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Proton nmr studies of hemo globin m milwaukee and their implications concerning the mechanism of cooperative oxygenation of hemo globin



Proton nmr studies of hemo globin m milwaukee and their implications concerning the mechanism of cooperative oxygenation of hemo globin



Biochemistry 16(7): 1452-1462



Hb M Milwaukee (.beta.67E11 Val .fwdarw. Glu) is a naturally occurring valency hybrid containing 2 permanently oxidized hemes on the .beta. chains. In this mutant, the 2 abnormal .beta. chains cannot combine with ligands, but the 2 .alpha. chains are normal and can combine with O2 with a Hill coefficient varying from 1.1-1.3. High-resolution proton nuclear magnetic resonance spectroscopy at 250 MHz was used to investigate the exchangeable, ring-current shifted, ferrous and ferric hyperfine shifted resonances of Hb M Milwaukee in the absence and presence of organic phosphates. The .alpha.-heme environment, as manifested by the ring-current shifted resonances in the liganded form as well as the ferrous hyperfine shifted resonances in unliganded form, and subunit interactions, as manifested by the exchangeable resonances, are similar in Hb M Milwaukee to those in normal adult human Hb. Organic phosphates can partially or completely inhibit the structural transformation which normally accompanies the binding of O2 or CO to Hb M Milwaukee. Upon stepwise addition of O2 to deoxy Hb M Milwaukee, the hyperfine shifted resonance spectra of ferric .beta. chains show features which cannot be attributed to either fully deoxy or oxy species. The spectra for partially oxygenated Hb M Milwaukee can be described as an appropriately weighted average of the spectra of zero, singly and doubly oxygenated species. The ferric hyperfine shifted resonance spectrum of the singly oxygenated intermediate was calculated by a method employing least-squares analysis of the spectra of partially oxygenated Hb M Milwaukee at several values of O2 saturation. The spectrum of this intermediate exhibits features which cannot be accounted for by a 2-structure model. The present results are consistent with a sequential model for the oxygenation of this mutant Hb. In view of the similarities between normal adult Hb and Hb M Milwaukee, it is suggested that a 2-state concerted allosteric model does not provide an adequate description of the structure-function relationship in normal adult Hb.

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