EPR spectra of type 3 copper centers in Rhus vernicifera laccase and Cucumis sativus ascorbate oxidase.

In order to reveal the detailed structure of the trinuclear site composed of type 2 copper and a pair of type 3 copper centers in multicopper oxidases, the action of inhibitors such as azide, thiocyanate, and fluoride on laccase and ascorbate oxidase has been investigated by absorption, CD, and EPR spectroscopies. Anaerobic reactions of inhibitor-treated laccase and ascorbate oxidase with pyrocatechol and L-ascorbate, respectively, gave EPR signals originating from the inhibitor-bound type 3 copper, except for the case of F(-)-laccase. The hyperfine splittings of these EPR signals (Az = 10.10(-3)-18.10(-3) cm-1) were smaller than those of type 2 copper centers (ca. 20.10(-3) cm-1), indicating that type 3 copper has a tetragonal geometry with tetrahedral distortion. The facile detection of a series of the inhibitor-bound type 3 copper centers indicates that the action of the exogenous anionic inhibitors is not only to interfere the access of dioxygen to the trinuclear site, but also to restrain the reduction of type 3 copper by lowering its reduction potential.