ELECTROCHEMICAL STUDY OF KINETICS OF ELECTRON-TRANSFER BETWEEN SYNTHETIC ELECTRON-ACCEPTORS AND REDUCED MOLYBDOHEME PROTEIN SULFITE OXIDASE

The electrocatalytic oxidation of sulfite using transition-metal complex or cytochrome c mediators and the molybdoheme protein sulfite oxidase (SO) as catalysts is used to measure the rate constant (k12) for cross electron transfer between mediator and enzyme. The enzyme heme is the reaction site for cobalt phenanthroline complex mediators; the Mo fragment obtained by tryptic cleavage of native enzyme is unable to reduce the Co species. Within a structurally similar set of cobalt complexes, when (mediator SO) DELTA-E-degrees' < 0.2 V, k12 varies with reaction free energy according to relative Marcus-Hush theory; we find k11 = 1 x 10(8) M-1 s-1 for the apparent electron self-exchange rate constant of the enzyme heme site. Attempts to achieve faster heme turnover rates result in a shift of rate control to another process, possibly the internal Mo --> Fe electron transfer. Ionic strength effects indicate a negative charge at the heme site and substantial electrostatic rate effects.