Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below:
H2 + Aox → 2H+ + Ared
()
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, 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 H2metalloenzymes 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).
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