Collman Group

20+ years of excellence in metalloporphyrin chemistry

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Professor Collman

Naturally Dangerous

Biomimetic studies of cytochrome c oxidase.

· Heme/Cu terminal oxidases (i.e., cytochrome c oxidase and ubiquinol oxidase) make aerobic life on Earth possible by catalyzing four-electron four-proton reduction of molecular oxygen to water. The biomimetic approach has proven quite productive in understanding the role that the distal Cu ion plays in O₂ reduction at the catalytic site of heme/Cu terminal oxidases.

• Over the past years we have developed several series of synthetic Fe porphyrins as biomimetic analogs of the heme/Cu site. Below is the structure of one of our more recent compounds that faithfully reproduce not only the stereoelectonic properties of the enzymatic site, but also its reactivity toward O₂ and H₂O₂ both under stoichiometric and physiologically-relevant catalytic conditions.

· Steady-state reduction of O₂ is studied by rotating ring-disk voltammetry. We have developed techniques that allow us to carry out catalysis when electrons are readily available and when arrival of electrons is slow compared to the reduction of bound O₂ to the redox level of H₂O (the latter reproduces the biological kinetics of electron flux to cytochrome c oxidase).

· We studied catalytic O₂ reduction by our biomimetic heme/Cu analogs in the bimetallic, FeCu and Cu-free forms, under both an excess of electrons and a slow flux of electrons from the electrode to the catalyst. Through these comparisons we determined that the distal Cu ion appears to serve mainly as an electron storage site, so that when electrons are readily accessible, the activation barrier for O₂ reduction at the heme is not affected by Cu.

· Even though Cu does not appear to chemically affect the main catalytic pathway, it still modifies the reactivity of the heme. Specifically, it stabilizes the heme-O₂ adduct against irreversible autooxidation, which releases, superoxide, a species that is toxic. Cu also decreases the affinity of the heme to CN^- and CO, so that the bimetallic catalyst is up to 5 times more resistant to inhibition by these poisons than the Cu-free analogs.