Construction and analysis of a semi-quantitative 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 + O₂ = RCHO + H₂O₂). An unusual crosslink between tyrosine 272 and cysteine 228 provides a modified tyrosine radical site which acts as a ligand for the active site copper and is believed to act as a one-electron redox center. The single active site copper is believed to act as a second one-electron redox center. The use of the tyrosine one-electron redox center and the copper one-electron redox center allows removal of two electrons from alcohol substrate for subsequent transfer to molecular oxygen. Previously, we and others have proposed a detailed step-by-step radical mechanism for the reaction catalyzed by galactose oxidase. The catalytic cycle can be divided into two half reactions. The first half reaction entails transfer of two electrons and two protons from the alcohol substrate to the enzyme to form aldehyde product and 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 O₂ to form H₂O₂ product and regenerate fully oxidized catalytically active enzyme ready for another catalytic cycle. In this paper, we describe the construction of a semi-quantitative energy profile for this radical mechanism. Several significant points emerge from this analysis. 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 distorted or entatic copper site causes the unusually low redox potential. A final point is that crosslinking of tyrosine 272 and cysteine 228 alters the redox properties of the tyrosine center to enhance catalysis compared to what would be expected for a normal tyrosine.