CALORIMETRIC STUDIES OF INOSITOL HEXAPHOSPHATE BINDING TO AQUOMETHEMOGLOBIN-A

Inositol hexaphosphate (IHP) binding to aquomethemoglobin A (metHb) ha s been studied by how microcalorimetry and compared to IHP interaction s with carbonmonoxyhemoglobin A (COHb) and deoxyhemoglobin A (deoxyHb) . At 25 degrees C and pH 6.8 in 0.05 M NaPhos containing 0.1 M NaQ and , after correction for heats of proton extraction from buffer, MP bind s to metHb, deoxyHb and COHb with enthalpies of -80.3, -104.2 and -105 .9 kJ/mol IHP, respectively. None of the above processes exhibits larg e apparent heat capacity changes. The binding to metHb is strongly lin ked to proton uptake, 1.5 H+ being absorbed per MP bound at pH 6.8, wi th an enthalpic contribution of -21 kJ/mol Hf absorbed. Inorganic anio ns, including chloride, phosphate and perchlorate, bind to metHb with dissociation constants in the 0.1 M range and are released upon IHP bi nding. At high salt concentrations 2-4 Such ions are released with ent halpic contributions of +13 to +29 kJ/mol anion released. Thus observe d heats of reaction represent a difference of large energetic terms, i ncluding not only intrinsic interaction of ligand with protein but als o heats of proton extraction from buffer, proton uptake by protein and anion displacement by ligand, each of which can exceed 42 kJ/mol. The se results are discussed in light of a possible allosteric transition accompanying MP binding to metHbA. (C) 1998 Published by Elsevier Scie nce B.V.