Pyruvate dehydrogenase (PDH) is a giant enzyme complex (three enzymes, ~60 subunits) that converts pyruvate to acetyl-CoA. It is the bridge between glycolysis (cytoplasm) and the TCA cycle (mitochondrial matrix). Without PDH, the pyruvate produced by glycolysis could not be fully oxidized - so the whole aerobic pipeline depends on this single committed step. Pyruvate from glycolysis enters the mitochondrion, PDH strips a CO2 and adds a CoA, and the resulting acetyl-CoA is handed off to the TCA cycle (Krebs / citric acid cycle) for complete oxidation.
Pyruvate dehydrogenase: the one-way bridge
Pathway map
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Junction metabolite Irreversible NADH made Turns the complex on
Why it only runs one wayThe reaction is an oxidative decarboxylation: a carbon leaves as CO₂ and is gone. That makes it irreversible, and it is the single reason fatty acids can never become glucose. Every carbon that crosses this bridge has left the sugar pool for good.
The five cofactorsTender Loving Care For Nancy: TPP, Lipoic acid, CoA, FAD, NAD⁺. Four of the five are vitamins, which is why thiamine deficiency stalls this step and causes the lactate build-up seen in beriberi and Wernicke-Korsakoff. α-ketoglutarate dehydrogenase uses exactly the same five.
How it is switchedTwo layers. Allosterically, the products (acetyl-CoA, NADH) and a full energy tank (ATP) slow it, while ADP, NAD⁺, CoA, and Ca²⁺ speed it up. On top of that, PDH kinase phosphorylates and switches it off, and PDH phosphatase (stimulated by insulin and by Ca²⁺ in contracting muscle) switches it back on.
One reaction, three enzymes, five cofactors, and no way back. PDH is not part of glycolysis and not part of the TCA cycle; it is the gate between them, and it is where the cell commits carbohydrate carbon to being burned rather than stored as sugar.
The Reaction
Pyruvate + CoA + NAD+→Acetyl-CoA + CO2+NADH
This is an oxidative decarboxylation. Pyruvate loses one carbon as CO2 and gains a CoA to become a 2-carbon acetyl-CoA. NADH is produced. The reaction is irreversible and commits carbon to the TCA cycle.
Five Required Coenzymes
PDH is one of three mitochondrial complexes that share the same five coenzymes:
TPP (thiamine pyrophosphate, from vitamin B1).
Lipoic acid (a covalent cofactor on E2).
CoA (coenzyme A, from vitamin B5 - pantothenate).
FAD (riboflavin, B2).
NAD+ (niacin, B3).
Alpha-ketoglutarate dehydrogenase (in the TCA cycle) and branched-chain alpha-ketoacid dehydrogenase (for branched amino acid catabolism) use the same five coenzymes.
Regulation
PDH is shut off when the cell has enough energy:
Inhibited by: ATP, NADH, acetyl-CoA (product inhibition), and by PDH kinase-mediated phosphorylation.
Activated by: ADP, NAD+, pyruvate, and dephosphorylation by PDH phosphatase.
Insulin activates PDH (favors oxidation of glucose). Glucagon inactivates PDH (in the liver).
What are the products of the pyruvate dehydrogenase reaction per pyruvate?
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Per pyruvate: 1 acetyl-CoA, 1 CO2, 1 NADH. The reaction is irreversible and commits carbon to the TCA cycle. Per glucose (2 pyruvates), 2 acetyl-CoA, 2 CO2, 2 NADH are produced by PDH.
Which vitamin is required as TPP for pyruvate dehydrogenase activity, and what happens with its deficiency?
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Vitamin B1 (thiamine) is required as thiamine pyrophosphate (TPP). Deficiency impairs PDH, alpha-ketoglutarate dehydrogenase, and other TPP-dependent enzymes. Clinical picture: beriberi (wet with heart failure, dry with neuropathy) and Wernicke-Korsakoff syndrome in chronic alcoholics (encephalopathy, memory loss). Pyruvate accumulates and is converted to lactate, causing lactic acidosis.
Why is the PDH reaction irreversible?
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PDH couples decarboxylation (CO2 released as gas, lost to the environment) with oxidation and thioester formation. The reaction has a large negative ΔG and the CO2 physically leaves the system. This is one of the defining features of metabolism - once acetyl-CoA is made from pyruvate, the cell cannot regenerate pyruvate from acetyl-CoA. That is why humans cannot convert fatty acids (which produce only acetyl-CoA) to glucose.
