Fatty acid synthesis is the opposite of beta-oxidation. It builds fatty acids two carbons at a time, using acetyl-CoA as the 2-carbon donor. It happens in the cytoplasm, which is a different compartment from beta-oxidation (the matrix) - deliberate separation to allow independent regulation.
Fatty acid synthesis and the citrate shuttle
Pathway map
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Scroll sideways to see the whole map.
Building direction NADPH consumed Acetyl-CoA carboxylase: the regulated step Mitochondrion
Getting the carbons outAcetyl-CoA cannot cross the inner membrane, so it condenses with oxaloacetate to make citrate, rides the citrate shuttle to the cytosol, and is cut back apart by ATP-citrate lyase. High citrate therefore means two things at once: plenty of fuel, and the raw material for fat.
Where NADPH comes fromMostly the pentose phosphate pathway, plus malic enzyme on the way back from the citrate shuttle. Each two-carbon addition costs 2 NADPH, so 14 NADPH go into one palmitate.
The reciprocal switchMalonyl-CoA is the first committed intermediate here and a direct inhibitor of CPT-I over in β-oxidation. Insulin activates acetyl-CoA carboxylase, glucagon and adrenaline switch it off, and citrate activates it allosterically while palmitate feeds back to shut it down.
Synthesis is not β-oxidation run backwards. It happens in the cytosol rather than the matrix, uses NADPH rather than making NADH, carries the growing chain on acyl carrier protein rather than CoA, and adds carbons via three-carbon malonyl-CoA rather than removing them two at a time. Only the chemistry of the four steps rhymes.
Getting Acetyl-CoA Out of the Matrix
Acetyl-CoA for synthesis is made in the matrix but synthesis happens in the cytoplasm. The citrate shuttle solves this: acetyl-CoA + OAA → citrate (via citrate synthase). Citrate exits the mitochondrion, and in the cytoplasm, citrate lyase regenerates acetyl-CoA + OAA. The cytoplasmic acetyl-CoA is then available for fatty acid synthesis.
Acetyl-CoA Carboxylase (ACC) - The Committed Step
Acetyl-CoA + CO2+ATP→Malonyl-CoA+ADP+Pi
ACC uses biotin (vitamin B7) as a cofactor to carboxylate acetyl-CoA, producing malonyl-CoA. This is the rate-limiting step of fatty acid synthesis. Regulation:
A single multifunctional enzyme (with 7 catalytic activities and an ACP carrier domain) that repeatedly:
Binds acetyl-CoA (or growing acyl chain) and malonyl-CoA.
Condenses them (releasing CO2), making a 4-carbon β-ketoacyl.
Reduces with NADPH.
Dehydrates.
Reduces again with NADPH.
Each cycle adds 2 carbons. Palmitate (C16) requires 7 cycles. After synthesis, palmitate can be elongated and desaturated by other enzymes.
NADPH Source
Fatty acid synthesis requires lots of NADPH (2 per cycle, 14 for palmitate). Main sources:
Pentose phosphate pathway (Chapter 9).
Malic enzyme (in the cytoplasm after citrate shuttle).
Where does fatty acid synthesis occur and where does beta-oxidation occur?
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Fatty acid synthesis: cytoplasm. Beta-oxidation: mitochondrial matrix. Compartmentalization prevents simultaneous synthesis and breakdown (futile cycling). Different cofactors also separate them: synthesis uses NADPH; beta-oxidation uses NAD+ and FAD.
What is the rate-limiting enzyme of fatty acid synthesis, and what cofactor does it need?
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Acetyl-CoA carboxylase (ACC) catalyzes the committed step: acetyl-CoA + CO2 + ATP → malonyl-CoA. It requires biotin (vitamin B7) as a cofactor. Regulation: citrate activates; palmitoyl-CoA inhibits; insulin dephosphorylates (activates); glucagon/epinephrine phosphorylate (inactivate).
Why does fatty acid synthesis use NADPH while beta-oxidation uses NAD+ and FAD?
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The two processes go in opposite directions. Beta-oxidation REMOVES electrons from fatty acids (oxidation), so it needs electron acceptors NAD+ and FAD (reduced to NADH and FADH2). Fatty acid synthesis ADDS electrons to build up the reduced fatty acid chain (reduction), requiring the reducing agent NADPH. Maintaining separate NADH and NADPH pools lets the cell run both without interference.
