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 Fe-Co hybrid hemoglobins, alpha(Fe-O2)2 beta(Co)2 and alpha(Co)2 beta(Fe-O2)2, have been determined as functions of pH, temperature and inositol hexaphosphate. The oxygen dissociation rates from alpha(Fe-O2)2 beta(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 degrees C in 50 mM-Tris or Bis-Tris buffer containing 0.1 M-Cl-, while those of alpha(Co)2 beta(Fe-O2)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.
Kitagishi
K
K
Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia 19104-6089.
Ikeda-Saito
M
M
Yonetani
T
T
D000818:Animals; D003035:Cobalt / Q000378:metabolism*; D006454:Hemoglobins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism*; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism*; D010833:Phytic Acid / Q000378:metabolism; D013696:Temperature
D006454:Hemoglobins; D003035:Cobalt; D010833:Phytic Acid; D007501:Iron; D010100:Oxygen