Incorporation of molybdenum in rubredoxin: models for mononuclear molybdenum enzymes.

Molybdenum is found in the active site of enzymes usually coordinated by one or two pyranopterin molecules. Here, we mimic an enzyme with a mononuclear molybdenum-bis pyranopterin center by incorporating molybdenum in rubredoxin. In the molybdenum-substituted rubredoxin, the metal ion is coordinated by four sulfurs from conserved cysteine residues of the apo-rubredoxin and two other exogenous ligands, oxygen and thiol, forming a Mo((VI))-(S-Cys)4(O)(X) complex, where X represents -OH or -SR. The rubredoxin molybdenum center is stabilized in a Mo(VI) oxidation state, but can be reduced to Mo(IV) via Mo(V) by dithionite, being a suitable model for the spectroscopic properties of resting and reduced forms of molybdenum-bis pyranopterin-containing enzymes. Preliminary experiments indicate that the molybdenum site built in rubredoxin can promote oxo transfer reactions, as exemplified with the oxidation of arsenite to arsenate.