Coupled Binuclear Copper Active Sites

Abstract

A coupled binuclear copper active site is found in a variety of different metalloproteins involved in dioxygen reactions. These include hemocyanin (reversible O2 binding), tyrosinase (O2 activation and hydroxylation) and laccase (O2 reduction to water). Unique excited state spectral features of active site derivatives and binuclear copper model complexes are used to define exogenous ligand bridging between the two coppers at the active sites of hemocyanin and tyrosinase. A chemical and spectroscopic comparison of the coupled site in hemocyanin to the binuclear center (Type 3, T3) in laccase indicates that the sites are similar with respect to an endogenous bridge responsible for antiferromagnetic coupling but differ in that exogenous ligands bind to only one copper at the T3 site. In addition to the T3 coupled binuclear copper site, laccase contains two additional coppers, the Type 1 and Type 2 centers. X-ray absorption edge spectral studies of the 1s→4p transition at 8984 eV are used to demonstrate that in the absence of the T2 copper, the T3 site is reduced and will not react with O2. The role of the T2 and T3 coppers in exogenous ligand binding has been probed through low temperature magnetic circular dichroism (MCD) which allows a correlation between the excited state and ground state spectral features. Through these low temperature MCD studies it is demonstrated that one azide produces charge transfer transitions to both the paramagnetic T2 and the antiferromagnetic T3 center defining a new trinuclear copper cluster active site which appears to be important in the irreversible multi-electron reduction of dioxygen to water.