Pyruvate carboxylase [EC 6.4.1.1] is a member of the family of biotin-dependent carboxylases and is found widely among eukaryotic tissues and in many prokaryotic species. It catalyses the ATP-dependent carboxylation of pyruvate to form oxaloacetate which may be utilised in the synthesis of glucose, fat, some amino acids or their derivatives and several neurotransmitters. Diabetes and hyperthyroidism increase the level of expression of pyruvate carboxylase in the long term, while its activity in the short term is controlled by the intramitochondrial concentrations of acetyl-CoA and pyruvate. Many details of this enzyme's regulation are yet to be described in molecular terms. However, progress towards this goal and towards understanding the relationship of pyruvate carboxylase structure to its catalytic reaction mechanism, has been enormously enhanced recently by the cloning and sequencing of genes and cDNAs encoding the approximately 130 kDa subunit of this homotetramer. Defects in the expression or biotinylation of pyruvate carboxylase in humans almost invariably results in early death or at best a severely debilitating psychomotor retardation, clearly reflecting the vital role it plays in intermediary metabolism in many tissues including the brain.