Bacterial Respiratory Chain Diversity Reveals a Cytochrome c Oxidase Reducing O2 at Low Overpotentials

Cytochrome c oxidases (C c Os) are the terminal enzymes in energy-converting chains of microorganisms, where they reduce oxygen into water. Their affinity for O 2 makes them attractive biocatalysts for technological devices in which O 2 concentration is limited, but the high overpotentials they display on electrodes severely limit their applicative use. Here, the C c O of the acidophilic bacterium Acidithiobacillus ferrooxidans is studied on various carbon materials by direct protein electrochemistry and mediated one with redox mediators either diffusing or co-immobilized at the electrode surface. The entrapment of the C c O in a network of hydrophobic carbon nanofibers permits a direct electrochemical communication between the enzyme and the electrode. We demonstrate that the C c O displays a μM affinity for O 2 and reduces O 2 at exceptionally high electrode potentials in the range of +700 to +540 mV vs NHE over a pH range of 4–6. The kinetics of interactions between the enzyme and its physiological partners are fully quantified. Based on these results, an electron transfer pathway allowing O 2 reduction in the acidic metabolic chain is proposed.