A calorimetric study of peptide-phospholipid interactions: the glucagon-dimyristoylphosphatidylcholine complex.
The isothermal enthalpy of reaction glucagon with dimyristoylphosphatidylcholine is found to be large and markedly temperature dependent, changing from -150 kcal/mol of glucagon at 25 degrees C to + 80 kcal/mol of glucagon at 23 degrees C. The observed enthalpy is shown to arise mainly from the glucagon-induced alteration of the distribution of the phospholipid between gel and liquid crystalline phases. The reaction is accompanied by a large change in the apparent heat capacity around the phase transition temperature of the pure lipid which explains the small variation of the free energy of binding with temperature despite the large changes in enthalpy. The equilibrium binding constant calculated from the enthalpy as a function of lipid-peptide ratio is in good agreement with that previously found from fluorescence titrations, but the number of lipid molecules bound per glucagon molecule at 25 degrees C is increased from 20 to 50, suggesting that glucagon can bind to 50 lipid molecules but that only 20 of these are close enough to affect the fluorescence of the hormone. Differential scanning calorimetry of dimyristoylphosphatidylcholine in the presence of glucagon shows a transition curve composed of two components. The major component, corresponding to 80% of the enthalpy change, is centered at 26.1 degrees C and has a cooperative unit of 45 lipid molecules. The total enthalpy change for the transition in the presence of glucagon is only +3.2 kcal/mol, compared with +4.8 kcal/mol for the pure lipid.