Structural Features of the Unready Ni-A State of [NiFe] Hydrogenase Revealed by X-Ray Crystallography and EPR Spectroscopy

The oxidized unready Ni-A state [NiFe] hydrogenases has been the subject of recent investigations by spectroscopic and crystallographic techniques. In the Ni-A state, the enzyme is inhibited and cannot turn over molecular hydrogen. Ideally, if the Ni-A state can be avoided, and if the oxidized enzyme exclusively resides in the ready Ni-B state, it can be quickly activated, thus increasing its activity of hydrogen production. In this study, recent observations by electron paramagnetic resonance spectroscopy as well as X-ray crystallography are combined and interpreted. Though it remains a mystery as to which differences exist on the geometric and electronic level between the Ni-A and Ni-B states, and in particular about the exact structure of the active site in the Ni-A state, the recent studies when combined do provide important hints.