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 interaction1,2. 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 interaction3,4. 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 Iwate5,6. 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.