Distal and Proximal Control of Ligand Reactivity: A Transient Raman Comparison of COHbA and COHb (Zürich)

When Raman scattering is used, it is shown that for a given solution condition, both the yield of picosecond geminate recombination and the quantum yield of photolysis averaged over a 10-ns laser excitation pulse are consistently lower for COHb (Zürich) than for COHbA. In going from pH 8.2 to pH 6.2 + 3 mM IHP, both hemoglobins exhibit the same proportional increase in the photolysis yield. The transient Raman spectrum of these photolyzed carboxyhemoglobins indicates that the behavior and geometry of the proximal side of the heme is very nearly the same in both proteins. The persistent difference in ligand-binding properties between these two proteins is therefore ascribable to a static variation in another structural degree of freedom, one that is likely to involve the distal side of the heme. The findings suggest that both distal and proximal variations in the heme environment account for base-line differences in ligand-binding properties between different hemoglobins, but within a given protein cooperativity are under proximal control.