A 2-STATE ANALYSIS OF COOPERATIVE OXYGEN-BINDING IN THE 3 HUMAN EMBRYONIC HEMOGLOBINS

The binding of oxygen to the three human embryonic haemoglobins, at pH 7.4, has been shown to occur as a co-operative process. Analysis of o xygen-binding curves obtained in the absence of organic phosphate allo steric effecters shows that the process can be described quite accurat ely by the two-state model of allosteric action. In the presence of or ganic phosphates, the binding affinity for oxygen to the T-state of th e alpha(2) epsilon(2) and zeta(2) epsilon(2) haemoglobins is significa ntly lowered. The values of the best-fit two-state parameters determin ed for each of the embryonic haemoglobins together with the temperatur e-dependence of the overall equilibrium binding process are discussed in terms of oxygen transfer from the maternal blood supply. Fast-react ion studies have been used to determine the rate constants of the oxyg en association and dissociation processes occurring in the R-state and the rate of the allosteric R > T conformational transition. Analysis of these data suggests a likely reason for the high affinity and low c o-operativity of the embryonic proteins and identifies the origins of the inability of equilibrium measurements to identify chain non-equiva lence in the R-state.