Transient haem–globin interactions in photodeligated carboxyhaemoglobin and subunits

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Abstract
Although the fixation of ligand to [human] Hb is accompanied by changes in protein conformation regulating O2 exchange in blood, the mechanism triggering these changes remains unknown. A dynamic approach to this problem using results obtained in a nanosecond laser photolysis study of HbCO and its isolated subunits is reported. The study is based on previous observation of a structural evolution of free Hb after photodeligation, manifested through slight variations of the protein spectrum in the microsecond range. The isolated subunits also show this behavior. The duration of the spectral evolution is .apprx. 2 .mu.s for the 3 proteins and the activation energy of the process .apprx. 9 kcal/mol. The spectral evolution is attributed to local conformational changes at the heme 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.