Identification of the Molybdenum Cofactor of Dimethyl Sulfoxide Reductase fromRhodobacter sphaeroidesf. sp.denitrificansas Bis(molybdopterin guanine dinucleotide)molybdenum

Chemical analysis of dimethyl sulfoxide reductase fromRhodobacter sphaeroidesf. sp.denitrificanshas shown that its molybdenum center contains two molybdopterin guanine dinucleotide molecules and a single atom of molybdenum. The enzyme, which exists as a monomer of 86 kDa, was shown to contain 1 mol of molybdenum, 4 mol of organic phosphate, and 2 mol of guanine per mole of protein. In addition, the relative yield of Form A, a fluorescent derivative of molybdopterin, was twice that obtained from sulfite oxidase, a protein which contains a single molybdopterin per molybdenum. These findings correlate with the recent report of the presence of two molybdopterin ligands in the tungsten cofactor of aldehyde ferredoxin oxidoreductase fromPyrococcus furiosus,providing the first example of a bis(pterin)molybdenum cofactor and extending this structural motif to the molybdopterin dinucleotide enzymes.