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Quaternary conformational changes in human oxy hemo globin studied by laser photolysis


, : Quaternary conformational changes in human oxy hemo globin studied by laser photolysis. Journal of Biological Chemistry 252(16): 5783-5788

Absorbance changes observed at the isobestic point of Hb and HbO2 near 421 nm following laser photolysis of HbO2 provide a direct measure of the rate of the conformational change between the rapidly reacting immediate product of full photolysis and normal deoxyhemoglobin. By analogy with the case of carboxyhemoglobin, this rapidly reacting Hb can be interpreted as deoxyhemoglobin remaining in the quaternary conformation of HbO2, following rapid removal of O2 by a laser pulse. In borate buffer pH 9, both the O2-rebinding kinetics observed at 436 nm and the conformational change kinetics observed at 421 nm can be well fit in terms of a Monod, Wyman and Changeux allosteric model. In this case the immediate product of full photolysis is taken to be rapidly reacting R state deoxyhemoglobin, which can either make the conformational change to slowly reacting T state Hb or bind ligand. The rate constant describing the ligand-free conformational change was 1080, 6400 and 40,000 s-1 at 4, 20, and 40.degree. C, respectively. Within experimental error the same spectrum and rate of conformational relaxation are found in borate buffer at pH 9 for the rapidly reacting deoxy material formed by laser photolysis of HbO2 and HbCO, suggesting that the quaternary conformations for these derivatives are similar.

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PMID: 885881


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