CHAPTER 2: Structure-function of [FeFe]- and [NiFe]-Hydrogenases: an Overview of Diversity, Mechanism, Maturation, and Bifurcation

[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 and metabolic contexts. Overviews of the mechanisms of hydrogen activation at the respective active sites are given and provide insight into the role of the surrounding protein coordination sphere in catalysis and tuning reactivity. The distinct maturation pathways of both the [FeFe]- and [NiFe]-hydrogenases are described, revealing unique processes for assembling diatomic carbon monoxide and cyanide ligands onto Fe atoms, a key feature of the active sites that facilitates hydrogen activation during turnover conditions. Finally, a discussion of bifurcating [FeFe]- and [NiFe]-hydrogenases sheds light on this recently characterized group of enzymes that may hold a profound advantage at energetically efficient hydrogen production.