Substrates and compartmentalization of fatty acid synthesis
Fatty acid synthesis takes place in the cytosol and requires three main substrates: acetyl-CoA as a source of carbon atoms, NADPH as a reducing equivalent, and ATP as an energy source. Acetyl-CoA is formed in the mitochondrial matrix via oxidative decarboxylation of pyruvate, whereas the assembly of fatty acids occurs outside the mitochondria. NADPH is supplied by the pentose phosphate pathway and the malic enzyme reaction. ATP is synthesized in the mitochondria.
Citrate shuttle: transport of acetyl residues into the cytosol
The inner mitochondrial membrane is impermeable to acetyl-CoA, so acetyl residues are transported into the cytosol as part of citrate.
- In the matrix, citrate synthase condenses acetyl-CoA with oxaloacetate to form citrate.
- Citrate is exported into the cytosol by a specific carrier.
- In the cytosol, ATP-citrate lyase cleaves citrate into acetyl-CoA and oxaloacetate, consuming ATP (forming ADP and Pi).
- Oxaloacetate is reduced by malate dehydrogenase to malate with the oxidation of NADH.
- Malate is decarboxylated by malic enzyme to pyruvate, yielding NADPH and CO₂; pyruvate returns to the mitochondrion.
Thus, the citrate shuttle simultaneously supplies acetyl-CoA for synthesis and a portion of the required NADPH.
Regulatory reaction: acetyl-CoA carboxylase
The first and key regulatory step is the carboxylation of acetyl-CoA to form malonyl-CoA. The reaction is catalyzed by acetyl-CoA carboxylase, an enzyme of the ligase class that uses biotin as a cofactor. The reaction requires CO₂ and ATP; the products are malonyl-CoA, ADP, and inorganic phosphate. Malonyl-CoA serves as an activated two-carbon donor for further chain elongation and simultaneously acts as an inhibitor of fatty acid transport into the mitochondria (via suppression of carnitine acyltransferase I), which uncouples synthesis from β-oxidation.
Fatty acid synthase and the elongation cycle
Fatty acid synthase (palmitate synthase) is a multienzyme complex consisting of two identical polypeptide chains, each containing an acyl carrier protein (ACP) with a phosphopantetheine SH-group and an SH-group of a cysteine residue in the active center of the condensing domain.
- Acetyltransacylase transfers the acetyl residue to the cysteine SH-group.
- Malonyltransacylase transfers the malonyl residue to the phosphopantetheine SH-group.
- Condensation: the acetyl group is transferred to the malonyl group with the simultaneous release of CO₂, forming acetoacetyl-ACP.
- First reduction of the keto group by NADPH to β-hydroxybutyryl-ACP.
- Dehydration with the elimination of H₂O.
- Second reduction of the double bond involving NADPH; butyryl-ACP is formed.
Each subsequent cycle elongates the chain by two carbon atoms using a new malonyl-CoA. After seven cycles, palmitoyl-ACP (C16) is formed. Thioesterase hydrolyzes the thioester bond, releasing free palmitate.
Further modifications and essential fatty acids
Other fatty acids are formed from palmitate via elongation and desaturation. In the human body, desaturases can only introduce double bonds between the carboxyl group and the ninth carbon atom. Therefore, polyunsaturated fatty acids with double bonds beyond C9 (linoleic, linolenic) are essential and must be obtained from the diet, predominantly as part of plant fats (recommended daily intake is 80–100 g).
Regulation of acetyl-CoA carboxylase
The activity of the regulatory enzyme is controlled by two mechanisms.
- Covalent modification. Insulin, via phosphoprotein phosphatase, causes dephosphorylation of the enzyme, converting it to the active form. Glucagon and epinephrine, via adenylyl cyclase, cAMP, and protein kinase A, phosphorylate the enzyme, inactivating it.
- Association/dissociation of protomers. Citrate (an allosteric activator) causes the association of protomers into an active polymer. Palmitoyl-CoA (the end product) dissociates the complex via feedback inhibition, inactivating the enzyme.
Induction of lipogenic enzymes by insulin
During prolonged carbohydrate intake, elevated levels of insulin induce the transcription of key lipogenic enzymes:
- glucose-6-phosphate dehydrogenase — supplies NADPH via the pentose phosphate pathway;
- malic enzyme — an additional source of NADPH;
- ATP-citrate lyase — supplies cytosolic acetyl-CoA;
- acetyl-CoA carboxylase — produces malonyl-CoA;
- fatty acid synthase — assembles palmitate.
Synthesized fatty acids do not accumulate in a free state, but are immediately used to form triacylglycerols.
Key points
- Fatty acid synthesis takes place in the cytosol and requires acetyl-CoA, NADPH, and ATP.
- Acetyl residues are transported from the mitochondria to the cytosol in the form of citrate; ATP-citrate lyase releases acetyl-CoA.
- Malic enzyme and the pentose phosphate pathway provide NADPH for reductive reactions.
- The regulatory reaction is the carboxylation of acetyl-CoA to malonyl-CoA by the enzyme acetyl-CoA carboxylase (a biotin-dependent ligase).
- Fatty acid synthase elongates the chain to C16 over seven cycles, yielding palmitate as the product, which is released by thioesterase.
- Citrate activates and palmitoyl-CoA inhibits acetyl-CoA carboxylase by altering its polymeric state.
- Insulin activates the enzyme via dephosphorylation and induces the synthesis of key lipogenic enzymes; glucagon inactivates it via phosphorylation.
- Polyunsaturated fatty acids with double bonds further than C9 cannot be synthesized and are essential.