29. The following statements are made regarding conversion of pyruvate to acetyl- CoA going from glycolysis to citric acid cycle:
    A. Oxidation of pyruvate to acetyl-CoA is reversible.
    B. Pyruvate is transported into the mitochondrion by a transporter.
    C. Pyruvate is carboxylated by pyruvate dehydrogenase.
    D. Acetyl lipoamide reacts with coenzyme A to form acetyl-CoA.
    E. The flavoprotein, dihydrolipoyl dehydrogenase, containing flavin adenine dinucleotide (FAD), is involved in conversion of pyruvate to acetyl-CoA.

Which one of the following options represents the combination of all correct statements?
(1) A, B and C                                           (2) B, C and D
(3) C, D and E                                           (4) B, D and E

The conversion of pyruvate to acetyl-CoA is a critical metabolic step linking glycolysis to the citric acid (TCA) cycle. This process, catalyzed by the pyruvate dehydrogenase complex, enables the complete oxidation of glucose-derived carbons and efficient energy production.

This article analyzes key statements about this conversion, clarifying which are correct and explaining the biochemical basis of each step.

Statement Analysis

A. Oxidation of pyruvate to acetyl-CoA is reversible.

• This is incorrect. The conversion of pyruvate to acetyl-CoA is an irreversible oxidative decarboxylation. Once pyruvate is converted, it cannot revert back, committing the molecule to aerobic respiration.

B. Pyruvate is transported into the mitochondrion by a transporter.

• This is correct. Pyruvate produced in the cytosol crosses the mitochondrial membranes via a specific pyruvate transporter (pyruvate carrier) into the mitochondrial matrix, where the pyruvate dehydrogenase complex resides.

C. Pyruvate is carboxylated by pyruvate dehydrogenase.

• This is incorrect. Pyruvate undergoes decarboxylation (removal of a carboxyl group as CO₂) by pyruvate dehydrogenase, not carboxylation (addition of CO₂). The enzyme catalyzes oxidative decarboxylation.

D. Acetyl lipoamide reacts with coenzyme A to form acetyl-CoA.

• This is correct. Within the pyruvate dehydrogenase complex, the acetyl group is transferred to the lipoamide cofactor forming acetyl lipoamide, which then reacts with coenzyme A to produce acetyl-CoA.

E. The flavoprotein, dihydrolipoyl dehydrogenase, containing flavin adenine dinucleotide (FAD), is involved in conversion of pyruvate to acetyl-CoA.

• This is correct. Dihydrolipoyl dehydrogenase is the third enzyme component of the complex. It regenerates the oxidized form of lipoamide by transferring electrons to FAD, then to NAD⁺, producing NADH.

Correct Combination of Statements

• Correct statements: B, D, and E

• Incorrect statements: A and C

Thus, the combination representing all correct statements is:

(4) B, D and E

Summary Table

Conclusion

The conversion of pyruvate to acetyl-CoA involves pyruvate transport into mitochondria, irreversible oxidative decarboxylation, acetyl group transfer to coenzyme A, and regeneration of enzyme cofactors involving FAD. The correct statements describing these steps are B, D, and E.

Correct answer: (4) B, D and E