Dioxygen activation in enzymatic systems and in inorganic models

Abstract

Cytochrome oxidase reduces O 2 to water quickly and with low overpotential. In addition, it uses the exergonicity of this reaction to pump protons against their thermodynamic gradient, thus contributing directly to the chemiosmotic potential that is used to synthesize ATP. In recent work, we have developed means by which to use time-resolved resonance Raman to study transient heme iron-bound oxygen intermediates that occur during the reduction of O 2 by cytochrome oxidase. Thus far, five different oxygen isotope sensitive modes have been observed; the temporal behavior of each during the reaction sequence has been characterized roughly. By combining the structure-specific vibrational results with optical data from other labs on the same reaction, we have constructed an overall working model for the dioxygen reduction reaction and have calculated concentration/time profiles for key intermediates. As opposed to most O 2 metabolizing enzymes, these calculations indicate that the oxidase/O 2 reaction is under proton control, which allows transient intermediates to build to detectable concentrations. We have linked this behavior to the proton-pump function of the enzyme and have postulated that proton control allows tight coupling between the oxygen chemistry and the proton translocations it drives. Recent findings from several laboratories have shown that a direct connection can be made between the intermediates that occur during oxygen reduction by fully reduced and mixed-valence oxidase and in its reaction with peroxide. This requires a branching reaction at the peroxide level in the reaction sequence. This modification to the mechanism is presented. In this article, these findings are reviewed and the mechanism by which O 2 reduction is catalyzed by cytochrome oxidase is compared and contrasted with other O 2-metabolizing heme enzymes. The continuing importance of vibrational spectroscopic approaches that rely on stable isotope substitution for mode identification is highlighted by reviewing recent developments in other laboratories on O 2 activation in the non-heme iron class of oxygen-metabolizing enzymes. This group of catalysts includes ribonucleotide reductase, methane monooxygenase, and fatty acid Δ 9-desaturase and has been recognized as a distinct, oxygen-metabolizing enzyme class only recently. Finally, recent inorganic model compound work on O 2 activation is briefly summarized.