Thermodynamics of lipid–proiein associations: the enthalpy of binding of Apo C-III to synthetic phosphatidylcholines

The association of synthetic phosphatidylcholines with apolipoprotein (apo) C-III, an apoprotein from human plasma very low density lipoproteins, has been studied by gel filtration, microcalorimetry, and differential scanning calorimetry. Apo C-III associates with a minimum of 35 molecules of dimyristoyl phosphatidylcholine (DMPC) and can accommodate up to 100 molecules of DMPC per apo C-III molecule. These complexes are readily isolated by chromatography on Sepharose CL-4B. Microcalcrimetry of apo C-III with DMPC or dipalmitoyl phosphatidylcholine (DPPC) vesicles reveals a high enthalpy of association of apo C-III with DMPC (−250 kcal∙mol apo C-III−1) and DPPC (−490 kcal∙mol apo C-III−1); these values translate into 2.8 and 4.65 kcal∙mol lipid−1, respectively. These high enthalpy values are observed only near the transition temperatures (Tc) of the lipids. Above and below Tc the enthalpy of association was practically zero. Differential scanning calorimetry of DMPC –apo C-III complexes shows them to be composed of 35–60 mol lipid∙mol apo C-III−1 in which no more than 45 molecules of DMPC form a boundary around apo C-III. It is shown that the effect of apo C-III on the thermal properties of DMPC is to elevate the Tc of the lipid and that an important component of the enthalpy of association of apo C-III with DMPC is due to the crystallization of the lipid acyl chains. Since the enthalpy of association is a term in the free energy of association, we suggest that the apo C-III induced crystallization of DMPC is important in the thermodynamics of this lipid–protein association. Generally, the sign and magnitude of the enthalpies of association should be important in predicting the distribution of lipids and proteins in more complex systems.