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 k_s 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 k_s 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, k_cat, of 56 ± 6 s^-1. The results obtained are of particular interest for applications of trHb in environmental biosensing and toxicity screening.