Pyruvate carboxylase <p> In the postabsorptive state, blood glucose concentration is kept constant by a combination of glycogenolysis and gluconeogenesis. During prolonged fasting, when hepatic glycogen is exhausted, gluconeogenesis becomes the only source of plasma glucose. Gluconeogenesis (the production of new glucose molecules) occurs mainly in liver and, to a small extent, in kidneys.</p> <p> Pyruvate carboxylase (<db_xref db="EC" dbkey="6.4.1.1"/>), a member of the biotin-dependent enzyme family, catalyses the ATP-dependent carboxylation of pyruvate to oxaloacetate, thus playing a crucial role in gluconeogenesis. <reaction> ATP + pyruvate + HCO3- = ADP + phosphate + oxaloacetate </reaction>Most well characterised forms of active enzyme 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 domain.</p>