Biosynthesis of higher fatty acids

Substrate transport into the cytosol

Lipid biosynthesis begins with the preparation of the basic building material. The primary substrate for constructing the carbon skeleton is Acetyl-CoA. The overwhelming majority of this substance is formed inside the mitochondria through the action of the pyruvate dehydrogenase complex (PDH), which converts pyruvate into Acetyl-CoA. However, the fatty acid synthesis process itself is localized in the cytosol, and the inner mitochondrial membrane is completely impermeable to Acetyl-CoA.

To deliver the substrate to the assembly site, the cell uses a special shuttle mechanism. Inside the mitochondrion, Acetyl-CoA condenses with oxaloacetate, turning into a citrate molecule. Citrate easily crosses the membrane via the tricarboxylate carrier and enters the cytoplasm. Here, the cytosolic enzyme ATP-citrate lyase comes into play. Utilizing ATP energy and attaching coenzyme A, this enzyme cleaves the citrate back. As a result, the required Acetyl-CoA and oxaloacetate are formed in the cytosol.

Preparation for synthesis and malonate formation

Direct joining of Acetyl-CoA molecules is impossible, so an activation step is required. From cytosolic Acetyl-CoA, under the action of the enzyme acetyl-CoA carboxylase, an activated carbon donor is formed — Malonyl-CoA. This is the key substrate of biosynthesis, which will supply carbon atoms for continuous chain elongation. It is important to understand the stoichiometry of the future product: only the two distal carbon atoms are taken from the starting Acetyl-CoA, while all other atoms are incorporated into the chain exclusively from Malonyl-CoA molecules in each subsequent cycle.

Next, the substrates are transferred to the active sites of fatty acid synthase:

• The enzyme acetyltransacylase binds the starting acetyl residue to one sulfhydryl (SH) group of the complex.
• The enzyme malonyltransacylase transfers the malonyl residue to another free SH group.

«Loading» of the complex occurs, after which assembly proper begins.

Elongation reaction cycle

The formation of the carbon skeleton is a strict cyclic process. During each such cycle, the growing fatty acid radical increases by exactly two carbon atoms. One turn includes four main reactions:

1. Condensation. The acetyl group is transferred to the malonyl group. Decarboxylation occurs simultaneously — carbon dioxide is released. Via the liberated valence, the groups bond, forming a 4-carbon ketoacyl (acetoacetyl) firmly bound to the enzyme.
2. First reduction. The keto group of the intermediate product is reduced to a hydroxyl group. NADPH + H⁺ serves as the hydrogen atom donor for this reaction.
3. Dehydration. A water molecule is split off from the resulting compound. As a result, a double bond is formed, producing an unsaturated radical (crotonyl).
4. Second reduction. The double bond is saturated with hydrogen again at the expense of oxidizing another NADPH molecule.

The outcome of the first cycle is the formation of butyryl — a fully saturated four-carbon residue that remains attached to the synthase.

Completion of synthase action

After the completion of the first cycle, the resulting butyryl radical does not go anywhere. A fresh Malonyl-CoA molecule attaches to the liberated SH group of the enzyme. Then butyryl condenses with the new malonyl (again releasing carbon dioxide), and the entire cascade of reactions — reduction, dehydration, second reduction — repeats.

To synthesize the final product, fatty acid synthase must complete exactly 7 elongation cycles. At each step, reduction reactions ensure the formation of a long saturated aliphatic radical. Ultimately, a 16-carbon residue — palmitoyl — is formed on the enzyme. When the chain reaches this length, the elongation process stops automatically.

At the final stage, thioesterase is activated — an enzymatic activity of the complex that hydrolyzes the thioester bond with the participation of a water molecule. The finished molecule is released, the enzyme complex is regenerated, and free palmitic acid is released into the cytosol.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of acetyl-CoA carboxylase function
• Structure of the multienzyme synthase complex
• Mechanism of action of ATP-citrate lyase in the cytosol
• Regulation of fatty acid biosynthesis at the cellular level
• The role of the tricarboxylate carrier in the shuttle mechanism

Other topics in the «Lipid metabolism» section

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