Much like β-oxidation, elongation occurs via four recurring reactions:
In the second step of elongation, butyryl ACP condenses with malonyl ACP to form an acyl ACP compound. This continues until a C16 acyl compound is formed, at which point it is hydrolyzed by a thioesterase into palmitate and ACP.
The first step is condensation of acetyl ACP and malonyl ACP, catalyzed by acyl-malonyl ACP condensing enzyme. This results in the formation of acetoacetyl ACP.
Although this reaction is thermodynamically unfavourable, the evolution of CO2 drives the reaction forward.
In this step, acetoacetyl ACP is reduced by NADPH into D-3-Hydroxybutyryl ACP. This reaction is catalyzed by β-Ketoacyl ACP reductase. The double bond is reduced to a hydroxyl group. Only the D isomer is formed.
In this reaction, D-3-Hydroxybutyryl ACP is dehydrated to crotonyl ACP. This reaction is catalyzed by 3-Hydroxyacyl ACP dehydratase.
During this final step, crotonyl ACP is reduced by NADPH into butyryl ACP. This reaction is catalyzed by enoyl ACP reductase.
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