Scheme 1.

Possible Reaction Pathways for the Catalytic Oxidation of Sulfite by SDH or SO^a

^aFor the sake of simplicity, the enzyme and substrate are depicted as Mo^VIO and SO₃^2-, respectively. The pathways differ in the sequence of steps which transform the enzyme-substrate complex, Mo^IV(OSO₃^2-), into product (sulfate) and the reoxidized Mo(VI) state of the enzyme. The pathway colored red is the one commonly proposed, in which product release precedes reoxidation of the enzyme. For the pathway colored black, the enzyme-substate complex is oxidized by two electrons prior to product release (34). See the text for additional discussion.