MODELS FOR THE REDOX-ACTIVE SITE IN GALACTOSE-OXIDASE

Modeling approaches have been used to develop insight into the nature of the redox active site of the free-radical-containing copper metalloenzyme galactose oxidase. The optical spectrum of the free radical generated by low-temperature UV irradiation of (methylthio)cresol is nearly identical to that observed for the free-radical site in metal-free apo galactose oxidase, supporting the assignment of the protein radical to a novel tyrosine-cysteine covalent cross-link structure recently reported from X-ray crystallographic studies. Basic characterization of the chemistry for this new type of biological redox group includes measurements of the substituent effects on phenolic proton acidic and observation of the oxygenation of (methylthio)cresol with peroxides. The latter reaction models a possible inactivation pathway for the enzyme. Low-temperature absorption and EPR spectra were obtained for free radicals formed from the methylthio and methylsulfinyl derivatives that are most important as models for the biological active site for comparison with the parent cresol radical. A series of simple copper complexes has been prepared with cresol, (methylthio)cresol, and (methylsulfinyl)cresol ligands to explore the effect of substitution on coordination chemistry. Structural and spectroscopic data obtained for these inorganic models contribute to an understanding of the active site metal interactions, providing evidence that the thio ether sulfur of the protein redox group is noncoordinating in the crystallographically defined enzyme complex.