a polyenzymatic complex that catalyzes the oxidative decarboxylation of pyruvic acid:

This reaction is the main pathway for the conversion of alpha keto acids in the tissues of animals, plants, and aerobic microorganisms; it links the two most important metabolic processes— glycolysis and the reactions of the tricarboxylic acid cycle. The vitamin derivatives thiamine pyrophosphate (TPP), coenzyme A (CoA), flavine-adenine dinucleotide (FAD), nicotinamide-adenine dinucleotide (NAD), lipoic acid, and Mg²⁺ ions all take part in the catalytic activity of pyruvate dehydrogenase. Magnesium ions are also necessary cofactors of the enzymes pyruvate decarboxylase, lipoyl transacetylase, and lipoyl dehydrogenase. The enzymes are present in specific quantitative relationships and fix the structure of the enzyme complex, which does not dissociate under normal conditions.

Pyruvate dehydrogenase that is extracted from animal organs and bacteria is a large protein aggregate with a molecular weight that ranges from 4,800,000 to 10,000,000 depending on its source. The complex ranges in size from 300 to 400 angstroms and is regulated by a variety of factors. In living organisms, pyruvate dehydrogenase is inhibited by phosphorylation of the decarboxylase component and reactivated upon splitting of the phosphate radical; the activity is also dependent on the energetic and hormonal state of the organism.