Transient kinetics of oxygen dissociation from ferrous subunits of iron-cobalt hybrid hemoglobins. The principal reaction controlling the co-operativity

The oxygen dissociation constants from Fe subunits in the half-ligated intermediate states of FeCo hybrid hemoglobins, α(FeO₂)₂β(Co)₂ and α(Co)₂β(FeO₂)₂, have been determined as functions of pH, temperature and inositol hexaphosphate. The oxygen dissociation rates from α(FeO₂)₂β(Co)₂ are estimated to be more than 1300 s^-1 for the deoxy quaternary state (T-state) and less than 3 s^-1 for the oxy quaternary state (R-state) at 15 °C in 50 mm-Tris or Bis-Tris buffer containing 0·1 m-Cl^-, while those of α(Co)₂β(FeO₂)₂ are more than 180 s^-1 and less than 5 s^-1 for the T and R-states, respectively. The pH dependence of the oxygen dissociation rate from Fe subunits is large enough to be accounted for by the R-T transition, and implies that those half-ligated intermediate hybrids mainly exist in the R-state at pH 8·8, and in the T-state at pH 6·6, while other studies indicated that the half-ligated hybrids are essentially in the R-state at pH 7. Large activation energies of the oxygen dissociation process of 19 to 31 kcal/mol determined from the temperature dependence suggest that the process is entropy-driven.