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
Rk. Poole et al., 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, Proceedings - Royal Society. Biological Sciences, 255(1344), 1994, pp. 251-258
The soluble flavohaemoglobin (Hmp) of Escherichia coli contains haem B
and FAD in a single 44 kDa polypeptide, and shows NADH oxidase activi
ty. The oxidized protein reacted rapidly with NADH in the presence of
0, to form an oxygenated species while the flavin remained largely oxi
dized. 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 physi
ological role for the protein. This was followed by a monophasic reduc
tion of the flavin with a relaxation time of 92 ms. On exhaustion of o
xygen, the oxygenated haem was converted into the deoxy form biphasica
lly with relaxation times of 43 and 170 s, followed by extensive reduc
tion of the flavin with corresponding relaxation times of 70 and 256 s
. Based on these observations, we propose that Hmp could act as an oxy
gen sensor in E. coli by combining with intracellular oxygen, thus lim
iting flavin reduction in the aerobic steady state. Lowering of the ox
ygen concentration causes dissociation of the oxy species and sustaine
d flavin reduction. Because Hmp can reduce Fe(III), such a mechanism m
ight control, for example, flavin-mediated Fe(III) reduction required
for activation of the anaerobic gene regulator, Fnr.