Kinetics of Oxygen Reduction by a Beta Barrel Heme Protein on Hybrid Bioelectrodes

Efficient catalysts are required to activate O2 at low overpotentials with improved reaction kinetics. The kinetics of electrocatalytic oxygen reduction by a β‐barrel, the nitrobindin heme protein (NBHP), is investigated using protein‐film electrochemistry. Structure‐guided strategies were developed to covalently attach NBHP and enable efficient interfacial electron transfer (ET) between the protein and electrode. The reduction of Fe(III) to Fe(II) with a rate constant ks of 25 s−1 is invoked as the rate‐limiting step of the catalytic process, as opposed to a proton‐coupled electron transfer (PCET) process. Further kinetic analyses revealed a complete 4e− reduction of O2 to H2O at lower pH, attributable to a higher H+ availability and more facile reduction of the heme in acidic conditions compared to the basic conditions where an incomplete 2e− reduction to H2O2 was observed. The rate constants kORR, k0, and the turnover numbers demonstrate that NBHP is an efficient biocatalyst for O2 electrolysis.