Posttranslationally modified tyrosines from galactose oxidase and cytochrome C oxidase

Abstract

The study of post-translationally modified redox-active amino acids is a new and continuing area of biochemistry. The model studies on compounds representing the galactose oxidase and cytochrome c oxidase cofactors have demonstrated that substitution at the ortho position of the tyrosine side chain may modity the redox potential, the bond dissociation energy, and the pKa of the hydroxyl group. Behavior attributable to such perturbations is evident in studies on both enzymes and the effect of the covalent modification may have different outcomes. In cytochrome c oxidase, the tyrosyl radical species in cytochrome c oxidase is proposed to arise as a result of donating a hydrogen atom (electron + proton) to bound dioxygen to assure O-O bond cleavage. This contrasts with galactose oxidase where the stabilized, cross-linked tyrosyl radical abstracts a hydrogen atom from the activated (by coordination to copper) substrate. Finally, it concludes that the cross-link may also serve a protective role, perhaps controlling the reactivity of the tyrosyl radical, and preventing deleterious ligand radical coupling reactions.