Three dimensional crystal structures of the oxidized (as isolated) and H2-reduced forms of [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F were determined. The active site of the enzyme in the oxidized form is a hetero binuclear Ni-Fe complex with four cysteinyl ligands and unusual four non-protein ligands (SO, CO, CN, S) . Among non-protein ligands, the monatomic S ligand, which makes a bridge between the Ni and Fe atoms has been confirmed to be liberated as a H2S upon reduction with H2. The possible mechanism of the activation process of the active site is discussed.
