The iron-proximal histidine linkage and protein control of oxygen binding in hemoglobin. A transient Raman study

Time-resolved Raman studies have shown that communication between the heme oxygen binding sites and the surrounding globin occurs through the iron-proximal histidine linkage. By comparing the frequency of the Fe-His stretching mode in equilibrium deoxy- and photoinduced transient deoxyhemoglobins, we have found that ligand binding induces protein structural changes that strengthen the Fe-His linkage. The extent of this increase is observed to depend upon the quaternary state. This dependence is reflected in the restricted set of values observed in transient and equilibrium studies for the frequency of the Fe-His stretching mode in both R and T state deoxyhemoglobins. For T state hemoglobins the frequency increases from 215 cm^-1 to ~222 cm^-1 in going from the stable to the nanosecond transient deoxy species. The corresponding change for R state human hemoglobins is from ~222 cm^-1 to ~230 cm^-1. Studies on transients derived from Fe-Co hybrid hemoglobins reveal no subunit heterogeneity associated with the R state transient (230 cm^-1) at high pH. We also observe that perturbations of the protein known to destabilize the structure of ligand-bound R state hemoglobins are associated with a decrease in the frequency of the iron-proximal histidine stretching mode in the corresponding transient species. Such effects can originate from changes in either quaternary state or solution conditions as well as species-specific variations in protein structure. These results indicate that modulation of the Fe-His linkage could be a general mechanism for regulating ligand binding properties in hemoglobin. A direct connection between ligand binding and the Fe-His bond is suggested from our finding that the structural parameter(s) regulating the barrier height for geminate recombination is likely to be the same as the one(s) modulating the frequency of the Fe-His stretching mode in the transient deoxy species. Based on the recent conclusion that the variation in the tilt of histidine with respect to the heme plane is primarily responsible for the spectrum of frequencies observed for the Fe-His stretching mode we have examined a model in which protein regulation of binding occurs via a protein-induced change in the histidine tilt.