About two dozen histidine residues in hemoglobin are involved in binding the protons produced by cellular metabolism. In this manner, hemoglobin contributes to buffering in the blood, and the imidazole groups able to bind and release protons contribute to the Bohr effect. One important and the imidazole groups able to contributor to the Bohr effect is His 146 on the beta chain of hemoglobin, whose side chain is in close proximity to the side chain of Asp 94 in the deoxy form of hemoglobin but not the oxy form.
a. What kind of interaction occurs between Asp 94 and His 146 in deoxyhemoglobin?
b. The proximity of Asp 94 alters the pK value of the imidazole ring of His. In what way? Draw chemical structures of histidine that predominate in deoxyhemoglobin form?