In Escherichia coli, what reaction is catalyzed by the pyruvate (pyruvic acid) dehydrogenase complex at the junction between glycolysis and the TCA cycle?

Introduction:
The pyruvate dehydrogenase complex (PDC) links glycolysis to the tricarboxylic acid (TCA) cycle by converting pyruvate into acetyl-CoA. This question checks recognition of the oxidative decarboxylation step that produces acetyl-CoA, CO2, and reduced cofactors crucial for aerobic metabolism.

Given Data / Assumptions:

• Substrate: pyruvate (end product of glycolysis).
• Products: acetyl-CoA and CO2; NAD+ is reduced to NADH.
• Multienzyme assembly: E1 (pyruvate dehydrogenase), E2 (dihydrolipoyl transacetylase), E3 (dihydrolipoyl dehydrogenase).

Concept / Approach:
PDC performs oxidative decarboxylation. Thiamine pyrophosphate (TPP) decarboxylates pyruvate, lipoamide accepts the acyl group, CoA forms acetyl-CoA, and FAD/NAD+ reoxidize the lipoamide. The net effect is oxidation of pyruvate coupled to the formation of acetyl-CoA and CO2 with generation of NADH.

Step-by-Step Solution:

Verification / Alternative check:
Stoichiometry: pyruvate + CoA + NAD+ → acetyl-CoA + CO2 + NADH + H+, matching oxidative decarboxylation rather than reduction.

Why Other Options Are Wrong:

• Decatalyze/retard: not meaningful biochemical actions.
• Reduction to acetyl-CoA: the reaction is oxidative, not reductive.
• Direct oxaloacetate formation: that is catalyzed by pyruvate carboxylase (anaplerosis), not PDC.

Common Pitfalls:
Confusing PDC with pyruvate carboxylase or lactate dehydrogenase; forgetting NADH production.

Final Answer:
Catalyze the oxidation of pyruvate to acetyl-CoA and CO2