Single crystal EPR studies of the oxidized active site of [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F

The Ni-A and the Ni-B forms of the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F have been studied in single crystals by continuous wave and pulsed EPR spectroscopy at different temperatures (280 K, 80 K, and 10 K). For the first time, the orientation of the g-tensor axes with respect to the recently published atomic structure of the active site at 1.8 Angstrom resolution was elucidated for Ni-A and Ni-B. The determined g-tensors have a similar orientation. The configuration of the electronic ground state is proposed to be Ni(III) 3d(z2)(1) for Ni-A and Ni-B. The g(z) principal axis is close to the Ni-S(Cys549) direction: the g(x) and the g(y) axes are approximately along the Ni-S(Cys546) and Ni-S(Cys81) bonds, respectively. It is proposed that the difference between the NI-A and Ni-B states lies in a protonation of the bridging ligand between the Ni and the Fe.