Self-association of oxyhaemoglobin. A nuclear magnetic relaxation study in H2O/D2O solutions

Exclusion chromatography of concentrated hemoglobin solutions. Comparison of the self-association behavior of the oxy and deoxy forms of the alpha 2 beta 2 species

Theory pertaining to the interpretation of partition chromatography results obtained with self-associating protein systems studied at high total concentrations is extended to permit consideration of situations in which both monomeric and dimeric states partition. This development, which includes considerations of thermodynamic nonideality effects, permits a quantitative correlation of human oxyhemoglobin results reported previously and obtained in this work employing a different stationary matrix of controlled-pore glass beads. The two sets of results, obtained at pH 7.3 and 20 degree C, indicate that the alpha 2 beta 2 species of oxyhemoglobin self-associates. Two types of association pattern, discrete dimerization and an indefinite self-association, are examined. This is done for a realistic range of values for the radius, r. of the effective hard sphere appropriate to the calculation of the covolume of the alpha 2 beta 2 species in the assessment of the thermodynamic nonideality contribution. Assessed values of the isodesmic association constant range from 66 +/- 23 M-1 (r - 2.84 nm) to 154 +/- 26 M-1 (r = 3.13 nm). This mode of indefinite association is marginally favored over dimerization when the larger value of r is considered, the two patterns becoming virtually indistinguishable for the lower value of r. Partition chromatography results are also presented for human deoxyhemoglobin up to a total concentration of 225 g/l, and are analyzed in a similar fashion to show that the indefinite self-association pattern is favored, governed by an isodesmic constant in the range 91 +/- 9 M-1 (r = 2.84 nm) to 223 +/- 84 M-1 (r = 3.13 nm). Comparison of the constants assessed for the oxy and deoxy systems permits discussion of the concept that oxygen binds preferentially to the alpha 2 beta 2 species of deoxyhemoglobin in comparison with its polymers.

Chromatographic evidence of the self-association of oxyhemoglobin in concentrated solutions: its biological implications

Expressions that take into account the effects of thermodynamic non-ideality, described in terms of a high-order virial expansion, are derived for the concentration-dependence of the weight-average partition coefficient in exclusion chromatography of a single solute and of a solute undergoing reversible self-association. Comparison of the concentration-dependences predicted by those expressions with results obtained for bovine and human oxyhemoglobins on CPG-10-120 porous glass beads in 0.156 I phosphate-chloride buffer, pH 7.3, shows that neither oxyhemoglobin conforms with the concept of it being a single alpha 2 beta 2 entity with Stokes radius of 3.13 nm, the experimental value. Previously published osmotic pressure and sedimentation equilibrium results are also shown to be inconsistent with this concept. On the other hand, both sets of exclusion chromatography results are consistent with the joint operation of thermodynamic non-ideality and reversible association of the alpha 2 beta 2 species. From the magnitude of the equilibrium constant, derived for either of two possible modes of association, it is calculated that only half of the oxyhemoglobin would be in the alpha 2 beta 2 states under conditions of oxygen saturation and a concentration of 320 g/liter, that pertaining in the red blood cell. The consequences of this association phenomenon are discussed in relation to the oxygen binding curves obtained by others in the presence and absence of 2,3-diphosphoglycerate (DPG). An explanation is provided of the observed dependence on hemoglobin concentration of oxygen-binding in the presence of DPG, and of the absence of such an effect in DPG-free solutions. It is concluded that the control of oxygen binding to hemoglobin in the physiological situation involves the joint operation of self-association and allosteric effects.