Interspecies variations in the transient heme species generated subsequent to CO photolysis from hemoglobins

The structure, ligand binding kinetics, and thermodynamics of hemoglobin have been the subject of a great deal of investigation. However, the exact pathway(s) by which cooperative energetics are communicated within the protein remain undefined. The effects of interspecies variations in quaternary and tertiary structure, oxygen affinity, cooperativity, and ligand binding kinetics upon the overall ligand binding process are, therefore, of great importance in understanding and solving these problems. The demonstrated sensitivity of resonance Raman spectroscopy to heme structure and environment make it an ideal probe of ligand binding dynamics. It is possible to examine how specific vibrational modes change with time and correlate this with solution conditions and protein structural and conformational differences. Those modes which exhibit the greatest change with ligand photolysis are also indicative of possible paths of cooperative energy dissipation within the protein. The changes which occur in the vibrational modes of the heme within 10 ns of CO photolysis have been determined for a wide variety of mammalian and reptilian hemoglobins. The modes most affected by this process are, without exception, v(Fe-His), v₄, and the substituent bending modes, δ(c(b) - s) and δ(c(b) - c(α) - c(β)). Furthermore, a direct correlation exists between the shift in porphyrin π* electron density upon CO photolysis (as indicated by the transient changes in v₄) and the Hill coefficient of cooperativity. The implications of those results concerning ligand binding cooperativity in hemoglobins are discussed.