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 metallo enzyme galactose oxidase. The optical spectrum of the free radical gen erated by low-temperature UV irradiation of (methylthio)cresol is near ly identical to that observed for the free-radical site in metal-free apo galactose oxidase, supporting the assignment of the protein radica l to a novel tyrosine-cysteine covalent cross-link structure recently reported from X-ray crystallographic studies. Basic characterization o f the chemistry for this new type of biological redox group includes m easurements of the substituent effects on phenolic proton acidic and o bservation of the oxygenation of (methylthio)cresol with peroxides. Th e latter reaction models a possible inactivation pathway for the enzym e. Low-temperature absorption and EPR spectra were obtained for free r adicals formed from the methylthio and methylsulfinyl derivatives that are most important as models for the biological active site for compa rison with the parent cresol radical. A series of simple copper comple xes has been prepared with cresol, (methylthio)cresol, and (methylsulf inyl)cresol ligands to explore the effect of substitution on coordinat ion chemistry. Structural and spectroscopic data obtained for these in organic models contribute to an understanding of the active site metal interactions, providing evidence that the thio ether sulfur of the pr otein redox group is noncoordinating in the crystallographically defin ed enzyme complex.