fatty Acid Synthesis

Allosteric Regulation of Acetyl CoA Carboxylase a) Citrate - Increases polymerization b) Palmitoyl CoA - A feedback inhibitor - Promotes depolymerization

Covalent Modification of Acetyl CoA Carboxylase Phosphorylation of the key Enzyme – Increases depolymerization

 High Calorie/High Carbohydrate diet and fat free diet-Increased synthesis of Acetyl CoA Carboxylase/ High Insulin -Enhanced Fatty acid synthesis  Low Calorie diet /Fasting/High Glucagon -Decreased synthesis of Acetyl CoA Carboxylase-Decreased Fatty acid synthesis

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