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Abstract

The three known types of hydrogenases catalyze reversible activation of H₂ and heterolytic
cleavage to a hydride and a proton . [NiFe]- and [FeFe]-hydrogenases further
cleave the hydride to two electrons and a proton. [Fe]-hydrogenase (Hmd), the enzyme covered in this chapter, instead transfers the hydride to a substrate, methenyltetrahydromethanopterin
(methenyl-H₄MPT⁺ ) (Figure 6.1) [1, 2]. The free-energy change of this reaction is – 5.5 kJ/mol. The reverse reaction, formation of H₂ and methenyl-
H₄MPT⁺, is favorable under acidic conditions [3, 4].
[Fe]-hydrogenase is involved in the methanogenic pathway of many hydrogenotrophic
methanogenic archaea , including members of the Methanopyrales, Methanobacteriales,
and Methanococcales. Under nickel-limiting conditions , [Fe]-hydrogenase is
overproduced and functions as the major hydrogenase enzyme in the methanogenic
pathway, in which it provides four electrons [5]. [Fe]-hydrogenase contains a mononuclear
iron-guanylyl pyridinol (FeGP) cofactor. In the cofactor, the low-spin Fe(II) is
coordinated with two cis-CO and fixed to the organic part, guanylyl pyridinol, with
its acyl-carbon and pyridinol-nitrogen; the iron site is covalently bound to a cysteine
residue [1].