Structural basis for the conformational states of nitrosyl hemoglobins M Saskatoon and M Milwaukee. Influence of distal histidine residues on proximal histidine-iron bonds.

The nitrosyl derivatives of hemoglobins M Saskatoon (a&63 His-Tyr) and M Milwaukee val+G'u) have been examined to determine the role that these 2 distal amino acid residues play in maintaining hemoglobin conformational states.Nitric oxide reduces the ferric 8 chains of hemoglobin M Saskatoon and the resultant complex is an R state corresponding to a six-coordinate ESR spectrum illustrating that the absence of a p-63 distal histidine-NO interaction does not disrupt the proximal histidine-iron bonds.The ferric 8 chains of hemoglobin M Milwaukee are not reduced by NO due to the strong intermolecular bonding between the 8-67 glutamic acid and the heme iron.When NO is ligated to dithionite-reduced hemoglobin M Milwaukee, the molecule initially assumes an R conformation.However, the complex is unstable and proceeds to a T state corresponding to an ESR spectrum of a mixture of fiveand six-coordinate structures as the p-hemes are oxidized progressively.Therefore, the p-67 valine does not influence the stability of the R state in nitrosyl hemoglobin.However, the uncompensated, negatively charged carboxyl group of glutamic acid in the immediate vicinity of the bound NO promotes the reoxidation of the 8-hemes in the absence of oxygen.The resulting high spin ferric p chains stabilize nitrosyl hemoglobin M Milwaukee in the T state.Thus, although both the p-63 histidine and 8-67 valine may influence the ligandbinding properties of hemoglobin (Tucker, P. W., Phil-