Reversible Active Site Sulfoxygenation Can Explain the Oxygen Tolerance of a NAD+-Reducing [NiFe] Hydrogenase and Its Unusual Infrared Spectroscopic Properties

Oxygen-tolerant [NiFe] hydrogenases are metalloenzymes that represent valuable model systems for sustainable H-2 oxidation and production. The soluble NAD(+) reducing [NiFe] hydrogenase (SH) from Ralstonia eutropha couples the reversible cleavage of H-2 with the reduction of NAD(+) and displays a unique O-2 tolerance. Here we performed IR spectroscopic investigations on purified SH in various redox states in combination with density functional theory to provide structural insights into the catalytic [NiFe] center. These studies revealed a standard-like coordination of the active site with diatomic CO and cyanide ligands. The long-lasting discrepancy between spectroscopic data obtained in vitro and in vivo could be solved on the basis of reversible cysteine oxygenation in the fully oxidized state of the [NiFe] site. The data are consistent with a model in which the SH detoxifies O-2 catalytically by means of an NADH-dependent (per)oxidase reaction involving the intermediary formation of stable cysteine sulfenates. The occurrence of two catalytic activities, hydrogen conversion and oxygen reduction, at the same cofactor may inspire the design of novel biomimetic catalysts performing H-2-conversion even in the presence of O-2.