Chapter 16 (Part 3) Fatty acid Synthesis

Fatty Acid Synthesis In mammals fatty acid synthesis occurs primarily in the liver and adipose tissues Also occurs in mammary glands during lactation. Fatty acid synthesis and degradation go by different routes There are four major differences between fatty acid breakdown and biosynthesis

The differences between fatty acid biosynthesis and breakdown Intermediates in synthesis are linked to -SH groups of acyl carrier proteins (as compared to -SH groups of CoA) Synthesis in cytosol; breakdown in mitochondria Enzymes of synthesis are one polypeptide Biosynthesis uses NADPH/NADP+; breakdown uses NADH/NAD+

ACP vs. Coenzyme A Intermediates in synthesis are linked to -SH groups of acyl carrier proteins (as compared to -SH groups of CoA)

Fatty Acid Synthesis Occurs in the Cytosol Must have source of acetyl-CoA Most acetyl-CoA in mitochondria Citrate-malate-pyruvate shuttle provides cytosolic acetate units and reducing equivalents for fatty acid synthesis Citrate synthase Citrate Lyase Malate dehydrogenase Malate Enzyme Pyruvate carboxylase

Fatty Acid Synthesis Fatty acids are built from 2-C units derived from acetyl-CoA Acetate units are activated for transfer to growing FA chain by conversion to malonyl-CoA Decarboxylation of malonyl-CoA and reducing power of NADPH drive chain growth Chain grows to 16-carbons (eight acetyl-CoAs) Other enzymes add double bonds and more Cs

Acetyl-CoA Carboxylase Acetyl-CoA + HCO3- + ATP  malonyl-CoA + ADP The "ACC enzyme" commits acetate to fatty acid synthesis Carboxylation of acetyl-CoA to form malonyl-CoA is the irreversible, committed step in fatty acid biosynthesis

Acetyl-CoA Carboxylase

Regulation of Acetyl-CoA Carboxylase (ACCase) ACCase forms long, active filamentous polymers from inactive protomers Accumulation of palmitoyl-CoA (product) leads to the formation of inactive polymers Accumulation of citrate leads to the formation of the active polymeric form Phosphorylation modulates citrate activation and palmitoyl-CoA inhibition

Regulation of Acetyl-CoA Carboxylase (ACCase) Unphosphorylated ACCase has low Km for citrate and is active at low citrate Unphosphorylated ACCase has high Ki for palmitoyl-CoA and needs high palmitoyl-CoA to inhibit Phosphorylated E has high Km for citrate and needs high citrate to activate Phosphorylated E has low Ki for palmitoyl-CoA and is inhibited at low palmitoyl-CoA

Fatty Acid Synthesis Step 1: Loading – transferring acetyl- and malonyl- groups from CoA to ACP Step 2: Condensation – transferring 2 carbon unit from malonyl-ACP to acetyl-ACP to form 2 carbon keto-acyl-ACP Step 3: Reduction – conversion of keto-acyl-ACP to hydroxyacyl-ACP (uses NADPH) Step 4: Dehydration – Elimination of H2O to form Enoyl-ACP Step 5: Reduction – Reduce double bond to form 4 carbon fully saturated acyl-ACP

Step 1: Loading Reactions

Step 2: Condensation Rxn

Step 3: Reduction

Step 4: Dehydration

Step 5: Reduction

Step 6: next condensation

Termination of Fatty Acid Synthesis Acyl-CoA synthetase

Organization of Fatty Acid Synthesis Enzymes In bacteria and plants, the fatty acid synthesis reactions are catalyzed individual soluble enzymes. In animals, the fatty acid synthesis reactions are all present on multifunctional polypeptide. The animal fatty acid synthase is a homodimer of two identical 250 kD polypeptides.

Animal Fatty Acid Synthase

Further Processing of Fatty acids: Desaturation and Elongation

Regulation of FA Synthesis Allosteric modifiers, phosphorylation and hormones Malonyl-CoA blocks the carnitine acyltransferase and thus inhibits beta-oxidation Citrate activates acetyl-CoA carboxylase Fatty acyl-CoAs inhibit acetyl-CoA carboxylase Hormones regulate ACC Glucagon activates lipases/inhibits ACC Insulin inhibits lipases/activates ACC

Allosteric regulation of fatty acid synthesis occurs at ACCase and the carnitine acyltransferase

Glucagon inhibits fatty acid synthesis while increasing lipid breakdown and fatty acid b-oxidation Insulin prevents action of glucagon