Biosynthesis of fatty acids

• Fatty acid synthesis occurs in cytosol of many organism and in chloroplast ( Stroma) in plants.
• Formed by condensation of two carbon units.
• Involves a separate series of reaction to build a long chain hydrocarbons from acetyl-CoA units.
• Uses NADPH as reductant.
• In vertebrate, occurs in cytosol but in plant and bacteria, occurs in chloroplast.
• Mainly two enzymes are required i.e. Acetyl CoA carboxylase and Fatty acid synthetase.

Steps

1. Activation:

• Synthesis starts with the formation of Acetyl ACP and malonyl ACP.
• Acetyl transacylase and malonyl transacylase catakyze these reaction

Acetyl CoA + ACP ⇌ Acetyl ACP + CoA

Malonyl CoA + ACP ⇌ Malonyl ACP + CoA

2. Condensation:

• Acetyl ACP and Malonyl ACP react to form Acetoacetyl ACP.

3. Reduction reaction:

• Acetoacetyl ACP is reduced to D-3-hydroxy butyryl ACP.

4. Dehydration reaction:

• D-3-hydroxybutyryl ACP is dehydrated to form Crotonyl ACP.

5. Reduction reaction:

• Crotonyl ACP then finally is reduced to butyryl ACP.
• This is the end of first elongation cycle.
• In second round, butyryl ACP condenses with malonyl ACP to form C6-β-ketoacyl ACP.
• Reduction, dehydration and second reduction converts C6-β-ketoacyl ACP into C6-acyl ACP, ready for third round of elongation.

6. Termination:

• Process continues until a C16 palmitoyl group is formed.