The oxygenated flavohaemoglobin from Escherichia coli: evidence from photodissociation and rapid-scan studies for two kinetic and spectral forms.

The kinetics of dissociation and reassociation of the oxygenated species of Escherichia coli flavohaemoglobin (Hmp) were studied using stopped-flow rapid-scan and flash photolysis spectrophotometry at 25 degrees C. The oxygenated compound(s) form rapidly on mixing oxygen with the NADH-reduced flavohaemoglobin. On exhaustion of NADH, with residual oxygen, decay occurs in two phases to give a form in which haem b and flavin are oxidized. Spectral changes during this process suggest a direct release of O2- from the oxy form. Photodissociation of the oxygenated species generates the unliganded protein, which recombines with oxygen to give two spectrally and kinetically distinct forms. The reversibility of the oxygen reaction and the rapid reassociation kinetics after photodissociation confirm the haemoglobin-like features of this protein.