Substrate Transport to the Cytosol
Lipid biosynthesis begins with the preparation of the basic building block. The primary substrate for constructing the carbon skeleton is acetyl-CoA. The overwhelming majority of this substance is produced within mitochondria through the action of the pyruvate dehydrogenase complex (PDH), which converts pyruvate into acetyl-CoA. However, the process of fatty acid synthesis itself is localized to the cytosol, and the inner mitochondrial membrane is completely impermeable to acetyl-CoA.
To deliver the substrate to the assembly site, the cell utilizes a special shuttle mechanism. Inside the mitochondrion, acetyl-CoA condenses with oxaloacetate, forming a citrate molecule. Citrate easily crosses the membrane via the tricarboxylate transporter and enters the cytoplasm. Here, the cytosolic enzyme ATP-citrate lyase comes into play. With the consumption of ATP energy and the addition of coenzyme A, this enzyme cleaves citrate back down. As a result, the necessary acetyl-CoA and oxaloacetate are formed in the cytosol.
Preparation for Synthesis and Malonyl-CoA Formation
Direct joining of acetyl-CoA molecules is impossible, so an activation step is required. Cytosolic acetyl-CoA is converted into the activated carbon donor, malonyl-CoA, by the action of the enzyme acetyl-CoA carboxylase. This is the key substrate of biosynthesis, which supplies carbon atoms for continuous chain elongation. It is important to understand the stoichiometry of the future product: only the distal two 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 centers 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 directly 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:
- Condensation. The acetyl group is transferred to the malonyl group. During this process, decarboxylation occurs—carbon dioxide is released. Through the freed valence, the groups join to form a 4-carbon ketoacyl (acetoacetyl), tightly bound to the enzyme.
- 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.
- Dehydration. A water molecule is split off from the resulting compound. As a result, a double bond forms, yielding an unsaturated radical (crotonyl).
- Second reduction. The double bond is saturated with hydrogen again through the oxidation of another NADPH molecule.
The result of the first cycle is the formation of butyryl—a fully saturated four-carbon residue that remains attached to the synthase.
Completion of Synthase Function
After the completion of the first cycle, the resulting butyryl radical does not leave. A fresh molecule of malonyl-CoA attaches to the freed SH group of the enzyme. Then, butyryl condenses with the new malonyl (again with the release of 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 automatically stops.
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.