Architecture of Allosteric Structure. Demonstration of the Fundamental Element of Allosteric Structure of Human Hemoglobin. O2-Equilibrium Binding Curve of Purified Human Hemoglobin in an Effector-Free Supporting Electrolyte.
The O2-affinity of human hemoglobin, free of effector molecules, was measured in 0.050 M BisTris, pH 7.0 with HCl, 20oC. A Hill plot of O2-equilibrium binding data reveals an initial slope of 2 and fails to demonstrate an upward inflection. The tetrameric hemoglobin structure in an effector-free supporting electrolyte can be described as two cooperative dimeric subunits. Equilibrium constants of these steps are related to intrinsic O2-binding constants for alpha and beta-chains, K(alpha) and K(beta). The equation of state is comprised of four unknown quantities. O2-Binding constants are not expected to be identical for each of the dimeric cooperative subunits. The first cooperative subunit binds O2 while bound to the second O2-free cooperative dimer. The second cooperative dimer binds O2 while bound to a fully oxygenated cooperative dimer. Effecter-free human hemoglobin is half saturated with1.9 micromolar oxygen. Addition of 0.1 M NaCl to the supporting electrolyte results in an approximately 4-fold increase in the concentration of O2 required for half saturation of hemoglobin, 8.1 micromolar oxygen. The mechanism of the response to chloride ions is attributed to neutralization of positively charged residues in the central cavity of Hb4 by the relatively high concentration of chloride ions.