Enthalpy change of the allosteric transition in human haemoglobin A

Abstract

Oxygen equilibria of haemoglobin were analysed using binding isotherms to obtain the intrinsic oxygen association constants to the haem moities of haemoglobin. Two sets of binding sites in haemoglobin were identified, which were ascribed to the R and T forms of the haems. The average intrinsic association constants, determined as a function of temperature, gave a heat of oxygenation of 76 ± 4 kJ mol −1 for the haemoglobin tetramer. A microcalorimetrically determined heat of dissociation of oxyhaemoglobin by dithionate was −267 ± 10 kJ mol −1 (tetramer). From these results, the heat of allostery of −343 ± 14 kJ mol −1 of the haemoglobin tetramer was obtained, yielding an allosteric energy per mole of salt-bridge of −42 ± 4 kJ mol −1. This results suggest strongly that the salt-bridges may be hydrogen bonds.