Ni and Se x-ray absorption spectroscopic studies of the [NiFeSeihydrogenases from Desulfovibrio baculatus are described. The Ni site geometry is pseudo-octahedral with a coordinating ligand composition of 3-4 (N,O) at 2.06 A, 1-2 (S,CI) at 2.17 A, and 1 Se at 2.44 A. The Se coordination environment consists of 1 C at 2.0 A and a heavy scatterer M (M = Ni or Fe) at -2.4 A. These results are interpreted in terms of a selenocysteine residue coordinated to the Ni site. The possible role of the Ni-Se site in the catalytic activation of H2 is discussed.Hydrogenases are vital to the anaerobic metabolism of sulfate-reducing bacteria and many other types of bacteria. Hydrogenases catalyze the bidirectional activation of molecular hydrogen H2 = 2H+ + 2e-and their activities are routinely determined by H2 production, H2 utilization, or the 2H2-H+ exchange assays (1). In the past, it was generally accepted that a single hydrogenase carried out these simple redox reactions (1), but in recent years the unveiling of the structural diversity of hydrogenases has promoted the idea that different hydrogenases may reflect differences in cellular localization and metabolic function (2). This specificity suggests that structural differences are the basis for tailoring the hydrogenase reaction to meet metabolic demands. The elucidation of the structural details of the active sites of hydrogenases is the first step toward a molecular understanding of the mechanisms involved in the hydrogenase reaction.In the genus Desulfovibrio, the metabolism of hydrogen involves at least three types of hydrogenases that may be distinguished by their heavy element composition, immunological reactivities, and gene structures (3-6). The [Fe]hydrogenases contain only iron-sulfur clusters (7-9), the [NiFe] As part of our effort to elucidate the structures of the nickel sites in the Ni-containing hydrogenases, and to define a structural basis for the functional differences between the Seand the non-Se-containing hydrogenases (19, 20), we report here the results of Ni and Se x-ray absorption spectroscopic measurements on the [NiFeSe]hydrogenases ofD. baculatus.
MATERIALS AND METHODS Sample Preparation. The D. baculatus [NiFeSe]hydrogenasesample was prepared according to the procedure described in ref. 19. To obtain a sample -2 mM Ni and Se, the periplasmic, cytoplasmic, and membrane-bound fractions were combined. Total iron was determined by the 2,4,6-tripyridyl-1,3,5-triazine method (21), and metals were quantified by plasma emission spectroscopy using a Mark II Jarrell-Ash model 965 AtomComp (Fisher). Nickel was also determined by atomic absorption spectroscopy. The sample for x-ray absorption spectroscopy