Molecular Biology of Microbial Hydrogenases

Hydrogenases (H₂ases) are metalloproteins. The great majority of them contain iron-sulfur clusters and two metal atoms at their active center, either a Ni and an Fe atom, the [NiFe]-H₂ases, or two Fe atoms, the [FeFe]-H₂ases. Enzymes of these two classes catalyze the reversible oxidation of hydrogen gas (H₂ <--> 2 H⁺ + 2 e^-) and play a central role in microbial energy metabolism; in addition to their role in fermentation and H₂ respiration, H₂ases may interact with membrane-bound electron transport systems in order to maintain redox poise, particularly in some photosynthetic microorganisms such as cyanobacteria. Recent work has revealed that some H₂ases, by acting as H₂-sensors, participate in the regulation of gene expression and that H₂-evolving H₂ases, thought to be involved in purely fermentative processes, play a role in membrane-linked energy conservation through the generation of a protonmotive force. The Hmd hydrogenases of some methanogenic archaea constitute a third class of H₂ases, characterized by the absence of Fe-S cluster and the presence of an iron-containing cofactor with catalytic properties different from those of [NiFe]- and [FeFe]-H₂ases. In this review, we emphasise recent advances that have greatly increased our knowledge of microbial H₂ases, their diversity, the structure of their active site, how the metallocenters are synthesized and assembled, how they function, how the synthesis of these enzymes is controlled by external signals, and their potential use in biological H₂ production.