The Effect of pH on the Rates of Ligand Replacement Reactions of Human Adult and Fetal Hemoglobins and Their Subunits

The rate of O2 dissociation with CO replacement in the presence of dithionite and the rate of CO replacement by NO have been measured at 20° for human adult and fetal hemoglobins and their isolated subunits over a pH range from 6 to 9. The rates of these reactions for the isolated subunits are pH dependent, as are the rates for the fully liganded structures of the tetrameric hemoglobins. However, the pH dependence exhibited by the isolated subunits is different from that of the tetramer. Although there is no simple relationship between the kinetic properties of the isolated subunits and those of the tetramer, a modification of the intrinsic properties of a subunit is clearly reflected in a similar modification of the properties of the ligand-saturated tetramer. The enthalpy values of these replacement reactions are reported for human adult hemoglobin and its subunits.