OXIDATIVE TURNOVER INCREASES THE RATE-CONSTANT AND EXTENT OF INTRAMOLECULAR ELECTRON-TRANSFER IN THE MULTICOPPER ENZYMES, ASCORBATE OXIDASE AND LACCASE

Using laser flash photolysis of lumiflavin/EDTA solutions containing ascorbate oxidase, we find that the rate constant for intramolecular electron transfer varies from one enzyme preparation to another and is generally a more sensitive measure of the state of the active site than are steady-state assays. Thus, type I copper is initially reduced in a second-order reaction followed by first-order reoxidation by the type II-III trinuclear copper center. The observed rate constant for this intramolecular process in presumably native enzyme is 160 s(-1) at pH 7, whereas an enzyme preparation which had less than 20% activity had a rate constant of 2.6 s(-1). Other samples of relatively active enzyme showed biphasic intramolecular kinetics intermediate between the above values. The inactive enzyme sample could be reactivated by dialysis against ascorbate or by treatment with ferricyanide, resulting in a corresponding increase in the intramolecular rate constant for type I copper reoxidation to a value comparable to that of native enzyme. Using this same methodology, we have determined that the type I copper in Japanese lacquer tree laccase is reoxidized by the type II-III trinuclear copper center in a first-order (intramolecular) process with rate constants of 1 s(-1) at pH 7.0 and 4.9 s(-1) at pH 6.0, values which are approximately two orders of magnitude smaller than for ascorbate oxidase. The intramolecular rate constant and enzyme activity for laccase also increased, but only by a factor of 2-6, when the enzyme was treated with ascorbate or ferricyanide, respectively. We further found that intramolecular electron transfer in laccase was completely inhibited by fluoride ion, in contrast to ascorbate oxidase which is unaffected by this ion. These differences in behavior for these two very similar enzymes are rather remarkable, when it is considered that the distance between copper atoms is constrained by the location of the protein-derived copper ligands in the three-dimensional structure, and that the redox potentials of the enzymes are similar. Our results may be interpreted in terms of an interconversion between active and inactive enzyme in which there is a rearrangement of the type II-III trinuclear copper center, resulting in a lowering of the redox potential and a block in electron transfer. Turnover restores the active enzyme conformation. The large difference between steady-state measurements of k(cat) and the observed rate constant for intramolecular electron transfer suggests that oxygen binding to the type II-III trinuclear copper center in both enzymes may dramatically increase the rate of intramolecular electron transfer.