Modeling the hemoglobin switchpoint with cyanomet valency hybrids: Raman spectroscopic probes of tertiary and quaternary structure.

Hybrid hemoglobins with cyanomet hemes in the alpha or in the beta chains have been investigated by resonance Raman (RR) spectroscopy, using ultraviolet (230 nm) and visible (441.6 nm) excitation. For the CO adducts, the UVRR spectra are identical with that of native HbCO, showing the tyrosine and tryptophan signals to be insensitive to ligand substitution within the R state. In the absence of CO, the doubly ligated hybrids show differences in the UVRR spectra, relative to the CO adducts, which are the superposition of two difference spectra: (1) the T-R difference spectrum obtained by subtracting the spectrum of HbCO from that of deoxyHb and (2) a perturbed R state spectrum, characteristic of deligated chains within the R state. These spectra arise from alterations, respectively, in the quaternary contacts of interface aromatic residues and in the tertiary contacts of interior aromatic residues. From the amplitudes of the difference spectra, the T state population was determined to be 30% for (alpha FeII beta FeIIICN)2 and 43% for (alpha FeIIICN beta FeII)2, in good agreement with the kinetic analysis of Cassoly and Gibson [Cassoly, R., & Gibson, Q. H. (1972) J. Biol. Chem. 247, 7332]. Addition of inositol hexaphosphate (IHP) increased the T state population, but only by a modest amount, to 40 and 53%, respectively, in contrast to the frequent assumption that the T state conversion is quantitative in the presence of IHP. Since current understanding of the quaternary state dependence of the Fe-histidine stretching frequency is based on that assumption, the RR band envelope for this vibration was reexamined.(ABSTRACT TRUNCATED AT 250 WORDS)