Resolution of the Equilibrium Constant for the T State → RState Conformational Change of Human Hemoglobin into Endothermic and Exothermic Component Reactions
The dimensionless equilibrium constant for the allosteric conformation change, KΔC = 0.02602 (Knowles & Magde, linked ms 2) following binding of O2 by α-chains in Tstate Hb4/BPG (whole blood under standard conditions) is shown to be comprised of: (i) an endothermic change in conformation, 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. Eq. (1) defines the component steps whereby the Tstate conformation is converted to the Rstate conformation.
ΔGo(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). ΔGo for the equilibrium constant for ΔGO(KΔC) and Σ ΔGo for binding of O2 by the pair of equivalent Tstate α-chains, ΔGO(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. ΔGo for reaction of a pair of equivalent Rstate α-chains with O2, ΔGO(RαO2), was estimated from the known affinity of myoglobin for O2 at 37oC (Theorell H. (1936). Biochem. Z., 268, 73-81), -63.4 kJ/mol. The unknown quantity, ∆GO(R(HbO2)4/BPG), was obtained by solving Eq. (1), being -10.5 kJ/mol, K (R(HbO2)4/BPG) = 58.4 L/mol. The value of the equilibrium constant for binding BPG to R-state conformations represents 0.0073% of the value of the binding constant of BPG to Tstate conformations: 800,000 L/mol. The value of KΔC; (i) accounts for the ability of O2 to escape, virtually unhindered from rbcs and (ii) provides a biophysical basis for manifestation of high resting rates of metabolism in warm blooded species.