Activation and inactivation of hydrogenase function and the catalytic cycle:: Spectroelectrochemical studies

Different redox states of the hydrogenase active sites of the catalysts Ni-Fe, Fe-Fe, and Fe-S cluster-free hydrogenases, their participation in the activation and inactivation processes of these enzymes and in the catalytic cycle with emphasis on the spectroelectrochemical studies were reviewed. The inactivation refers to the loss of activity, either reversible or irreversible, induced by extrinsic compounds. The catalysts were treated separately although they could be united on the basis of mechanism of reaction and common organometallic chemistry. The common feature that is responsible for hydrogenase activity is the conserve low-spin Fe coordinated to CN and CO. The existence of two different types of hydrogenases is the result of two enzymes of different origins have been selected to carry out the same reaction. Strategies have been adopted to operate in the presence of oxygen: exclusion of O2 from the active site by restricting the gas channels, or having a catalytic center partially blocked by additional ligands. Under anaerobic environment, these mechanisms protected the active sites from Co while in aerobic conditions, hydrogenases are converted to a dormant state but are arranged that it can reactivated under reducing conditions.