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De-Constructing the ^TState/^RState Structure Change of Human Hemoglobin

Francis Knowles*, Douglas Magde

The dimensionless equilibrium constant for the allosteric structure change is shown to be comprised of: (i) An endothermic change in structure, from ^Tstate to ^Rstate, of 24.3 kJ/mol; (ii) Exothermic conversion of ^Tstate ^TαO₂- chains to ^Rstate ^RαO₂-chains of-13.8 kJ/mol; (iii) Exothermic binding of BPG by R-states. Equation (1) defines the component steps whereby the ^Tstate structure is converted to the ^Rstate structure. ΔG°(^R(Hb₄), BPG) describes the endothermic decomposition of the binary complex, ^THb₄/BPG into ^RHb₄ and BPG, equal to +33.7 kJ/mol (DeBruin et al. (1973). J. Biol. Chem. 248, 2774-2777). ΔG° of the equilibrium constant for ΔG° (K_Δ) and Ʃ ΔG° for binding of O₂ by the pair of equivalent ^Tstate α-chains, ΔG°(^Tα*O₂), +9.41 kJ/mol and-49.6 kJ/mol, respectively, are determined by fitting of O₂ equilibrium binding data to the Perutz-Adair equation.