page_title: Hydrogenase
section_title: [NiFe] hydrogenase
hierarchical_section_title: Hydrogenase / Structural classification / [NiFe] hydrogenase
caption_attribution_description: English: Structure of [NiFe] hydrogenase
context_page_description: A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen, as shown below:
  Hydrogen uptake is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide, and fumarate. On the other hand, proton reduction is coupled to the oxidation of electron donors such as ferredoxin, and serves to dispose excess electrons in cells. 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.
context_section_description: The [NiFe] 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 [NiFe] hydrogenases, one of the Ni-bound cysteine residues is replaced by selenocysteine. On the basis of sequence similarity, however, the [NiFe] and [NiFeSe] hydrogenases should be considered a single superfamily.
  To date, periplasmic, cytoplasmic, and cytoplasmic membrane-bound hydrogenases have been found. The [NiFe] 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, [NiFe] hydrogenases are more active in oxidizing H₂. A wide spectrum of H₂ affinities have also been observed in H₂-oxidizing hydrogenases.
  Like [FeFe] hydrogenases, [NiFe] 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 [NiFe] 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.
caption: Crystal structure of [NiFe] hydrogenase
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