Sulfoxygenation of Active Site Models of [NiFe] and [FeFe] Hydrogenases - A Commentary on Possible Chemical Models of Hydrogenase Enzyme Oxygen Sensitivity

The organometallic active sites in [NiFe]- and [FeFe]H(2)ases are sensitive to oxygen in varying degrees. The microorganisms that utilize these enzymes for their hydrogen metabolism, and the enzymes themselves, have evolved from a reducing to an oxidizing environment in ways to avoid competition with oxygen, primarily by burying the active site machinery deeply within the protein matrix. In the case of [NiFe]H(2)ase, biological studies indicate that repair mechanisms exist for reversible O-2-inhibition processes. This Microreview explores the possibility that S-oxygenation may represent reparable O-damaged enzyme active sites. Such S-oxygenation has precedent in chemical models for the terminal thiolate sulfur atoms of the nickel site in [NiFe]H(2)ase as well as the bridging thiolate sulfur in the [FeFe]H(2)ase active site. A discussion of the processes of O-2 damage leading to both reversible and irreversible enzyme inhibition, and reclamation of activity in the H(2)ases, as explored by various biochemical assays and spectroscopic methods, is also presented.