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(-)/1H(+) - 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 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 O-2 catalyzed by trHb-Bs was one of the most efficient hitherto reported for Hbs, with an apparent catalytic rate constant, k(cat), of 56 +/- 6s(-1). The results obtained are of particular interest for applications of trHb in environmental biosensing and toxicity screening. (C) 2013 Elsevier Ltd. All rights reserved.