Differential scanning calorimetric studies of the effects of ions and pH on ribulose 1,5-bisphosphate carboxylase/oxygenase.

Lucerne rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase, EC 4.1.1.39) was purified by ammonium sulfate fractionation and gel chromatography. Differential scanning calorimetry (d.s.c.) was used to study the effects of pH in the range 3.5 to 11.4, and the effects, at pH 7.5, of various salts at ionic strength lower than 0.3 (NaH2PO4, (NH4)2SO4, Na2SO4, NaCl, MgCl2, CaCl2) on the thermal denaturation of the protein. Van't Hoff and calorimetric enthalpies, and the change in heat capacity between the native and denatured states were calculated from the experimental data. The effects of salts on the thermal denaturation seem to follow the Hofmeister series. In some cases, the thermal denaturation may be interpreted as a two-step transition of the polypeptide chains. Moreover, comparison of the results with literature data suggests that the thermal denaturation parameters depend on the botanical origin of rubisco and are affected by the conditions of its purification.