Oxidation states, molybdenum center sulfite oxidase

Recently, Mo K-edge EXAFS data have been obtained at low pH/high Cl and high pH/low Cl for each of the three oxidation states of the molybdenum center of sulfite oxidase (69) by poising the potential with redox dyes (78). This latter EXAFS study provides strong evidence that one chloride ion binds to the Mo(IV) and Mo(V) states of the enzyme at low pH/high Cl", but that Cl" does not appear to bind to the Mo(VI) state of the enzyme. Combination of the EXAFS and EPR data for sulfite oxidase yields the minimal structures for the molybdenum center shown in Fig. 6. 

Physical studies on oxidized and reduced enzymes show that all of the enzymes studied to date possess an oxo-molybdenum center that cycles between the Mo(VI) and the Mo(IV) states during catalysis and that the Mo(V) state can be detected as a transient species by EPR spectroscopy. Scheme 3 shows a simple cycle of reactions that describes the oxidation (or in reverse, reduction) of a substrate at an oxo-molybdenum center, such as that present in sulfite oxidase. 

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