Fluorescence Enhancement of Laccase Induced by Reduction of Cu(II) Sites

The intrinsic fluorescence of laccase (p-diphenol:O(2) oxidoreductase, EC 1.10.3.2), emitted by its tyrosinyl and tryptophanyl residues, underwent significant enhancement upon reduction of the enzyme redox sites. The increase in quantum yield reached its maximum after the addition of approximately four reduction equivalents and depended on the wavelength of excitation:54% increase for lambda(ex) = 250 nm and 76% for lambda(ex) = 305 nm.A linear correlation between this enhancement and the reduction of the type 1 Cu(II) was observed both by direct measurement and by calculation on the basis of added reductant. The implications of the fluorescence enhancement and its correlation with the reduction of the type 1 copper are discussed in terms of the possible quenching mechanisms. The possibility of a redox-induced structural transition in the protein is suggested.