Medical Biochemistry II: Fatty Acid Synthesis PDF

Medical Biochemistry II Synthesis of Fatty AcidsTAGs & membrane lipids Student Learning Outcomes: 1 2 3 4 Describe basic steps for synthesis of fatty acids from dietary glucose (or amino acids) in the liver Explain the role of VLDL lipoprotein particles Explain the role of citrate, PPP and malic enz...

Medical Biochemistry II Synthesis of Fatty AcidsTAGs & membrane lipids Student Learning Outcomes: 1 2 3 4 Describe basic steps for synthesis of fatty acids from dietary glucose (or amino acids) in the liver Explain the role of VLDL lipoprotein particles Explain the role of citrate, PPP and malic enzyme in FA synthesis Explain the use of fatty acids for synthesis of glycerophospholipids and sphingolipids FLASH BACK: TCA cycle intermediates and anaplerotic paths TCA cycle intermediates - biosynthesis precursors Liver ʻopen cycleʼ high efflux of intermediates: Specific transporters inner mitochondrial membrane for pyruvate, citrate, a-KG, malate, ADP, ATP. After high carb meal → citrate efflux for FA synthesis During fasting → malate exits for cytosolic gluconeogenesis Overview lipogenesis Synthesis of triacylglycerols from glucose: If excess calories; citrate moved from mitochondrion Building blocks of liver FA: Acetyl CoA, Malonyl CoA, NADPH Occurs in cytosol TAG packaged as VLDL Regulated pathway OAA, oxaloacetate TG, triacylglycerol Overview Fate of VLDL-TG TAG is digested by LPL (lipoprotein lipase) on surface of capillaries forming FA and glycerol Some fatty acids are oxidized by muscle Most are converted to triacylglycerols in adipose cells, stored, released during fasting Fate of FA during fed state Some used as components of membrane lipids unsaturated glycerophospholipid plasmalogen Fatty acid synthesis Fatty acid synthesis from excess carbohydrates Glucose to cytosolic Acetyl CoA via OAA + Acetyl CoA → citrate Citrate transported to cytosol Cleave to OAA + Acetyl CoA Note: PDH is only in mitochondrion Acetyl CoA can’t cross the membrane Fig. 4 Citrate in cytosol Citrate in cytosol to Acetyl CoA: OAA and Acetyl CoA generated in cytosol Acetyl CoA will be used to form FA OAA will regenerate pyruvate ↑, inducible enzymes Fatty acid synthesis needs Acetyl CoA, NADPH Oxaloacetate is converted back to pyruvate in two steps The NADPH is generated by 1. malic enzyme 2. pentosephosphate pathway and is used for the reduction reactions that occur on the fatty acid synthase complex Malate DH + NADH Acetyl CoA to Malonyl CoA One Acetyl CoA and many Malonyl CoA are needed Malonyl CoA is immediate donor of the 2-C units to growing FA chain Acetyl CoA carboxylase requires biotin and ATP Figs. 8,9 Fatty acid synthase complex (FASC) Has many active sites in different domains 1 large single polypeptide chain β-ketoacyl-ACP synthase (KS): SH DOMAIN Malonyl/acetyl-CoA-ACP transferase (MAT) β-hydroxyacyl-ACP reductase (KR) ACP is the acyl carrier protein: SH DOMAIN TE is the thioesterase Fig. 10 FASC Sequentially adds 2-C units from 3-C malonyl CoA (CO2 is released) 2 reduction reactions after each addition (NADPH) 16-C Palmitate is typical product ACP is the shuttle that holds the system together ACP prosthetic group is 4’-phosphopantetheine (contains B vitamin pantothenic acid, vitamin B5) that is covalently attached to the hydroxyl of Ser residue in ACP ACP is the site of malonyl entry Fig. 10 4 steps to add two carbons to growing fatty acyl chain 1. Condensation 1. Reduction 1. Dehydration 1. Reduction Fatty Acid synthase – st 1 step: condensation 1. Claisen condensation to form acetoacetyl-ACP 2. Catalyzed by β-ketoacyl-ACP synthase 3. Acetyl is transferred from the KS – SH group to malonyl group (becomes the methyl terminal 2-C unit (ω)) 4. Malonyl CoA releases CO2; 2-C unit condenses with the Acetyl CoA, and a 4-C product is produced on ACP Fig. 11* 2. Fatty Acid synthesis Reduction reactions Reduction reactions convert bketoacyl group to alcohol Catalyzed by β-ketoacyl-ACP reductase (KR) NADPH is reducing agent (electron donor) C=O → HCOH Fig. 12 3. Fatty Acid synthesis Dehydration (Elimination) of water Elimination of water generates double bond Catalyzed by β-hydroxyacylACP dehydratase Fig. 12 4. Fatty Acid synthesis Reduction of double bond Double bond is reduced to form saturated fatty acyl group Catalyzed by enoyl-ACP reductase (ER) NADPH is reducing agent C=C → CH2-CH2 The 4-C unit will transfer to the SH of the Cys on other subunit Costs 1 ATP to form Malonyl CoA Costs 2 NADPH per addition Fig. 12 Fatty Acid synthesis to palmitate (C16) Fatty acid synthesis: cycles of 2-C addition From 1 2-C Acetyl CoA and rest 3-C malonyl CoA End C was first added (last unit is the COOH end from acetyl coA) Elongates on ACP, then moves to Cys SH of KS subunit Cleavage at end by thioesterase (TE) to release palmitate 2 NADPH/cycle 1 ATP/cycle 1 CO2 added/ released Fatty Acid synthesis to palmitate (C16) Fig. 13 Key concepts Fatty acids synthesized mainly in liver, from glucose Glucose to pyruvate in mitochondrion, forms Ac CoA, OAA, which form citrate Citrate in cytosol then to Ac CoA, malonyl CoA Fatty acid synthesis involve series 2-C additions from malonyl CoA to the w-C of Ac CoA onto FA synthase. Costs 2 NADPH and 1 ATP per cycle addition Fatty acids packaged as TG in liver as VLDL with proteins and other lipids; digested by LPL on capillaries and FA enter cells (oxidized or stored)