Correct Answer: D. Pyruvate dehydrogenase and dihydrolipoyl transacetylase
The pyruvate dehydrogenase complex (PDC) is a multi-enzyme complex that catalyzes the irreversible conversion of pyruvate to acetyl-CoA, a critical hub linking glycolysis to the citric acid cycle. E1 is pyruvate dehydrogenase (PDH), the first component enzyme that catalyzes the oxidative decarboxylation of pyruvate using thiamine pyrophosphate (TPP) as a cofactor. E2 is dihydrolipoyl transacetylase, the second component enzyme that accepts the acetyl group from the lipoyl arm of E1 and transfers it to CoA, forming acetyl-CoA. The PDC contains three catalytic enzymes (E1, E2, E3) and five cofactors (TPP, lipoic acid, CoA, NAD+, FAD). This complex is essential for energy production in all tissues, particularly in the brain and heart. In India, PDC deficiency causes severe lactic acidosis and developmental delay in infants, and understanding its structure is crucial for recognizing metabolic disorders. The sequential action of E1 (decarboxylation) followed by E2 (transacetylation) is the defining mechanism of the complex.
Why the other options are wrong
A. Pyruvate carboxylase and dihydrolipoyl dehydrogenase — Pyruvate carboxylase catalyzes pyruvate → oxaloacetate (gluconeogenesis/anaplerosis), not pyruvate → acetyl-CoA. Dihydrolipoyl dehydrogenase (E3) is the third component of PDC, not the second. This option confuses gluconeogenic enzymes with oxidative decarboxylation and misidentifies the E2 component. B. Pyruvate carboxylase and dihydrolipoyl transacetylase — While dihydrolipoyl transacetylase is correctly identified as E2, pyruvate carboxylase is wrong for E1. Carboxylase adds CO₂ to pyruvate (anaplerosis), whereas PDC removes CO₂ (oxidative decarboxylation). This is a classic NBE trap pairing the correct E2 with an incorrect E1 to test whether students know the first step. C. Pyruvate dehydrogenase and dihydropoyl dehydrogenase — Pyruvate dehydrogenase (E1) is correct, but dihydrolipoyl dehydrogenase (E3) is the wrong component for E2. E3 catalyzes the final step (regeneration of oxidized lipoic acid using NAD+), not the transacetylation step. This option confuses the sequence of PDC component enzymes.
High-Yield Facts
- Pyruvate dehydrogenase (E1) catalyzes oxidative decarboxylation of pyruvate using TPP as cofactor; it is the rate-limiting and regulated step of PDC.
- Dihydrolipoyl transacetylase (E2) transfers the acetyl group from lipoyl-E1 to CoA, forming acetyl-CoA; it is the core catalytic enzyme of the complex.
- Dihydrolipoyl dehydrogenase (E3) regenerates oxidized lipoic acid by transferring electrons to NAD+, completing the catalytic cycle.
- PDC is inhibited by acetyl-CoA, NADH, and ATP (feedback inhibition) and activated by ADP and Ca²⁺ (allosteric regulation).
- PDC deficiency causes lactic acidosis and neurological damage in infants; thiamine (vitamin B1) deficiency impairs PDH function, causing Wernicke-Korsakoff syndrome.
Mnemonics
PDC Enzyme Sequence: D-T-D Dehydrogenase (E1, removes CO₂) → Transacetylase (E2, transfers acetyl) → Dehydrogenase (E3, regenerates lipoate). Use this to recall the three catalytic steps in order. PDC Cofactors: TPP-LAC-NAD Thiamine pyrophosphate (E1), Lipoic acid (E1/E2), Adenine nucleotides (regulation), CoA (E2 product), NAD⁺ (E3). Covers all five essential cofactors.
NBE Trap
NBE pairs pyruvate carboxylase (gluconeogenic enzyme) with the correct E2 (option B) to trap students who recognize transacetylase but confuse carboxylase with dehydrogenase—a common error when students conflate anaplerotic and oxidative pathways. Similarly, option C pairs the correct E1 with E3 (dihydrolipoyl dehydrogenase) to test whether students know the sequential order of PDC component enzymes.
Clinical Pearl
In Indian pediatric practice, infants with PDC deficiency present with severe lactic acidosis, developmental delay, and seizures—a medical emergency. Recognizing that E1 (pyruvate dehydrogenase) is the rate-limiting step helps clinicians understand why PDC deficiency is so devastating: the block at E1 prevents pyruvate entry into the citric acid cycle, forcing it toward lactate production. Thiamine supplementation in suspected deficiency can partially restore PDH function and is a critical bedside intervention.
_Reference: Harper Biochemistry Ch. 11 (Carbohydrate Metabolism); KD Tripathi Pharmacology Ch. 12 (Thiamine deficiency); Robbins Pathology Ch. 1 (Metabolic disorders)_