O ₂ activation by binuclear Cu sites: Noncoupled versus exchange coupled reaction mechanisms

Abstract

Binuclear Cu proteins play vital roles in O2 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 O2 to a -2 : 2 side-on peroxide bridged Cu II 2 (O 2 2؊ ) species. This side-on bridged peroxo-Cu 2 II species is activated for electrophilic attack on the phenolic ring of substrates. Noncoupled binuclear Cu proteins include peptidylglycine ␣-hydroxylating monooxygenase and dopamine ␤-monooxygenase. These proteins have binuclear Cu active sites that are distant, that exhibit no exchange interaction, and that activate O2 at a single Cu center to generate a reactive Cu II ͞O2 species for H-atom abstraction from the C-H bond of substrates. O2 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 ͞O2 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.

Mechanism of O2activation and substrate hydroxylation in noncoupled binuclear copper monooxygenases

“…Cu H as an ET Site. The lack of exchange coupling between Cu H and Cu M has been explained as the key factor directing the reactivity of the noncoupled binuclear copper enzymes toward 1e -HAA/ hydroxylation, as opposed to 2e -electrophilic aromatic attack in the coupled binuclear Cu enzymes (32). This control allows the overall two-electron oxidation of substrate to be accomplished by delivering electrons to the active Cu M site one at a time, which permits one-electron reduction of O 2 to the superoxide intermediate to "turn on" substrate HAA rather than two-electron reduction of O 2 to a (hydro)peroxo intermediate that is not activated toward HAA.…”

A temperature independent pH (TIP) buffer for biomedical biophysical applications at low temperatures

“…[1][2][3][4][5][6][7] Buffers are invariably used in the sample storage and analysis process. It is widely known that the pH of a buffer solution can change at low temperatures, and this has been ascribed to enthalpic effects on the proton equilibrium as well as selective precipitation of buffer components upon cooling.…”

Reaction Mechanism of the Bicopper Enzyme Peptidylglycine α-Hydroxylating Monooxygenase

“…This experimental finding contradicts the mechanism suggested by Crespo et al (21), because a very small energy barrier does not lead to a significant KIE. Klinman and co-workers (10,25) suggested a Cu(II)-superoxo mechanism, similar to the one proposed by Solomon and co-workers (19,20), based on experimental indications but without computational backup. Based on recent QM/MM calculations (12), it was argued that the high barrier for the Cu(II)-superoxo case is a spurious effect of the lack of relaxation of the solvent.…”

“…Prigge et al (7) suggested Cu M ϩ -O 2 Ϫ 7 Cu M 2ϩ -O 2 2Ϫ as the abstracting species based on the crystal structure. Gas phase density functional theory calculations with a truncated model of the protein by Chen and Solomon (19,20) suggested a Cu(II)-superoxo species to be responsible for the abstraction of the glycine hydrogen. This mechanism was ruled out by Crespo et al (21) because including the environment of the protein in a QM/MM fashion enlarged the energy barrier up to 84 kJ mol Ϫ1 (gas-phase calculations by Chen and Solomon (19,20) gave a value of ϳ60 kJ mol Ϫ1 for the energy barrier).…”

“…Gas phase density functional theory calculations with a truncated model of the protein by Chen and Solomon (19,20) suggested a Cu(II)-superoxo species to be responsible for the abstraction of the glycine hydrogen. This mechanism was ruled out by Crespo et al (21) because including the environment of the protein in a QM/MM fashion enlarged the energy barrier up to 84 kJ mol Ϫ1 (gas-phase calculations by Chen and Solomon (19,20) gave a value of ϳ60 kJ mol Ϫ1 for the energy barrier). Instead, Crespo et al (21) suggested Cu-oxo as a possible species, because of the small energy barrier involved, similar to the reaction mechanism suggested for the homologous dopamine ␤-monooxygenase (EC 1.14.17.1) by Kamachi et al (22).…”