CONSTRUCTION AND ANALYSIS OF A SEMIQUANTITATIVE ENERGY PROFILE FOR THE REACTION CATALYZED BY THE RADICAL ENZYME GALACTOSE-OXIDASE

Galactose oxidase (GOase) is a mononuclear type 2 copper enzyme which oxidizes primary alcohols to aldehydes using molecular oxygen (RCH2OH + O-2 = RCHO + H2O2). An unusual crosslink between tyrosine 272 and cy steine 228 provides a modified tyrosine radical site which acts as a l igand for the active site copper and is believed to act as a one-elect ron redox center. The single active site copper is believed to act as a second one-electron redox center. The use of the tyrosine one-electr on redox center and the copper one-electron redox center allows remova l of two electrons from alcohol substrate for subsequent transfer to m olecular oxygen. Previously, we and others have proposed a detailed st ep-by-step radical mechanism for the reaction catalyzed by galactose o xidase. The catalytic cycle can be divided into two half reactions. Th e first half reaction entails transfer of two electrons and two proton s from the alcohol substrate to the enzyme to form aldehyde product an d two-electron-reduced enzyme (one electron at the tyrosine center and one at the copper center). The second half reaction entails transfer of two electrons and two protons from the two-electron-reduced enzyme to O-2 to form H2O2 product and regenerate fully oxidized catalyticall y active enzyme ready for another catalytic cycle. In this paper, we d escribe the construction of a semi-quantitative energy profile for thi s radical mechanism. Several significant points emerge from this analy sis. One point is the prediction that galactose oxidase should have an unusually low redox potential for copper, to our knowledge lower than any other redox active copper protein. Another point is that the dist orted or entatic copper site causes the unusually low redox potential. A final point is that crosslinking of tyrosine 272 and cysteine 228 a lters the redox properties of the tyrosine center to enhance catalysis compared to what would be expected for a normal tyrosine. (C) 1998 El sevier Science B.V.