Structure and reactivity in hemoglobin: implications of recent picosecond transient raman and absorption studies

Recent picosecond transient absorption studies¹ show that the yield of geminate rebinding is sensitive to protein structure. Picosecond transient Raman studies² reveal that over the time course (Շ ≃ 200 ps) of the geminate rebinding, the effective tertiary structure about the heme is not relaxing. The results of this study also imply strong coupling among the elements of tertiary structure that control both the frequency of iron-proximal histidine stretching motion and the ligand binding properties of the heme. These findings and conclusions are used to further develop a working model in which reactivity and structure about the heme are controlled by an effective tertiary structure associated with what Karplus and co-workers³call an "allosteric core" comprised of the heme, the proximal histidine (F-8), a portion of the F-helix and the α₁-β₂ interface.