DIRECT MEASUREMENT OF INTRAMOLECULAR ELECTRON-TRANSFER BETWEEN TYPE-I AND TYPE-III COPPER CENTERS IN THE MULTI-COPPER ENZYME ASCORBATE OXIDASE AND ITS TYPE-II COPPER-DEPLETED AND CYANIDE-INHIBITED FORMS

Transient kinetics of reduction of zucchini squash ascorbate oxidase (AO) by lumiflavin semiguinone have been studied by using laser flash photolysis. Second-order kinetics were obtained for reduction of the type I copper with a rate constant of 2.7 x 10(7) M-1 s-1, which is comparable to that obtained with other blue copper proteins such as plastocyanin. Following reduction, the type I copper was reoxidized in a protein concentration independent (i.e., intramolecular) reaction (k(obs) = 160 s-1). Comparison with literature values for limiting rate constants in transient single-turnover kinetic experiments suggests that intramolecular electron transfer probably is the rate-limiting step in enzyme catalysis. The extent of reoxidation of type I copper was approximately 55%, which is consistent with the approximately equal redox potentials of the type I and type III copper centers. Neither azide nor fluoride caused any significant changes in kinetics, although they are enzyme inhibitors and are thought to bind to the type II copper. In contrast, cyanide caused a concentration-dependent decrease in the extent of intramolecular electron transfer (with no change in rate constant), and decreased the rate constant for reduction of the type I copper by a factor of 2. The apparent dissociation constant for cyanide (0.2-0.4 mM) is similar to that reported for inhibition of enzyme activity. Removal of the type II copper from AO only marginally affected the kinetics of electron transfer to type I copper (k = 3.2 x 10(7) M-1 s-1) and slightly increased the extent but di not alter the rate constant of intramolecular electron transfer. This provides a direct confirmation that type III copper is the immediate electron acceptor from type I copper. Cyanide also inhibits intramolecular electron transfer in type II copper-depleted protein just as in the holoprotein, with a similar apparent dissociation constant. This suggests that cyanide binds to the type III copper center rather than to type II copper.