Suggest an explanation. suggest an explanation. unlike histidine imidazole binds to atp forming a fairly stable complex. atp binds to hemoglobin more strongly than does the atp-imidazole complex. imidazole, therefore, tends to increase oxygen affinity of hemoglobin by blocking the atp-hemoglobin interaction. in native hb, the binding of oxygen is actually hindered by the fact that pulling on helix f must move it against constraints within the molecule. in the imidazole replacement, there is no need to do the extra work of moving helix f. this difference shows up as a more favorable free energy for binding. imidazole is significantly more acidic (lower pka) than histidine. thus, at the ph of blood, native hemoglobin is more acidic than the imidazole replacement. this lowering in ph causes the oxygen affinity of the hemoglobin to increase. in native hemoglobin, the initial binding of one o2 to a heme group promotes the binding of o2 to the other heme site on the molecule. in the imidazole replacement, the initial binding of one o2 to a heme group promotes the binding of two o2 to the other heme sites on the molecule. this effectively increases the oxygen affinity of the hemoglobin.