Hydrogenases (H2ase), which catalyze the reversible reduction of protons to hydrogen (H2),
are ancient enzymes that play a critical role in metabolism. 1–3 H2ases are categorized
according to the type of metal found in their active site, and there are several distinct types.
These include the [FeFe]-H2ases, the [NiFe]-H2ases (see also Chapter 4), and the [Fe]-only
H2ases.[Fe]-only H2ases are only found in a subset of methanogenic archaea, and their
catalysis relies on an additional substrate, methenyltetrahydromethanopterin. 4 The [Fe] …

[FeFe]-and [NiFe]-hydrogenases are two classes of enzymes that couple the reversible
reduction of protons to the production of hydrogen gas. While these enzymes have unique
taxonomic distributions and differences in their structure, function, maturation, and
evolutionary history, they nevertheless share important similarities and may both be of
critical importance for photobiological hydrogen production. Here, the structural and
functional diversity of both types of hydrogenases are discussed in light of their phylogenetic …