The crystal structure of the xanthine oxidase-related molybdenum-iron protein aldehyde oxidoreductase from the sulfate reducing anaerobic Gram- negative bacterium Desulfovibrio gigas (Mop) was analyzed in its desulfo-, sulfo-, oxidized, reduced, and alcohol-bound forms at 1.8-Å resolution. In the sulfo-form the molybdenum molybdopterin cytosine dinucleotide cofactor has a dithiolenebound fac-[Mo, =O, =S, ···(OH...