pyruvate dehydrogenase complex
Summary
Pyruvate dehydrogenase complex (PDC) is a mitochondrial multi-enzyme complex that converts pyruvate to acetyl-CoA, linking glycolysis to the TCA cycle. It requires five cofactors: thiamine (B1/TPP), riboflavin (B2/FAD), niacin (B3/NAD+), pantothenic acid (B5/CoA), and lipoic acid. Deficiency (genetic or thiamine-related) causes lactic acidosis and neurologic dysfunction.
Detail
PDC catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, producing NADH and CO2, in the mitochondrial matrix. This is the key regulatory step connecting glycolysis to the citric acid (TCA) cycle. The complex has three catalytic components: E1 (pyruvate dehydrogenase, requires TPP), E2 (dihydrolipoyl transacetylase, requires lipoic acid and CoA), and E3 (dihydrolipoyl dehydrogenase, requires FAD and NAD+).
Regulation: PDC is activated by NAD+, CoA, ADP, Ca2+, and pyruvate (via pyruvate dehydrogenase phosphatase which dephosphorylates and activates it). It is inhibited by ATP, NADH, and acetyl-CoA (via pyruvate dehydrogenase kinase, which phosphorylates and inactivates E1). This mirrors regulation of the analogous alpha-ketoglutarate dehydrogenase complex in the TCA cycle.
Clinical significance: - Arsenic poisoning inhibits lipoic acid-dependent enzymes (PDC and alpha-KG dehydrogenase), causing vomiting, rice-water stools, and garlic breath odor. - Thiamine (B1) deficiency (e.g., alcoholism, malnutrition) impairs PDC activity, contributing to lactic acidosis and Wernicke-Korsakoff syndrome; also affects alpha-ketoglutarate dehydrogenase and branched-chain ketoacid dehydrogenase. - Pyruvate dehydrogenase deficiency (genetic, X-linked E1-alpha subunit mutation) causes congenital lactic acidosis, neurologic defects, and can present with build-up of pyruvate that gets shunted to alanine (via alanine transaminase) and lactate (via lactate dehydrogenase). Treatment includes a ketogenic diet (high fat) since acetyl-CoA production is bypassed via fatty acid oxidation, and supplementation with lysine/leucine (ketogenic amino acids) may help bypass the block. - Dichloroacetate can be used to stimulate PDC activity by inhibiting pyruvate dehydrogenase kinase, used experimentally in lactic acidosis treatment.
High-yield board association: PDC and alpha-ketoglutarate dehydrogenase are the two mitochondrial dehydrogenase complexes requiring the same 5 cofactors (TPP, FAD, NAD+, CoA, lipoic acid) - remember with mnemonic 'Tender Loving Care For Nancy.'
Sources
- First Aid for the USMLE Step 1
- Lippincott Biochemistry
- Harper's Illustrated Biochemistry
- Robbins Basic Pathology
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