Biochimie & Biochimie Analytique

Abstrait

Allosteric Structure of Effector-free Human Hemoglobin

Francis Knowles

The O₂-affinity of human hemoglobin, free of E-molecules, was measured in 0.050 M Bis-Tris, pH 7.0 with HCl, 20°C. A Hill plot of O₂-equilibrium binding data reveals an initial slope of 2 and fails to demonstrate an upward inflection. Four O₂-binding reactions account for these results. The tetrameric hemoglobin structure in an E-free supporting electrolyte can be described as two cooperative dimeric subunits: (^T,1α, ^R,2β) and (^T,2α, ^R,1β). Subunits of human Hb₄ are α- and β-chains. Within the hemoglobin tetramer: (i) Parenthetical inclusions describe the composition of cooperative dimeric subunits; (ii) Superscripts in the upper left identify the position of the subunit in the tetramer, and conformational state; R for the high affinity state and T for the low affinity state. Equilibrium constants of these four steps are related to intrinsic O₂-binding constants for α and β chains, K_α and K_β, respectively, by statistical factors: The equation of state contains these four unknown quantities.