Fatty Acid Synthesis Notes

Lecture 31: Fatty Acid Synthesis

Learning Objectives

• Tricarboxylate transport system
• Acetyl CoA carboxylase reaction
• Reaction sequence for the biosynthesis of fatty acids
• Comparison of degradation and synthesis of fatty acids
• Mammalian fatty acid synthase
• Regulation of lipid metabolism

Fatty Acid Synthesis Stages

1. Transfer of acetyl CoA: From the mitochondria to the cytoplasm.
2. Activation of acetyl CoA: Forms malonyl CoA.
3. Repetitive addition and reduction of two-carbon units: Synthesizes C16 fatty acid. This synthesis occurs on an acyl carrier protein (ACP), which acts as a molecular scaffold.

Transfer of Acetyl Groups to the Cytoplasm

• Fatty acid synthesis occurs in the cytoplasm.
• Acetyl CoA is formed in mitochondria, but it is not membrane permeable.
• Acetyl CoA is transported as citrate, formed by the condensation of oxaloacetate and acetyl CoA.
• Citrate is cleaved by ATP-citrate lyase.
• Oxaloacetate, resulting from the cleavage, is also not permeable and is converted to pyruvate via bypass reactions.
• Pyruvate enters the mitochondria.

Acetyl CoA Carboxylase Reaction

• Acetyl CoA carboxylase (ACC1) catalyzes the formation of malonyl CoA in two steps.
• Malonyl CoA is the activated form of acetyl CoA.
• The formation of malonyl CoA is the committed step in fatty acid synthesis.
• Fatty acid synthesis begins with the carboxylation of acetyl CoA to generate malonyl CoA.

Intermediates and Acyl Carrier Protein (ACP)

• Fatty acid synthesis occurs on the acyl carrier protein (ACP).
• ACP is a polypeptide linked to CoA.
• Intermediates are linked to the sulfhydryl group of the pantothenate attached to ACP.
• In fatty acid degradation, the phosphopantetheine group is part of CoA.
• In fatty acid synthesis, the group is attached to a serine residue of ACP.
• Formation of acetyl ACP and malonyl ACP are catalyzed by acetyl transacetylase and malonyl transacetylase.

Reaction Sequence for Fatty Acid Biosynthesis

1. Condensation: Acetyl ACP and malonyl ACP react to form acetoacyl ACP. β-ketoacyl synthase catalyzes this condensing reaction.

2. Reduction: Acetoacyl ACP is reduced to D-3-hydroxybutyryl ACP by β-ketoacyl reductase.

   • The D isomer is formed, unlike fatty acid degradation where the L isomer is formed.
   • $NADPH$ is the reducing agent, whereas $NAD^+$ is the oxidizing agent in β-oxidation.

3. Dehydration: D-3-hydroxybutyryl ACP is dehydrated to form crotonyl ACP, catalyzed by 3-hydroxyacyl dehydratase.

4. Reduction: Crotonyl ACP is reduced to butyryl ACP (trans-Δ2-enoyl ACP) by enoyl reductase.

   • These four reactions (condensation, reduction, dehydration, reduction) are repeated until palmitate (a 16-carbon fatty acid) is formed.

Comparison of β-Oxidation (Degradation) and Biosynthesis of Fatty Acids

β-Oxidation

• Occurs in the mitochondrion.
• Acyl group carrier: CoA
• Intermediate: Fatty acyl-CoA $(C_{n+2})$
• Electron Acceptor: $FAD$
• Product: $FADH_2$
• Intermediate: Enoyl-CoA
• Reactant: $H_2O$
• Intermediate: 3-L-Hydroxyacyl-CoA
• Electron Acceptor: $NAD^+$
• Product: $NADH + H^+$
• Intermediate: B-Ketoacyl-CoA
• C2 unit product: Acetyl-CoA
• Fatty acyl-CoA (Cn)

Biosynthesis

• Occurs in the cytoplasm.
• Acyl group carrier: ACP
• Intermediate: Fatty acyl-ACP $(C_{n+2})$
• Electron Donor: NADPH
• Product: NADP+
• Intermediate: Enoyl-ACP
• Reactant: $H_2O$
• Intermediate: 3-D-Hydroxyacyl-ACP
• Electron Donor: NADPH
• Product: NADP+
• Intermediate: B-Ketoacyl-ACP
• C2 unit donor: Malonyl-CoA
• Reactant: $CoA + CO_2$
• Fatty acyl-ACP (Cn)

Mammalian Fatty Acid Synthase Structure

• The human fatty acid synthase is a large multi-enzyme complex.
• Key components include:
  • KS (ketosynthase)
  • MAT (malonylacetyl transferase)
  • DH (dehydratase)
  • Ψ-MT (methyl transferase, inactive)
  • Ψ-KR (ketoreductase, inactive)
  • ER (enoyl reductase)
  • KR (ketoreductase)
  • ACP (acyl carrier protein)
  • TE (thioesterase)

Catalytic Cycle of Mammalian Fatty Acid Synthase

The cycle involves:

• Condensation
• Reduction
• Dehydration
• Reduction
• Malonyl binding

Regulation of Lipid Metabolism

• Fatty acid oxidation is largely controlled by the concentration of fatty acids in the blood.
  • The concentration is controlled by hormone-sensitive triacylglycerol lipase.
• β-oxidation is inhibited when fatty acid synthesis is activated.
  • Malonyl-CoA inhibits carnitine palmitoyl transferase.
• Fatty acid synthesis is inhibited by cAMP and AMP-dependent phosphorylation of acetyl CoA carboxylase.
  • cAMP allosterically activates PKA.
  • AMP-dependent kinase (AMPK) is activated upon AMP binding and inhibited by ATP.
  • AMPK senses the energy level of the cell.
• Fatty acid synthesis is activated by citrate, which stimulates acetyl CoA carboxylase in a feed-forward loop.
• Insulin-dependent dephosphorylation of acetyl CoA carboxylase activates fatty acid synthesis.