Fatty Acid Synthesis Fatty Acid Synthase AcetylCo A

Fatty Acid Synthesis • Fatty Acid Synthase – Acetyl-Co. A serves as a primer – Addition of two-carbon units from malonyl-Co. A – Each two-carbon unit added must be reduced by 2 NADPH + 2 H+ – Reaction for the synthesis of Palmitic acid (C: 16): Acetyl-Co. A + 7 Malonyl-Co. A + 14 NADPH + 14 H+ Palmitic acid + 7 CO 2 + 14 NADP+ + 8 Co. A + 6 H 2 O

Cytosolic Acetyl-Co. A & NADPH Generation (presented as in most text books, this scheme ignores the specificities of mitochondrial transporters; a more accurate description is in the handout) Glycolysis Mitochondrion Acetyl-Co. A Pyruvate Oxaloacetate TCA cycle Citrate Malate Citrate Cytosol Malic enzyme Pyruvate Malate dehydrogenase Malate NADPH NADP+ + H+ + CO 2 Citrate lyase ATP + Co. A ADP + Pi Oxaloacetate + Acetyl-Co. A NAD+ NADH+H+ Fatty acid synthesis Acetyl-Co. A carboxylase ATP + CO 2 ADP + Pi Malonyl-Co. A

Fatty Acid Synthesis • Malonyl-Co. A is produced by Acetyl-Co. A carboxylase O || CH 3 -C-S-Co. A O O || || - O-C-CH -C-S-Co. A 2 Acetyl-Co. A (cytoplasmic) + HCO 3 ATP Malonyl-Co. A ADP + Pi Acetyl-Co. A Carboxylase Requires Biotin

Fatty Acid Synthesis • Acetyl-Co. A Carboxylase – Rate limiting reaction for fatty acid synthesis – ACC 1 is a liver isozyme – Small amounts of ACC 2 are present in muscle where malonyl-Co. A has a regulatory function (Fatty acid oxidation)

Fatty Acid Synthesis • Acetyl-Co. A Carboxylase 1 – Highly regulated • Allosteric activation by citrate; inhibition by palmitoyl. Co. A. • Inhibited by phosphorylation in the fasting state. – (low blood glucose inhibits; phosphorylation state is determined by both glucagon activation of a kinase and insulin activation of a phosphatase). • Transcriptional up regulation by Ch. REBP (high carbohydrate diet increases amount of ACC 1 and most other enzymes of fatty acid synthetic pathway)

Fatty Acid Synthesis Transcriptiona l control Acetyl-Co. A Carboxylase 1 Xylulose-5 phosphate + Insulin H 2 O PP Phosphorylated Acetyl Co. A carboxylase (Inactive) Acetyl-Co. A Transcription Citrate Palmitoyl-Co. A Pi + + Protein phosphatase PKA ADP + Pi + Glucagon Covalent modification Acetyl Co. A carboxylase + (Inactive) AMP CO 2 ATP AMPK ATP ─ Acetyl Co. A carboxylase (Active) ADP + Pi Malonyl-Co. A Allosteric regulation

Triacylglycerol Synthesis • Long-term transcriptional regulation by Ch. REBP (Carbohydrate Regulatory Element Binding Protein). – In addition to short term regulation of Acetyl-Co. A carboxylase – Many enzymes of fatty acid & triacylglycerol synthetic pathway are coordinately regulated by Ch. REBP. – Ch. REBP is inhibited by Protein Kinase A dependent phosphorylation. – Ch. REBP is activated by Protein Phosphatase 2 A dependent dephophorylation (PP 2 A is stimulated by Xyulose-5 -P). Low Glucose: Glucagon c. AMP Protein kinase A Inactive Ch. REPB-P Fatty acid synthesis High Glucose: Xyulose-5 -P Protein Phosphatase A 2 Active Ch. REPB-OH Fatty acid synthesis

Fatty Acid Synthesis • The main product of fatty acid synthase is palmitic acid (16: 0). • Fatty acids can be elongated by other enzymes that add two carbon units from malonyl-Co. A. Elongation is particularly important in brain. • Still other enzymes can add double bonds (usually at 9 ). Omega-3 and omega-6 fatty acids can not be synthesized by humans.

Triacylglycerol Synthesis • Fatty acids must be activated to Acyl-Co. A Fatty acid + Co. A + ATP Acyl-Co. A + AMP + PPi Acyl-Co. A synthetase PPi + H 2 O 2 Pi Pyrophosphatase

Triacylglycerol Synthesis • Glycerol-3 -phosphate is required for triacylglycerol synthesis. H 2 C-OH | HOCH O | | H 2 C-O-P-O || O- H 2 C-OH | O=C O | | H 2 C-O-P-O || O- Glycerol-3 -phosphate dehydrogenase Dihydroxyacetone Phosphate Glycerol-3 -phosphate NADH + H+ NAD+ Glycerol-3 -phosphate dehydrogenase

Triacylglycerol Synthesis • Addition of 3 Acyl groups from Acyl-Co. A to Glycerol-3 -phosphate 2 Acyl-Co. A Phosphatidate Co. A Acyl-Co. A Triacylglycerol Co. A + Pi

VLDL formation Apolipoprotien B-100 has a repeating -helix/ -sheet structure: Lipids are packaged as apolipoprotein B-100 is being synthesized: From Shelness & Sellers (2001) Curr Opin Lipidology 12: 151 -157

VLDL formation • VLDL stands for Very Low Density Lipoprotein • As it is synthesized, VLDL contains: • • One molecule of apoliprotein B-100 Triacylglycerol Phospholipid Cholesterol ester • Microsomal Triacylglycerol Transfer Protein(MTP) assists in the formation of the VLDL • Other components are added to the VLDL in the blood.

VLDL formation • Apolipoprotein B-100 synthesis is required for the transport of lipid out of the liver – If protein synthesis is reduced (e. g. by malnutrition) fat droplets accumulate in the liver. – If the rate of lipid synthesis is greatly elevated with respect to protein synthesis (e. g. in type I diabetes or glucose 6 -phosphatase deficiency) fat droplets accumulate in the liver.