A STRUCTURE-BASED CATALYTIC MECHANISM FOR THE XANTHINE-OXIDASE FAMILYOF MOLYBDENUM ENZYMES

The crystal structure of the xanthine oxidase-related molybdenum-iron protein aldehyde oxido-reductase from the sulfate reducing anaerobic G ram-negative bacterium Desulfovibrio gigas (Mop) was analyzed in its d esulfo-, sulfo-, oxidized, reduced, and alcohol-bound forms at 1.8-Ang strom resolution. In the sulfo-form the molybdenum molybdopterin cytos ine dinucleotide cofactor has a dithiolene-bound fac-[Mo, =O, =S, ---( OH2)] substructure, Bound inhibitory isopropanol in the inner compartm ent of the substrate binding tunnel is a model for the Michaelis compl ex of the reaction with aldehydes (D-C=O, -R). The reaction is propose d to proceed by transfer of the molybdenum-bound water molecule as OH- after proton transfer to Glu-869 to the carbonyl carbon of the substr ate in concert with hydride transfer to the sulfide group to generate [MolV,=O, -SH, ---(O-C=O, -R)), Dissociation of the carboxylic acid pr oduct may be facilitated by transient binding of Glu-869 to the molybd enum, The metal-bound water is replenished from a chain of internal wa ter molecules. A second alcohol binding site in the spacious outer com partment may cause the strong substrate inhibition observed. This comp artment is the putative binding site of large inhibitors of xanthine o xidase.