Biohydrogenases

Biohydrogenases are enzymes that catalyze the reversible oxidation and production of molecular hydrogen (H2), linking hydrogen metabolism to cellular electron transfer and energy conservation. They are widespread in bacteria and archaea and play key roles in hydrogen cycling, fermentation, respiration, and, in some phototrophic organisms, in light-driven hydrogen production.

The two largest families are [NiFe]-hydrogenases and [FeFe]-hydrogenases, with a smaller group of [Fe]-hydrogenases reported in a few organisms. [NiFe]-hydrogenases feature an active site with a nickel-iron center coordinated by cysteine ligands and diatomic ligands such as CO and CN−. [FeFe]-hydrogenases contain a highly reactive diiron center within the H-cluster, connected to a [4Fe-4S] subcluster, and require a dedicated set of maturation proteins for assembly. Both families catalyze the two-way reaction: H2 can be oxidized to feed electrons into electron carriers (such as ferredoxin or NAD(P)H), or protons can be reduced to form H2 during fermentation or photosynthetic processes.

Localization within cells varies; hydrogenases may be cytoplasmic, periplasmic, or membrane-associated, and their activity is closely linked to the organism’s metabolic state. Most hydrogenases are sensitive to oxygen, but several oxygen-tolerant [NiFe]-hydrogenases have been identified and are studied for potential applications.

Ecologically, biohydrogenases support anaerobic energy metabolism, hydrogen uptake in respiration, and, in some photosynthetic microbes, light-driven hydrogen production. They are of interest for renewable-energy applications, including biohydrogen production and enzymatic fuel cells, though challenges such as oxygen sensitivity, maturation requirements, and stability need to be addressed for practical use.