Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 85

back of the substrate-binding pocket. Upon aerobic exposure to excess
phenylethyleneamine, the structure of the intermediate reveals dioxygen
replacing the axial water ligand to the copper ion while the product
phenylactaldehyde remains bound at the back of the active site. The kinetic
studies of the oxidative half-reaction for bovine serum amine oxidase suggest
that the rate-limiting electron transfer to is possibly from aminoquinol. A
single electron transfer results in the formation of superoxide and semiquinol,
which is absent based on the visible spectrum, suggesting formation of the
hydrogen peroxide species through the second electron transfer from the
cofactor. The crystal structure shows that the two oxygen atoms of are 2.8
and 3.0 Å from the copper ion and the Cu-O-O angle is 88 °, which the
authors state is consistent with a peroxide, or perhaps hydroperoxide species.
Galactose oxidase is an extracellular fungal enzyme, catalyzing the
stereospecific two-electron oxidation of D-galactose and other primary
alcohols. Prior to any knowledge from a protein x-ray structure, Whittaker

and co-workers 35,36 deduced that a stabilized ligand-protein radical-cation
was involved, which explained a long-standing puzzle of how a single active
site copper ion could effect a two-electron process. The x-ray structure of the
protein 37 revealed a novel protein co-factor consisting of a thioether bond
linking a cysteine to the -position of a copper-coordinated tyrosine

ligand, with the latter also undergoing a interaction with a
tryptophan residue. A recent report indicates that the formation of the

ortho

Cys-Tyr linked active-site cofactor is a self-processing reaction, i.e., mediated
by copper-dioxygen reactivity occuring at the active site of the pro-enzyme
38

following the binding of copper(II) ion.