Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 86

The reaction to oxidize alcohols (eqs. 1 & 2) can be thought of as
proceeding by oxidation of a reduced protein and copper(I) center by
dioxygen, in a two-electron process, to give oxidized copper(II) and protein-
ligand radical cation The “stored” oxidizing equivalents can then oxidize
a coordinated alcohol substrate, producing aldehyde and restoring the reduced
enzyme center. Proton transfers from substrate to dioxygen (as product
peroxide) are suggested to be mediated via protonation-deprotonation of the
coordinated tyrosinate/tyrosine ligands, which are more likely to be
protonated and not coordinated when the copper ion is reduced.

1.2.3 Cytochrome c Oxidases
These are ubiquitous proteins that occur in anaerobic organisms and
are part of a superfamily of heme-copper oxidases which function as proton
pumps; the proton and charge gradient thus produced is utilized for ATP
synthesis by ATPase enzymes. 39,40 A heme-copper binuclear active site
functions to bind and receive electrons and protons in the four-electron,
four-proton reduction of to water. A copper-dioxygen 1:1 initial adduct is
implicated 39,41 in the earliest stage of the protein reaction of the
active site. An subsequent intermediate has been
considered in the past, but more recent studies abate the importance of such a
species, suggesting that an iron-bound receives electrons from the
heme, and a modified active site tyrosine (Y) phenol, directly cleaving the
O-O bond and forming a ferryl and (plus radical) 'P'
intermediate. 42,43 X-ray structures are now available for various states of three
cytochrome oxidases. 44-47 In one case, a protein form with peroxo species
bridging the heme and copper ion is claimed. 44 Further details can be found
in the references cited.

2. COPPER-DIOXYGEN ADDUCTS

As mentioned above, there are several cases of protein dioxygen-
copper adducts which have been characterized only by spectroscopy (i.e.,
tyrosinase) 12 and/or protein x-ray crystallography. The latter cases include
oxyhemocyanins, 21,22 and lower resolution examples are reported for amine
oxidase 34 and cytochrome c oxidase. 44 Far more detailed structures are now
available for small molecule synthetic complexes, as described below.
There are four principal binding modes previously established for the
interaction of transition metal complexes with dioxygen (Chart 1), and all are
suggested or proven for copper. These include end-on binding, as found in