LIGAND-BINDING KINETICS AND DISSOCIATION OF THE HUMAN EMBRYONIC HEMOGLOBINS

The three human embryonic haemoglobins have been studied using a range of stopped-flow and flash photolysis experiments. The association and dissociation kinetics and equilibrium constants for the tetramer-dime r reactions of the deoxy and oxygenated forms have been investigated a nd found to be characterized by constants similar to those of the huma n adult protein. The rates of oxygen dissociation from the embryonic h aemoglobins have been measured and appear to be responsible for the hi gh oxygen-binding affinity associated with the embryonic proteins comp ared with the adult protein. The pH dependence of the oxygen dissociat ion rate constants also accounts for the rather unusual, previously de scribed, Bohr effects characteristic of the embryonic haemoglobins. A general scheme has been developed coupling both the dimer-tetramer equ ilibria and ligand-binding steps observed following photolysis of the liganded forms of the human embryonic haemoglobins.