The EPR characterization of the molybdenum(V) forms obtained on formate reduction of both as-prepared and inhibited formate dehydrogenase from Desulfovibrio desulfuricans ATCC 27774, an enzyme that catalyzes the oxidation of formate to CO2, is reported. The Mo(V) EPR signal of the as-prepared formate-reduced enzyme is rhombic (g(max) = 2.012, g(mid) = 1.996, 9,g(min) = 1.985) and shows hyperfine coupling with two nuclear species with I = 1/2. One of them gives an anisotropic splitting and is not solvent exchangeable (A(max) = 11.7, A(mid) = A(min) = non-detectable, A-values in cm(-1) x 10(-4)). The second species is exchangeable with solvent and produces a splitting at the three principal g-values (A(max) = 7.7, A(mid) = 10.0, A(min) = 9.3). The hyperfine couplings of the non-solvent and solvent exchangeable nuclei are assigned to the hydrogen atoms of the P-methylene carbon of a selenocysteine and to a Mo ligand whose nature, sulfydryl or hydroxyl, is still in debate. The Mo(V) species obtained in the presence of inhibitors (azide or cyanide) yields a nearly axial EPR signal showing only one detectable splitting given by nuclear species with I = 1/2 (g(max) = 2.092, g(mid) = 2.000, g(min) = 1.989, A(max) = non-detectable, A(mid) = A(min) = 7.0), which is originated from the alpha-proton donated by the formate to a proximal ligand of the molybdenum. The possible structures of both paramagnetic molybdenum species (observed upon formate reduction in presence and absence of inhibitors) are discussed in comparison with the available structural information of this enzyme and the structural and EPR properties of the closely related formate dehydrogenase-H from Escherichia coli. (C) 2007 Elsevier Inc. All rights reserved.