Structure, function and regulation of pyruvate carboxylase

Pyruvate carboxylase (PC; EC 6.4.1.1), a member of the biotin-dependent enz yme family, catalyses the ATP-dependent carboxylation of pyruvate to oxaloa cetate. PC has been found in a wide variety of prokaryotes and eukaryotes. In mammals, PC plays a crucial role in gluconeogenesis and lipogenesis, in the biosynthesis of neurotransmitter substances, and in glucose-induced ins ulin secretion by pancreatic islets. The reaction catalysed by PC and the p hysical properties of the enzyme have been studied extensively. Although no high-resolution three dimensional structure has yet been determined by X-r ay crystallography, structural studies of PC have been conducted by electro n microscopy, by limited proteolysis, and by cloning and sequencing of gene s and cDNA encoding the enzyme. Most well characterized forms of active PC consist of four identical subunits arranged in a tetrahedron-like structure . Each subunit contains three functional domains: the biotin carboxylation domain, the transcarboxylation domain and the biotin carboxyl carrier domai n. Different physiological conditions, including diabetes, hyperthyroidism, genetic obesity and postnatal development, increase the level of PC expres sion through transcriptional and translational mechanisms, whereas insulin inhibits PC expression. Glucocorticoids, glucagon and catecholamines cause an increase in PC activity or in the rate of pyruvate carboxylation in the short term. Molecular defects of PC in humans have recently been associated with four point mutations within the structural region of the PC gene, nam ely Val(345) --> Ala, Arg(451) --> CYs, Ala(610) --> Thr and Met(743) --> T hr.