Pyruvate Dehydrogenase Complex (Link Reaction, connecting glycolysis to the TCA cycle)

1. 1

   Pyruvate + TPP → hydroxyethyl-TPP + CO2

   Pyruvate dehydrogenase E1 1.2.4.1 ST-0264 Irreversible TPP (B1) Mg2+
   In Pyruvate Thiamine pyrophosphate (TPP)

   Out Hydroxyethyl-TPP CO2
   › Notes

   Required coenzyme: thiamine pyrophosphate (TPP), derived from vitamin B1, and Mg2+ for TPP binding. This is the first irreversible chemical event in the overall reaction; release of CO2 helps make PDC flux effectively irreversible in vivo.

2. 2

   Hydroxyethyl-TPP + oxidized lipoamide-E2 → acetyl-dihydrolipoamide-E2 + TPP

   Dihydrolipoyl transacetylase (E2) 2.3.1.12 ST-0265 Irreversible/directional TPP (B1) Lipoamide
   In Hydroxyethyl-TPP Oxidized lipoamide-E2

   Out Acetyl-dihydrolipoamide-E2 Thiamine pyrophosphate (TPP)
   › Notes

   Required cofactors: TPP and lipoic acid/lipoamide. The lipoamide “swinging arm” channels intermediates among active sites.

3. 3

   Acetyl-dihydrolipoamide-E2 + CoA-SH → acetyl-CoA + dihydrolipoamide-E2

   Dihydrolipoyl transacetylase (E2) 2.3.1.12 ST-0266 Irreversible CoA-SH Lipoamide
   In Acetyl-dihydrolipoamide-E2 CoA-SH

   Out Acetyl-CoA Dihydrolipoamide-E2
   › Notes

   Required cofactor: CoA, whose reactive thiol forms the acetyl thioester. Acetyl-CoA formation is part of the overall irreversible, committed entry of pyruvate carbon into oxidative metabolism; acetyl-CoA cannot be converted back to pyruvate in humans.

4. 4

   Dihydrolipoamide-E2 + FAD → oxidized lipoamide-E2 + FADH2

   Dihydrolipoyl dehydrogenase (E3) 1.8.1.4 ST-0267 Irreversible/directional FAD FADH2 Lipoamide
   In Dihydrolipoamide-E2 FAD

   Out Oxidized lipoamide-E2 FADH2
   › Notes

   Required prosthetic group: tightly bound FAD; electrons are transferred from dihydrolipoamide to FAD.

5. 5

   FADH2 + NAD+ → FAD + NADH + H+

   Dihydrolipoyl dehydrogenase (E3) 1.8.1.4 ST-0268 Irreversible NAD+ NADH FAD FADH2
   In FADH2 NAD+

   Out FAD NADH H+
   › Notes

   Required cofactor: NAD+. The summed PDC reaction is: pyruvate + CoA-SH + NAD+ → acetyl-CoA + CO2 + NADH + H+. It is irreversible and is a major regulatory/committed step for oxidative disposal of pyruvate.

Showing all 5 steps.

Overview

The pyruvate dehydrogenase complex (PDC) catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, thereby committing glycolytic carbon to mitochondrial oxidative metabolism. This irreversible link reaction couples pyruvate oxidation to reduction of NAD+ and conserves the remaining two-carbon acetyl group in the high-transfer-potential thioester of acetyl-CoA. PDC flux is especially important in the fed-to-fasting transition and in tissues that depend heavily on aerobic glucose oxidation, including brain, cardiac muscle, and working skeletal muscle.

Cellular location

In human cells, PDC is a large multienzyme assembly in the mitochondrial matrix; pyruvate first crosses the inner mitochondrial membrane through the mitochondrial pyruvate carrier (MPC). The matrix location places the acetyl-CoA product adjacent to the TCA cycle and the NADH product near Complex I of the respiratory chain. Brain and myocardium are highly dependent on oxidative pyruvate metabolism under usual conditions; erythrocytes lack mitochondria and therefore cannot perform this reaction.

Net energetics

For each pyruvate, PDC produces 1 acetyl-CoA, 1 NADH, and 1 CO2, with no ATP or GTP produced directly. Per glucose, two pyruvate molecules yield 2 acetyl-CoA, 2 NADH, and 2 CO2. The two matrix NADH can yield approximately 5 ATP through oxidative phosphorylation using the modern P/O estimate of about 2.5 ATP per NADH. When glycolysis, PDC, the TCA cycle, and oxidative phosphorylation are integrated, complete aerobic oxidation of one glucose yields approximately 30–32 ATP, depending chiefly on the cytosolic NADH shuttle; older textbooks often cite approximately 36–38 ATP.

Clinical significance

PDC determines whether pyruvate is oxidized to acetyl-CoA or instead diverted toward lactate, alanine, or gluconeogenic precursors. Its product acetyl-CoA also supports fatty-acid synthesis in the fed state after citrate export from mitochondria. Thiamine deficiency impairs the TPP-dependent E1 reaction and may cause impaired cerebral energy metabolism and lactic acidosis, notably in severe malnutrition or alcoholism. Inherited PDC deficiency or activating PDK states limit pyruvate oxidation, producing elevated lactate and neurologic dysfunction; dichloroacetate can activate residual PDC by inhibiting PDK in selected settings.