O2 activation by binuclear Cu sites: noncoupled versus exchange coupled reaction mechanisms

Binuclear Cu proteins play vital roles in O(2) binding and activation in biology and can be classified into coupled and noncoupled binuclear sites based on the magnetic interaction between the two Cu centers. Coupled binuclear Cu proteins include hemocyanin, tyrosinase, and catechol oxidase. These proteins have two Cu centers strongly magnetically coupled through direct bridging ligands that provide a mechanism for the 2-electron reduction of O(2) to a mu-eta(2):eta(2) side-on peroxide bridged Cu(II)(2)(O(2)(2-)) species. This side-on bridged peroxo-Cu(II)(2) species is activated for electrophilic attack on the phenolic ring of substrates. Noncoupled binuclear Cu proteins include peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase. These proteins have binuclear Cu active sites that are distant, that exhibit no exchange interaction, and that activate O(2) at a single Cu center to generate a reactive Cu(II)/O(2) species for H-atom abstraction from the C-H bond of substrates. O(2) intermediates in the coupled binuclear Cu enzymes can be trapped and studied spectroscopically. Possible intermediates in noncoupled binuclear Cu proteins can be defined through correlation to mononuclear Cu(II)/O(2) model complexes. The different intermediates in these two classes of binuclear Cu proteins exhibit different reactivities that correlate with their different electronic structures and exchange coupling interactions between the binuclear Cu centers. These studies provide insight into the role of exchange coupling between the Cu centers in their reaction mechanisms.