Enthalpy Change of Allosteric Transition in Human Haemoglobin A

Abstract

AbstractOxygen equilibria of haemoglobin were analysed according to a binding isotherm proposed by Amire (Bull. Chem. Soc. Jpn. 1994, 67, 7)1 to obtain the intrinsic oxygen association constants to the molecule. Two sets of binding sites in haemoglobin were identified, which were ascribed to R2 and T forms of the molecule. The average intrinsic association constants determined as a function of temperature gave a heat of oxygenation of‐76 ± 4 kJ mol−;1 (tetramer). A microcalorimetrically determined heat of deoxygenation of oxyhaemoglobin by dithionite gave −267 ± 10 kJ mol−1 (tetramer). From these results, the heat of allostery of −234 ± 24 kJ mol−1 for haemoglobin tetramer was obtained, yielding allosteric energy per salt bridge of‐29 ± 3 kJ. This result suggests that salt‐bridge may, in fact, be thermochemically equivalent to hydrogen bonds.