RADICAL COPPER OXIDASES, ONE-ELECTRON AT A TIME

Radical copper oxidases (including the fungal enzymes galactose oxidas e and glyoxal oxidase) are emerging as an important family of metalloe nzymes based on the free radical-coupled copper catalytic motif. The a ctive sites of these enzymes combine a redox active copper ion with a stable protein free radical, forming a two-electron redox unit capable of oxidizing a variety of alcohols and aldehydes with reduction of di oxygen to hydrogen peroxide. This active site is remarkable in the ext ent to which the ligands participate in catalysis. One of the tyrosine residues dissociates from the metal center and abstracts a proton to activate substrate for oxidation, while a second tyrosine (post-transl ationally modified to form a tyrosine-cysteine dimer) serves as the fr ee radical redox site. Computational studies of the substituted phenox yl indicates that in the ground state the majority of the unpaired ele ctron is localized on the phenoxyl oxygen and thioether sulfur atoms, accounting for the special properties of this site. Isotope kinetics h ave been investigated for the substrate oxidation half-reaction indica ting a dramatic isotope effect (k(H)/k(D) similar to 20) consistent wi th homolytic cleavage of the methylene C-H bond and hydrogen atom tran sfer to the phenoxyl in the transition state.