On the allosteric transition between the structures of high and low ligand affinity in carp hemoglobin.

The variation of magneto-optical rotatory dispersion with pH for carp deoxyhemoglobin in the presence and absence of inositol hexaphosphate was interpreted as a pH-induced allosteric transition between the structures of high and low ligand affinity (the R and T states in terms of the two state model of cooperativity). Increasing the pH from 6 to 11 causes a decrease in the fraction of molecules in the T state from 1 to 0.65. In the absence of inositol hexaphosphate the pH dependence of this fraction has a midpoint at 7.8, addition of inositol hexaphosphate shifts this midpoint by 1.5 units toward high pH. From the analysis of the data obtained and the pH dependences of functional properties (Tan, A.L., Noble, R.W. and Gibson, Q.H. (1973) J. Biol. Chem. 248, 2880-2888) the parameters of the two state model of cooperativity for carp hemoglobin were estimated.