page_title: Hydrogenase
section_title: Structural classification
hierarchical_section_title: Hydrogenase / Structural classification
caption_attribution_description: English: Corrected picture of active sites of hydrogenases
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: 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 [FeFe] and [NiFe] hydrogenases are true redox catalysts, driving H₂ oxidation and proton (H⁺) reduction (equation 3), the [Fe] hydrogenases catalyze the reversible heterolytic cleavage of H₂ shown by reaction (4).
  Until 2004, the [Fe]-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 [Fe]-only hydrogenases. This protein contains only a mononuclear Fe active site and no iron-sulfur clusters, in contrast to the [FeFe] hydrogenases. [NiFe] and [FeFe] 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.
caption: The structures of the active sites of the three types of hydrogenase enzymes.
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