EXAFS INVESTIGATIONS OF THE NI SITE IN THIOCAPSA-ROSEOPERSICINA HYDROGENASE - EVIDENCE FOR A NOVEL NI,FE,S CLUSTER

The results of the analysis of Ni K-edge EXAFS data from the Ni site in Thiocapsa roseopersicina hydrogenase poised in form C are presented. Form C represents a single, reduced, active form of the enzyme. The results obtained from the analysis are consistent with a Ni ligand environment composed of 3 +/- 1 N,O donors at 2.06 angstrom and 2 +/- 1 S donors at 2.21 angstrom. These results are compared with structural data from EXAFS and crystallographic studies of a number of Ni complexes with mixed ligand environments and with the results of EXAFS analyses of the Ni sites of other hydrogenases. The latter comparisons reveal a strong similarity between the data from T. roseopersicina and the data from the Fe,Ni,Se enzyme from Desulfovibrio baculatus. Data from scattering atoms in the second and third coordination spheres indicate that the Ni center in the enzyme is near to, but not part of, an Fe,S cluster. The data are consistent with Ni-Fe distances of 4.3 and 6.2 angstrom. Although scattering from S atoms at distances greater than 4 angstrom makes a negligible contribution to the overall EXAFS spectrum, the fits of Fourier-filtered second coordination sphere EXAFS are improved by the addition of S-scattering atoms at 4.2 angstrom. These data indicate that Ni-containing hydrogenases contain a novel Fe,S,Ni cluster.