Alteration of functional properties associated with the change in quaternary structure in unliganded haemoglobin

Abstract

The preparation of a number of specifically modified haemoglobins lacking various C-terminal residues is described. These haemoglobins can be changed from the unliganded R (or oxy type) quaternary structure to the unliganded T (or deoxy type) on addition of inositol hexaphosphate. This paper shows that this transition is associated with a lowering of the oxygen affinity and an increase in the Hill's coefficient, n, except in the case of des-(Arg141α, Tyr140α) haemoglobin where addition of inositol hexaphosphate lowers the oxygen affinity but does not increase the Hill's coefficient, n. This shows that Tyr140α plays a more important role than Tyr145β in generating co-operativity. The transition between unliganded R and unliganded T is associated with a lowering of the reactivity of the sulphydryl group Cys93β; this is due both to the change in quaternary structure per se and to the formation of the salt bridge between His146β and Asp94β. The Bohr effect associated with the transition from the unliganded to liganded R structure was less than one-tenth of the normal Bohr effect.