Modification of human haemoglobin with glucose 6-phosphate enhances tetramer-dimer subunit dissociation

Studies using equilibrium gel-permeation chromatography demonstrate that formation of the covalent adduct of D-glucose 6-phosphate (G6P) with human haemoglobin promotes dissociation of the haemoglobin tetramer into its component alpha beta dimer pairs [Kdoxy = 2.57 X 10(-6) versus Kdoxy (G6P) = 11.22 X 10(-6) M-haem]. On the other hand, Kd for glucosylated haemoglobin is identical with those of the O2- and CO-liganded forms of intact haemoglobin A0. These data are consistent with the phosphate moiety alone being responsible for a 4.5-fold increase in the tetramer-to-dimer apparent Kd. This suggests the glucose 6-phosphate moiety does not bind to the same sites on haemoglobin as do the free organic phosphates, as suggested by ligand-binding kinetics data or structural analysis. My study presents a working model for studying changes in protein subunit assembly as altered by protein phosphorylations.