Identification of a putative histidine base and of a non-protein nitrogen ligand in the active site of Fe-hydrogenases by one-dimensional and two-dimensional electron spin-echo envelope-modulation spectroscopy.

The active H-cluster of the Fe-hydrogenases from Megasphaera elsdenii and Desulfovibrio vulgaris (strain Hildenborough) has been investigated with one- and two-dimensional pulsed EPR spectroscopy. In both complexes the coordination of a nitrogen-containing ligand was found. The unusual quadrupole interaction parameters (D. vulgaris: quadrupole coupling constant, K = 1.20 MHz, asymmetry parameter eta = 0.32, M. elsdenii: K = 1.23 MHz, eta = 0.25) indicate a non-protein type of nitrogen and are consistent with cyanide as ligand to the H-cluster. The additional interactions measured on the EPR signal of the inactivated H-cluster in D. vulgaris hydrogenase are consistent with an imidazole interaction similar to that found in Rieske-type iron-sulfur clusters. Since a His residue near the putative H-cluster binding motif of Cys residues, His371, is the only conserved His in Fe-hydrogenases, it is a likely candidate for the base that accepts the proton in the heterolytic cleavage of molecular hydrogen. The inactivation of the enzyme is accompanied by direct binding of the imidazole ring to the H-cluster.