Question

The removal of two-carbon units from a fatty acyl coenzyme A involves four
sequential reactions. Which of the following best describes the reaction
sequence?​

Answer 1

Answer:

.

First oxidation. In the initial step, acyl coenzyme A loses two hydrogen atoms from its α and β carbon atoms (C2 and C3). This dehydrogenation is catalyzed by acyl-CoA dehydrogenase that uses FAD as a hydrogen acceptor. The product is an unsaturated α-β (or 2–3) acyl-CoA derivative in trans configuration.

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There are three isozymes of acyl-CoA dehydrogenase. One uses long chain FAs (of more than 12C) as substrate, the other acts on FAs of 6–12C in length, a third one catalyze the dehydrogenation of FAs of 4–6C in length. Complete oxidation of a long FA chain requires the action of all three isoforms.

2.

Hydration. Water is added to saturate the double bond and to form β-hydroxyacyl-CoA (or 3-hydroxyacyl-CoA). The reaction is catalyzed by enoyl hydratase also called crotonase.

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3.

Second oxidation. The β-hydroxy derivative undergoes a new β carbon dehydrogenation to form the corresponding β-keto-acyl-CoA. The β-hydroxyacyl-CoA dehydrogenase is responsible for this reaction, with NAD acting as a hydrogen acceptor.

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4.

Chain rupture and acetyl-CoA release. Finally, the β-ketoacyl-CoA is cleaved at the junction between α and β carbons by the action of thiolase (ketothiolase). This thiolytic reaction requires the presence of another molecule of coenzyme A. The products formed are acetyl-CoA and acyl-CoA, which are two carbons smaller than the original compound.

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Fatty acids of 12 carbon residues or more are subjected to the enzymes that catalyze the last three stages (reactions 2, 3, and 4), which are located in the same trifunctional protein.

The series of reactions included in β-oxidation are summarized in Fig. 15.4.