Fatty Acid Synthesis

Key points

• • Fatty acid synthesis builds long fatty acids from two-carbon units (as acetyl-CoA) when energy and carbohydrate are plentiful, mainly in the liver and fat tissue.
• • It happens in the cytoplasm, the opposite compartment to fatty acid breakdown, which happens in the mitochondria.
• • The committed, rate-limiting step is the making of malonyl-CoA by the enzyme acetyl-CoA carboxylase, which needs the vitamin biotin.
• • Acetyl-CoA is made in the mitochondria but is needed in the cytoplasm, so it is shuttled out as citrate (the citrate shuttle).
• • The building reactions are carried out by a large multi-part enzyme, fatty acid synthase, using reducing power supplied by NADPH.
• • Insulin (the fed-state hormone) switches synthesis on, while malonyl-CoA also blocks the carrier (CPT-I) that takes fat into the mitochondria for breakdown, so the cell does not build and burn fat at once.