A possible allosteric communication pathway identified through a resonance raman study of four β337 mutants of human hemoglobin A

The highly conserved tryptophan at position β37 occupies a key locus at the hinge region within the α₁β₂ interface of the mammalian hemoglobins. This residue is thought to play an important role in mediating the heme-heme interaction associated with the cooperative binding of oxygen; however, its explicit function is unclear. In this study, the proximal heme environments of several β37 mutants of adult human hemoglobin (HbA) are probed using visible (Soret band enhanced) resonance Raman spectroscopy. In the equilibrium deoxy derivatives of these mutants, a systematic variation in proximal strain, as reflected in the iron-proximal histidine (F8) stretching frequency, ν(Fe-His), is seen upon mutation of the β37 residue. The variation in proximal strain correlates with both the ligand binding rates [Kwiatkowski et al. (1998) Biochemistry 37, 4325-4335] and conformational changes observed at the FG corner through X-ray crystallography [Kavanaugh et al. (1998) Biochemistry 37, 4358-4373]. The results from the deoxy samples indicate a plasticity of the tertiary structure within the T quaternary state. The correlation between the X-ray data and the Raman supports the idea that the proximal strain at the heme within the T state can be modulated by a combination of forces including those arising from the hinge region of the α₁β₂ interface, from the binding of allosteric effectors, and from the degree of iron displacement from the heme plane. Each of these contributors appears to operate through a shifting of the F helix either away from or toward the FG corner. The Raman spectra obtained from the 10 ns CO photoproduct of the β37 mutant Hb's indicate that these mutants contain an altered coupling between the R state α₁β₂ interface and the proximal heme environment. This altered coupling could be due to either dissociation of the ligated mutant tetramers into dimers or the formation of an R state tetramer with significantly weakened hydrogen bonds and van der Waals contacts between the α₁ and β₂ subunits at the interface. In either case, the results reveal a clear-cut structural basis for the quaternary enhancement effect in which the normal R state quaternary structure produces a higher affinity ligand binding site than that which occurs in the corresponding dimeric form of the protein. The normal R state interface is shown to be important for stabilizing a favorable ligand binding environment that persists long enough after laser photolysis to enhance the geminate rebinding process within the photoproduct. The addition of IHP to the solution of mutant COHb proteins results in photoproduct spectra that are all identical and are consistent with the ligand-bound derivatives having either a T state structure or a very strained and anomalous R state structure.