Reactions of the Escherichia coli flavohaemoglobin (Hmp) with oxygen and reduced nicotinamide adenine dinucleotide: evidence for oxygen switching of flavin oxidoreduction and a mechanism for oxygen sensing.

The soluble flavohaemoglobin (Hmp) of Escherichia coli contains haem B and FAD in a single 44 kDa polypeptide, and shows NADH oxidase activity. The oxidized protein reacted rapidly with NADH in the presence of O2 to form an oxygenated species while the flavin remained largely oxidized. Spectral and kinetic analyses revealed rapid biphasic reduction and oxygenation of high-spin haem with apparent relaxation times of 6 and 64 ms at pH 8 and 25 degrees c, suggestive of a significant physiological role for the protein. This was followed by a monophasic reduction of the flavin with a relaxation time of 92 ms. On exhaustion of oxygen, the oxygenated haem was converted into the deoxy form biphasically with relaxation times of 43 and 170 s, followed by extensive reduction of the flavin with corresponding relaxation times of 70 and 256 s. Based on these observations, we propose that Hmp could act as an oxygen sensor in E. coli by combining with intracellular oxygen, thus limiting flavin reduction in the aerobic steady state. Lowering of the oxygen concentration causes dissociation of the oxy species and sustained flavin reduction. Because Hmp can reduce Fe(III), such a mechanism might control, for example, flavin-mediated Fe(III) reduction required for activation of the anaerobic gene regulator, Fnr.