On the spectral features associated with peroxide reactivity of the coupled binuclear copper active site in type 2 depleted and native Rhus laccase.

We report herein an X-ray absorption spectroscopic (XAS) determination of the oxidation state of the copper sites in T2D and native Rhus laccase. The increase in intensity of the 330 nm absorption feature which results from peroxide titration of T2D laccase (T3: [Cu(I)Cu(I)], T1: [Cu(II)]) is found to correlate linearly with the percent of oxidation of the binuclear copper site (determined by XAS analysis). This indicates that peroxide oxidizes but does not bind to the T3 site. We have used this correlation to determine that native laccase, as isolated, contains approximately 25% reduced T3 sites and that all spectral changes observed upon peroxide addition to native laccase can be accounted for by oxidation of these reduced sites. The importance of this result to previous reports of peroxide binding at the laccase active site is discussed.