The structure and the mechanism of action of pyruvate carboxylase.

Pyruvate carboxylase plays an important role in intermediary metabolism, catalysing the formation of oxaloacetate from pyruvate and HCO3-, with concomitant ATP cleavage. It thus provides oxaloacetate for gluconeogenesis and replenishing tricarboxylic acid cycle intermediates for fatty acid, amino acid and neurotransmitter synthesis. The enzyme is highly conserved and is found in a great variety of organisms including fungi, bacteria and plants as well as higher organisms. It is a member of a group of biotin-dependent enzymes and the biotin prosthetic group is covalently bound to the polypeptide chain of the enzyme, there normally being four such chains in the native, tetrameric enzyme. The overall reaction catalysed by pyruvate carboxylase involves two partial reactions that occur at spatially separate subsites within the active site, with the covalently bound biotin acting as a mobile carboxyl group carrier. In the first partial reaction, biotin is carboxylated using ATP and HCO3- as substrates whilst in the second partial reaction, the carboxyl group from carboxybiotin is transferred to pyruvate. The chemical mechanisms of the partial reactions and some of the roles played by amino acid residues of the enzyme in catalysing the reaction have been elucidated. The domain structure of the yeast enzyme has been deduced by comparing its amino acid sequence with those of enzymes that have similar catalytic functions. The quaternary structures of the pyruvate carboxylases studied so far, all involve a tetrahedron-like arrangement of the subunits. The major regulator of enzyme activity, acetyl CoA, stimulates the cleavage of ATP in the first partial reaction and in addition it has been shown to induce a conformational change in the tetrameric structure of the enzyme. In the past, the lack of any detailed structural information on the enzyme has hampered efforts to fully understand how this and other biotin-dependent enzymes function and are regulated. With the recent cloning of the enzyme from a variety of sources and the performance of three-dimensional structural studies, the next few years should see much progress in our understanding the mechanism of action of this enzyme.