Chapter 17, Problem 1P

Interpretation Introduction

Interpretation: The catalytic cofactors required by the pyruvate dehydrogenase complex are to be determined.

Concept Introduction:

Pyruvate dehydrogenase enzyme complex is required for the linking of glycolysis and TCA. It helps in the conversion of pyruvate into Acetyl CoA. Acetyl CoA then enters into the series of the citric acid cycle.

Answer to Problem 1P

Correct answer:

The correct answer is the option (a) NAD⁺, lipoic acid, thiamine pyrophosphate.

Explanation of Solution

Explanation/justification for the correct answer:

Option (a) NAD+ , lipoic acid, thiamine pyrophosphate. An irreversible catabolic reaction is catalyzed by the pyruvate dehydrogenase complex. It occurs at the entry point of pyruvate into the TCA cycle. It is a multifunctional enzyme complex. It catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, CO2, and NADH.

It uses the NAD⁺, lipoic acid, thiamine pyrophosphate as cofactors. NADH and NAD+ are both forms of a coenzyme nicotinamide adenine dinucleotide. NAD+ get reduced by picking up the electrons lost in the oxidation and forms NADH. Pyruvate dehydrogenase uses the thiamine pyrophosphate or TPP as its prosthetic group. Lipoic acid acts as a cofactor for about five enzyme systems. Two of these enzymes systems are in the Krebs cycle. Hence, the option (a) is correct.

Conclusion

Explanation for incorrect answer:

Option (b) NAD⁺, biotin, thiamine pyrophosphate. Biotin is involved in some essential metabolic processes such as gluconeogenesis, amino acid catabolism, and fatty acid synthesis. Hence, the option (b) is incorrect.

Option (c) NAD⁺, FAD, lipoic acid. Lipoic acid acts as a cofactor for about five enzyme systems. Two of these enzymes systems are in the Krebs cycle. These help many organisms to produce energy. Hence, the option (c) is incorrect.

Option (d) FAD, lipoic acid, thiamine pyrophosphate. Lipoic acid acts as a cofactor for about five enzyme systems. Two of these enzymes systems are in the Krebs cycle. These help many organisms to produce energy. Hence, the option (d) is incorrect.

Thus, an irreversible catabolic reaction is catalyzed by the pyruvate dehydrogenase complex. It occurs at the entry point of pyruvate into the TCA cycle. It uses the NAD⁺, lipoic acid, thiamine pyrophosphate as cofactors. Hence, the option (a) is correct.