Contributions of Asparagine at α97 to the Cooperative Oxygenation Process of Hemoglobin^†

According to the X-ray crystallographic results from human deoxyhemoglobin, beta 99Asp at the alpha(1) beta(2) interface forms hydrogen bends with alpha 42Tyr and alpha 97Asn. To clarify the structural and functional roles of the hydrogen bond between alpha 97Asn and beta 99Asp, we have engineered a recombinant hemoglobin in which alpha 97Asn is replaced by Ala, and have investigated its oxygen-binding properties, and have used proton nuclear magnetic resonance spectroscopy to determine the structural consequences of the mutation. Recombinant Hb (alpha 97Asn-->Ala) shows a milder alteration of functional properties compared to the severely impaired beta 99 mutants of the human abnormal hemoglobins, The addition of inositol hexaphosphate, an allosteric effector, causes recovery of the functional properties of recombinant Hb (alpha 97Asn-->Ala) almost to the level of human normal adult hemoglobin without this allosteric effector. r l-lb (alpha 97Asn-->Ala) shows very similar tertiary structure around the heme pockets and quaternary structure in the alpha(1) beta(2) interface compared to those of human normal adult hemoglobin. The proton nuclear magnetic resonance spectrum of the deoxy form of this recombinant hemoglobin shows the existence of an altered hydrogen bond which is believed to be between alpha 42Tyr and beta 99Asp at the alpha(1) beta(2) interface. Thus, the present results suggest that the intersubunit hydrogen bond between alpha 97Asn and beta 99Asp at the alpha(1) beta(2) interface is not as crucial as the one between alpha 42Tyr and beta 99Asp in the deoxy quaternary structure. Preliminary molecular dynamics simulations have been used to calculate the contributions of specific interactions of several amino acid residues in r Hb (alpha 97Asn-->Ala) to the free energy of cooperativity of this recombinant hemoglobin. The results of these calculations are consistent with the experimental results.