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Phosphoenolpyruvate carboxylase (also known as PEP carboxylase, PEPCase, or PEPC; (EC 4.1.1.31 ), PDB ID: 3ZGE) is an enzyme in the family of carboxy-lyases found in plants and some bacteria that catalyzes the addition of bicarbonate (HCO3−) to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate and inorganic phosphate: :PEP + HCO3− → oxaloacetate + Pi This reaction is used for carbon fixation in CAM (crassulacean acid metabolism) and C4 organisms, as well as to regulate flux through the citric acid cycle (also known as Krebs or TCA cycle) in bacteria and plants. The enzyme structure and its two step catalytic, irreversible mechanism have been well studied. PEP carboxylase is highly regulated, both by phosphorylation and allostery. ==Enzyme structure== The PEP carboxylase enzyme is present in plants and some types of bacteria, but not in fungi or animals (including humans). The genes vary between organisms, but are strictly conserved around the active and allosteric sites discussed in the mechanism and regulation sections. Tertiary structure of the enzyme is also conserved. The crystal structure of PEP carboxylase in multiple organisms, including ''Zea mays'' (maize), and ''Escherichia coli'' has been determined.〔 The overall enzyme exists as a dimer-of-dimers: two identical subunits closely interact to form a dimer through salt bridges between arginine (R438 - exact positions may vary depending on the origin of the gene) and glutamic acid (E433) residues. This dimer assembles (more loosely) with another of its kind to form the four subunit complex. The monomer subunits are mainly composed of alpha helices (65%),〔 and have a mass of 106kDa each. The sequence length is about 966 amino acids.〔http://www.pdb.org/pdb/explore/explore.do?structureId=3ZGE〕 See figure 1 for a PyMOL generated structure of the enzyme’s single subunit from the organism ''Flaveria trinervia''. The enzyme active site is not completely characterized. It includes a conserved aspartic acid (D564) and a glutamic acid (E566) residue that non-covalently bind a divalent metal cofactor ion through the carboxylic acid functional groups.〔 This metal ion can be magnesium, manganese or cobalt depending on the organism,〔〔 and its role is to coordinate the phosphoenolpyruvate molecule as well as the reaction intermediates. A histidine (H138) residue at the active site is believed to facilitate proton transfer during the catalytic mechanism.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Phosphoenolpyruvate carboxylase」の詳細全文を読む スポンサード リンク
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