The pyruvate dehydrogenase complex (PDHc) is a crucial multi-enzyme assembly that links glycolysis to the citric acid (TCA) cycle by catalyzing the irreversible conversion of pyruvate into acetyl-CoA. This complex enables the cell to fully oxidize glucose and generate energy efficiently.

Understanding the enzymatic components of the PDH complex is essential for grasping how metabolic pathways are interconnected. This article clarifies which enzymes constitute the PDH complex and identifies which enzyme listed is not part of it.

The Pyruvate Dehydrogenase Complex: An Overview

The PDH complex is composed of three enzymatically active subunits, each catalyzing a specific step in the conversion of pyruvate to acetyl-CoA:

1. Pyruvate Dehydrogenase (E1):
   This enzyme catalyzes the decarboxylation of pyruvate, forming a hydroxyethyl-TPP intermediate and releasing CO₂.

2. Dihydrolipoyl Transacetylase (E2):
   Also known as dihydrolipoyl transferase, this enzyme transfers the acetyl group from the lipoamide cofactor to Coenzyme A, forming acetyl-CoA.

3. Dihydrolipoyl Dehydrogenase (E3):
   This enzyme regenerates the oxidized form of lipoamide by transferring electrons to NAD⁺, producing NADH.

These three enzymes work in a coordinated manner, with substrate channeling facilitated by flexible lipoamide “arms” that shuttle intermediates between active sites.

Enzymes Not Part of the PDH Complex

Among the options provided, one enzyme is not a recognized component of the PDH complex:

• Dihydrolipoyl Oxidase:
  This enzyme is not a standard subunit of the pyruvate dehydrogenase complex. The correct third enzyme is dihydrolipoyl dehydrogenase, which performs the oxidation of dihydrolipoamide and reduction of NAD⁺.

Summary of Enzymatic Components

Conclusion

The pyruvate dehydrogenase complex consists of three enzymatic components: pyruvate dehydrogenase, dihydrolipoyl transferase, and dihydrolipoyl dehydrogenase. The enzyme dihydrolipoyl oxidase is not a part of this complex.

Correct answer: (4) Dihydrolipoyl oxidase