Pyruvate Oxidation / Link Reaction (Pyruvate Dehydrogenase Complex)

1. 1

   Pyruvate + thiamine pyrophosphate (TPP) → hydroxyethyl-TPP + CO2

   Pyruvate dehydrogenase E1 1.2.4.1 ST-0011 Irreversible TPP (B1) Mg2+
   In Pyruvate Thiamine pyrophosphate (TPP)

   Out Hydroxyethyl-TPP CO2
   › Notes

   The E1 component, pyruvate dehydrogenase (pyruvate decarboxylase), requires tightly bound TPP and Mg2+. It decarboxylates pyruvate and retains the two-carbon hydroxyethyl group on TPP; this is the decarboxylation portion of the overall irreversible reaction.

2. 2

   Hydroxyethyl-TPP + oxidized lipoamide (E2) → acetyl-lipoamide + TPP

   Pyruvate dehydrogenase E1 1.2.4.1 ST-0012 Irreversible/directional TPP (B1) Lipoamide
   In Hydroxyethyl-TPP Oxidized lipoamide

   Out Acetyl-lipoamide Thiamine pyrophosphate (TPP)
   › Notes

   E1 transfers and oxidizes the hydroxyethyl group to the lipoyllysine swinging arm of E2, dihydrolipoyl transacetylase. Lipoic acid/lipoamide serves as the redox-active prosthetic group.

3. 3

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

   Dihydrolipoyl transacetylase (E2) 2.3.1.12 ST-0013 Irreversible/directional CoA-SH Lipoamide
   In Acetyl-lipoamide CoA-SH

   Out Acetyl-CoA Dihydrolipoamide
   › Notes

   E2 transfers the acetyl group to coenzyme A (CoA-SH), creating acetyl-CoA. This is a transesterification reaction; CoA is derived from pantothenate (vitamin B5).

4. 4

   Dihydrolipoamide + FAD → oxidized lipoamide + FADH2

   Dihydrolipoyl dehydrogenase (E3) 1.8.1.4 ST-0014 Irreversible/directional FAD FADH2 Lipoamide
   In Dihydrolipoamide FAD

   Out Oxidized lipoamide FADH2
   › Notes

   The E3 component, dihydrolipoyl dehydrogenase, reoxidizes reduced lipoamide using its tightly bound FAD prosthetic group. This reaction restores the oxidized lipoyl arm required for another catalytic cycle.

5. 5

   FADH2 + NAD+ → FAD + NADH + H+

   Dihydrolipoyl dehydrogenase (E3) 1.8.1.4 ST-0015 Irreversible/directional NAD+ NADH FAD FADH2
   In FADH2 NAD+

   Out FAD NADH H+
   › Notes

   E3 transfers reducing equivalents from FADH2 to NAD+, generating NADH. Together, the E1–E3 sequence constitutes PDC catalysis; the complex is the regulated, effectively rate-controlling entry point of pyruvate carbon into acetyl-CoA.

Showing all 5 steps.

Overview

The pyruvate dehydrogenase complex (PDC) oxidatively decarboxylates pyruvate to acetyl-CoA, coupling cytosolic glycolysis to the mitochondrial tricarboxylic acid (TCA) cycle. It is a multienzyme complex that conserves the oxidation energy as NADH and a high-energy thioester, and its overall reaction is physiologically irreversible. Thus, even-carbon acetyl-CoA cannot provide net carbon for glucose synthesis in humans.

Cellular location

PDC is located in the mitochondrial matrix, where it associates functionally with TCA-cycle enzymes. It is highly active in aerobic tissues such as cardiac and skeletal muscle, brain, liver, and adipose tissue; pyruvate crosses the inner mitochondrial membrane through the mitochondrial pyruvate carrier before oxidation. Erythrocytes lack mitochondria and therefore cannot perform pyruvate oxidation.

Net energetics

For each pyruvate: pyruvate + CoA-SH + NAD+ → acetyl-CoA + CO2 + NADH + H+. The reaction yields 1 NADH per pyruvate (therefore 2 NADH per glucose-derived pair of pyruvate), consumes no ATP directly, and produces no ATP, NADPH, FADH2, or GTP directly. Oxidation of the NADH by the respiratory chain yields approximately 2.5 ATP under typical conditions.

Clinical significance

PDC commits carbohydrate-derived carbon to oxidative metabolism or fatty-acid synthesis and is inhibited when mitochondrial energy charge and acetyl-CoA/NADH are high. Thiamine deficiency impairs E1 activity and can contribute to lactic acidosis and neurologic dysfunction, particularly in alcoholism or severe malnutrition. Inherited PDC deficiency, often involving the X-linked PDHA1 gene, produces lactic acidosis, developmental delay, and neurologic disease because pyruvate is diverted to lactate and alanine.