Determination of the seven apparent equilibrium constants for the binding of oxygen by hemoglobin from measured fractional saturations.
Subunit dissociation has to be taken into account in the determination of the oxygen binding constants of hemoglobin, as described by Ackers and Halvorson in 1974. The seven apparent equilibrium constants for a particular set of conditions can be determined by using extrapolations to determine the fractional saturations YT of tetramer and YD of dimer from measured values of the fractional saturation Y of partially dissociated hemoglobin. Analytical methods are used to show that YT as a function of [O2] for tetramers can be calculated from Y of hemoglobin by linear extrapolation of measured Y values at high [heme] versus [heme]-1/2 to [heme]-1/2 = 0. YD for dimers can be calculated from measured Y values by linear extrapolation of Y versus [heme] to [heme] = 0 if sufficiently low [heme] can be used. These extrapolations have been tested with numerical calculations of Y for a particular hemoglobin as a function of [heme] and [O2] by using the seven apparent equilibrium constants determined by Mills, Johnson, and Ackers in 1976. The proposed procedure also yields the apparent association constant K" for 2TotD = TotT, where TotD is the sum of the dimers and TotT is the sum of the tetramers. This thermodynamic analysis of experimental data to determine the seven apparent equilibrium constants is independent of the model used to interpret the values of the thermodynamic parameters.
Alberty
R A
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Department of Chemistry, Massachusetts Institute of Technology, Cambridge 02139, USA.
D055598:Chemical Phenomena; D002627:Chemistry, Physical; D006418:Heme / Q000737:chemistry; D006454:Hemoglobins / Q000737:chemistry*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010100:Oxygen / Q000737:chemistry*; D013816:Thermodynamics
D006454:Hemoglobins; D006418:Heme; D010100:Oxygen