ELECTROCHEMICAL STUDY OF THE RATE OF ACTIVATION OF THE MOLYBDOHEME PROTEIN SULFITE OXIDASE BY ORGANIC ELECTRON-ACCEPTORS

The rates of electron transfer between the molybdoheme protein sulfite oxidase and electrochemically generated organic mediator-oxidants have been investigated under sulfite-saturated conditions. The organic mediators are tetramethylphenylenediamine (TMPD), the indo dye 2,6-dichloroindophenol (DCIP), and the phenothiazine methylene blue (MB). Analysis of the mediator and enzyme concentration dependencies of electrocatalytic currents generated by reaction of the enzyme with oxidized TMPD, MB, and DCIP yields values of k12, the electron-transfer rate constant for reaction of mediator-oxidant with an enzyme heme site, and k(in), the rate constant for intraenzyme electron transfer between the enzyme Mo and heme sites. k12 depends on the choice of mediator-oxidant whereas k(in) does not.