Electron transfer reactions, cyanide and O2 binding of truncated hemoglobin from Bacillus subtilis

The truncated hemoglobin from Bacillus subtilis (trHb-Bs) possesses a surprisingly high affinity for oxygen and resistance to (auto)oxidation; its physiological role in the bacterium is not understood and may be connected with its very special redox and ligand binding reactions. Electron transfer reactions of trHb-Bs were electrochemically studied in solution and at graphite electrodes. Spectrophotometrical potentiometric titration and direct electrochemical measurements gave a heme iron redox potential of −103 ± 4 mV and −108 ± 2 mV vs. NHE, at pH 7, respectively. The redox potential of the heme in trHb-Bs shifted −59 mV per pH unit at pH higher than 7, consistently with a 1e−/1 H+ - transfer reaction. The heterogeneous rate constant ks for a quasi-reversible 1e− - 1H+ - transfer reaction between graphite and trHb-Bs was 10.1 ± 2.3 s−1. Upon reversible cyanide binding the ks doubled, while the redox potential of heme shifted 21 mV negatively, presumably reflecting changes in redox activity and in vivo signaling functions of trHb-Bs associated with ligand binding. Bioelectrocatalytic reduction of O2 catalyzed by trHb-Bs was one of the most efficient hitherto reported for Hbs, with an apparent catalytic rate constant, kcat, of 56 ± 6 s−1. The results obtained are of particular interest for applications of trHb in environmental biosensing and toxicity screening.