Thermodynamic studies on subunit assembly in human hemoglobin. Calorimetric measurements on the reconstitution of oxyhemoglobin from isolated chains.

Calorimetric heats generated upon mixing solutions of alphaSH and betaSH chains of human hemoglobin have been studied by isothermal heatburst microcalorimetry as a function of mixture composition. Based upon studies described in accompanying papers, the contributions to the measured heats arising from (a) alpha chain self-association, (b) beta chain self-association, (c) association of dimers to form tetramers, have been evaluated. Taking these processes into account, the calorimetric data have been used to determine the enthalpy of formation for alphabeta dimers, yielding a value of -15.71 +/- 0.96 kcal in the fully oxygenated state at 21.5 degrees in 0.1 M Tris/HCl, 0.1 M NaCl, 1 mM Na2EDTA, pH 7.4. The total enthalpy for assembly of a mole of hemoglobin tetramers from oxygenated chains is -27.6 +/- 2.1 kcal. Combining results of this study with independently determined information, limits can be placed upon the magnitude of the enthalpy for dimer formation in unliganded hemoglobin. The total enthalpy for assembly of a mole of unliganded hemoglobin tetramers from unliganded chains is -61.6 +/- 3.5 kcal, or approximately twice the value for oxygenated hemoglobin. This difference lies entirely in the dimer-tetramer stage of assembly. There are essentially no oxygenation-linked thermodynamic quantities (deltaG, deltaH, deltaS) associated with alphabeta dimer formation from isolated chains.