Inositol hexaphosphate is the strongest allosteric effector
even for the metform of haemoglobin. Its effects upon the
quaternary structure of the tetramer have been studied in relation
to the overall conformational state(s) of the haem-pockets in
aqueous solutions of human haemoglobins. The method useci,
proton magnetic relaxation, yields information about the accessibility
of solvent pl.\u27otons towards the haem-iron. No differences
in the relaxation rates were detected by this method between the
unstripped carbonmonoxyhaemoglobin and the phosphate-stripped
sample in the presence and absence of IHP. There are considerable
changes in those relaxation rate·s due to the paramagnetic
haem-iron of aquomethaemoglobin when IHP is added to the
stripped adult haemoglobin, but none is observed for the foetal
haemoglobin, although a similar shift in the spin-state equilibrium
ts expected for both haemoglobins on addition of ,!HP.
Neither was there any change with IHP in solutions of adult
fluoromethaemoglobin. It is concluded thart there is no tightening
of the haem-pockets upon addition of IHP to solutions of any
of the three haemoglobin samples. An increase in the accessibility
of the haem-pockets is probable only for the aquometfom1 of
the adult haemoglobin. It is suggested that the structural aspect
of ligand affinity, i.e. the haem-pocket conformation, is not as
decisive in altering the affinity by IHP as is possibly the change
in the haem-iron spin-state induced by !HP-binding
