Transient haem-globin interactions in photodeligated carboxyhaemoglobin and subunits

Although the fixation of ligand to haemoglobin (Hb) is known to be accompanied by changes in protein conformation regulating the oxygen exchange in blood, the mechanism triggering these changes remains undecided. We now report a dynamic approach to this problem using results obtained in a nanosecond laser photolysis study of carboxyhaemoglobin (HbCO) and its isolated subunits. The study is based on our previous observation of a structural evolution of free Hb after photodeligation, manifested through slight variations of the protein spectrum in the microsecond range. It is now found that the isolated subunits also show this behaviour. The duration of the spectral evolution is approximately 2 microseconds for the three proteins and the activation energy of the process approximately 9 kcal mol-1. The spectral evolution is attributed to local conformation changes at the haem region, occurring during the structural relaxation of the freshly deliganded protein. The results for the isolated chains show that such changes exist even in the absence of the R-T transition.