Biochemistry Module 6 Practice Exam

Which molecule serves as the two-carbon donor in the first committed step of fatty acid synthesis?

Acetyl-CoA Acetoacetyl-CoA Malonyl-CoA Succinyl-CoA

Correct Answer:

Malonyl-CoA

Explanation:

In fatty acid synthesis, the chain is extended by a two-carbon unit donated from malonyl-CoA during the first condensation with the starter acetyl group. Malonyl-CoA is decarboxylated as it condenses with the growing chain on the fatty acid synthase complex, delivering two carbons and forming a four-carbon product. This decarboxylative condensation step is what makes malonyl-CoA the two-carbon donor in the initial committed step. Acetyl-CoA provides the initial primer, not the donor in this step. Succinyl-CoA isn’t part of this fatty acid synthesis pathway, and acetoacetyl-CoA is the product of the condensation, not the donor.

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