High cooperativity of haemoglobin M Boston in the completely reduced state

Abstract

HAEMOGLOBIN (Hb) M Boston and Hb M Iwate, in which distal and proximal histidines, respectively, in the haem pockets of α chains are replaced by tyrosines, bind ligands only to the normal β chains, and have α chains stabilised in the ferric form. These M Hbs contain mutant α chains and have abnormally low oxygen affinity, almost no Bohr effect and virtually no haem–haem interaction^1,2. The β chain mutants, Hb M Hyde Park and M Saskatoon, have normal oxygen affinity and a normal Bohr effect but show almost no haem–haem interaction^3,4. Thus among the M Hbs, the proximal and distal mutants of the same chain display similar ligand binding properties. When abnormal α chains of Hb M Iwate are completely reduced using dithionite, the affinity for CO and Bohr effect were almost normal, but Hill's constant was still one¹. It has been suggested that the ligand binding properties of completely reduced Hb M Boston are approximately the same as those of Hb M Iwate^5,6. We have studied the equilibria of Hb M Boston and Hb M Iwate in both the half ferric and completely reduced states using ethylisocyanide (EIC), a non-oxidising ligand which equilibrates rapidly with haemoglobin, and here report that the completely reduced Hb M Boston shows ligand binding properties similar to those of Hb A.