Origin of the anomalous Fe-CO stretching mode in the CO complex of Ascaris hemoglobin

We report an unusually high frequency (543 cm^-1) for an Fe-CO stretching mode in the CO complex of Ascaris suum hemoglobin as compared to vertebrate hemoglobins in which the frequency of the Fe-CO mode is much lower. A second Fe-CO stretching mode in Ascaris hemoglobin is observed at 515 cm^-1. We propose that these two Fe-CO stretching modes arise from two protein conformers corresponding to interactions of the heme-bound CO with the B10-tyrosine or the E7-glutamine residues. This postulate is supported by spectra from the B10-Tyr → Phe mutant in which the 543 cm^-1 line is absent. Thus, a strong polar interaction, such as hydrogen bonding, of the CO with the distal B 10 tyrosine residue is the dominant factor that causes this anomalously high frequency. Strong hydrogen bonding between O₂ and distal residues in the oxy complex of Ascaris hemoglobin has been shown to result in a rigid structure, rendering an extremely low oxygen off rate [Gibson, Q. H., and Smith, M. H. (1965) Proc. R. Soc. London B 163, 206-214]. In contrast, the CO off rate in Ascaris hemoglobin is very similar to that in sperm whale myoglobin. The high CO off rate relative to that of O₂ in Ascaris hemoglobin is attributed to a rapid equilibrium between the two conformations of the protein in the CO adduct, with the off rate being determined by the conformer with the higher rate.