Hydrogenase

A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below:H2 + Aox → 2H+ + Ared (1)2H+ + Dred → H2 + Dox (2)H2 ⇌ 2 H+ + 2 e− (3)H2 ⇌ H+ + H− (4) A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below: Hydrogen uptake (1) is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide (CO2), 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 c3, and cytochrome c6 can act as physiological electron donors or acceptors for hydrogenases. 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 3), the hydrogenases catalyze the reversible heterolytic cleavage of H2 shown by reaction (4). 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. The hydrogenases are heterodimeric proteins consisting of small (S) and large (L) subunits. The small subunit contains three iron-sulfur clusters while the large subunit contains the active site, a nickel-iron centre which is connected to the solvent by a molecular tunnel. In some hydrogenases, one of the Ni-bound cysteine residues is replaced by selenocysteine. On the basis of sequence similarity, however, the and hydrogenases should be considered a single superfamily.To date, periplasmic, cytoplasmic, and cytoplasmic membrane-bound hydrogenases have been found. The hydrogenases, when isolated, are found to catalyse both H2 evolution and uptake, with low-potential multihaem cytochromes such as cytochrome c3 acting as either electron donors or acceptors, depending on their oxidation state. Generally speaking, however, hydrogenases are more active in oxidizing H2. A wide spectrum of H2 affinities have also been observed in H2-oxidizing hydrogenases. Like hydrogenases, hydrogenases are known to be usually deactivated by molecular oxygen (O2). Hydrogenase from Ralstonia eutropha, and several other so-called Knallgas-bacteria, were found to be oxygen-tolerant. The soluble hydrogenase from Ralstonia eutropha H16 be conveniently produced on heterotrophic growth media. This finding increased hope that hydrogenases can be used in photosynthetic production of molecular hydrogen via splitting water. The hydrogenases containing a di-iron center with a bridging dithiolate cofactor are called hydrogenases. Three families of hydrogenases are recognized: In contrast to hydrogenases, hydrogenases are generally more active in production of molecular hydrogen. Turnover frequency (TOF) in the order of 10,000 s−1 have been reported in literature for hydrogenases from Clostridium pasteurianum. This has led to intense research focusing on use of hydrogenase for sustainable production of H2.