Neutron diffraction studies of [NiFe]-hydrogenase from Desulfovibrio vulgaris Miyazaki

[NiFe]-hydrogenase contains several metal centers, including the bimetallic Ni-Fe active site, iron-sulfur clusters and an Mg2+ center. X-ray structure analysis of the [NiFe]-hydrogenase from Desulfovibrio vulgaris Miyazaki F elucidated that Ni is coordinated by four sulfur atoms of cysteinyl residues, and two of them coordinate Fe making a bridge between Ni and Fe. The third bridging ligand between Ni and Fe presents depending on the oxidation states. In the oxidized (inactive) form, the third bridging ligand is assigned as an oxygen species from X-ray structural analysis. In the reduced (active) form, single crystal EPR analyses showed that a hydride binds between two metal atoms. Recently, a hydride between Ni and Fe has been reported by subatomic resolution X-ray structure analysis. However, the assignment of a hydride by x-ray crystallography is still controversial, and proton transfer pathways in the enzyme are still unclear.We aim to collect neutron diffraction data of the [NiFe] hydrogenase in both oxidized and reduced forms to obtain precise information of the bridging ligand species between Ni and Fe in the reduced form and the proton transfer pathways. We have already succeeded in preparation of large crystals (>1 mm3) in both forms from D2O buffer solutions in order to reduce the background noise during neutron diffraction experiments. Neutron diffraction experiments were carried out under cryogenic condition in order to preserve the reduced form of the enzyme during the data collection using iBIX diffractometer at MLF/J-PARC and MaNDi diffractometer at SNS/ORNL. For experiments at 100K, crystals were mounted on cryo-loops, which was optimized for large crystals, and then frozen under the cold N2 gas stream. We could observe diffraction spots up to 2.0 Å resolution from a crystal in the oxidized form.12th International Hydrogenase Conferenc