Dimers, tetramer partial dissociation into

The tetramer (x2fi2) of hemoglobin is partially dissociated into dimers (xfS). Mills, Johnson, and Ackers (1976) give the following value for the apparent equilibrium constant °K 2 (their symbol) for the association reaction in the absence of oxygen ... 

The fractional saturation of tetramer YT and the fractional saturation of dimer YD are functions only of [02] at specified T, P, pH, etc., as shown by equations 7.1-18 and 7.3-6. However, since the tetramer form is partially dissociated into dimers, the fractional saturation of heme Y is a function of both [02] and [heme]. Ackers and Halvorson (1974) derived an expression for the function Y([02], [heme]). When Legendre transforms are used, a simpler form of this function is obtained, and it can be used to derive limiting forms at high and low [heme]. These limiting forms are of interest because they show that if data can be obtained in regions where Y is linear in some function of [heme], extrapolations can be made to obtain YT and YD. These fractional saturations can be analyzed separately to obtain the Adair constants for the tetramer and the dimmer (Alberty, 1997a). 

At specified concentration of molecular oxygen, the five forms of hemoglobin are pseudoisomers, and they have the same further transformed Gibbs energy of formation at equilibrium. The further transformed thermodynamic properties of the tetramer can be calculated from experimental measurements of the fractional saturation, but in order to interpret experimental data, it is necessary to provide for the partial dissociation of tetramer 02 Bz into dimers a/3 (3). Seven apparent equilibrium constants are required to describe experimental data, and it is shown that all seven can be determined using limiting forms (4). 

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