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A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below: : (1) H2 + Aox → 2H+ + Ared : (2) 2H+ + Dred → H2 + Dox Hydrogen uptake (1) is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide, and fumarate. On the other hand, proton reduction (2) is coupled to the oxidation of electron donors such as ferredoxin (FNR), and serves to dispose excess electrons in cells (essential in pyruvate fermentation). Both low-molecular weight compounds and proteins such as FNRs, cytochrome ''c''3, and cytochrome ''c''6 can act as physiological electron donors or acceptors for hydrogenases. == Structural classification == It has been estimated that 99% of all organisms utilize dihydrogen, H2. Most of these species are microbes and their ability to use H2 as a metabolite arises from the expression of H2 metalloenzymes known as hydrogenases.〔 〕 Hydrogenases are sub-classified into three different types based on the active site metal content: iron-iron hydrogenase, nickel-iron hydrogenase, and iron hydrogenase. All hydrogenases catalyze reversible H2 uptake, but while the () and () hydrogenases are true redox catalysts, driving H2 oxidation and proton (H+) reduction (equation 1), the () hydrogenases catalyze the reversible heterolytic cleavage of H2 shown by reaction (2). :H2 2 H+ + 2 e- (1) :H2 H+ + H- (2) Until 2004, the ()-only hydrogenase was believed to be "metal-free". Then, Thauer ''et al.'' showed that the metal-free hydrogenases in fact contain iron atom in its active site. As a result, those enzymes previously classified as "metal-free" are now named ()-only hydrogenases. This protein contains only a mononuclear Fe active site and no iron-sulfur clusters, in contrast to the () hydrogenases. () and () hydrogenases have some common features in their structures: Each enzyme has an active site and a few Fe-S clusters that are buried in protein. The active site, which is believed to be the place where catalysis takes place, is also a metallocluster, and each metal is coordinated by carbon monoxide (CO) and cyanide (CN−) ligands. 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「Hydrogenase」の詳細全文を読む スポンサード リンク
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