Volume and compressibility changes accompanying thermally-induced native-to-unfolded and molten globule-to-unfolded transitions of cytochrome c: a high pressure study

We have measured the transition temperatures, T(M), and van't Hoff enthalpies, DeltaH(M), of the thermally induced native-to-unfolded (N-to-U) and molten globule-to-unfolded (MG-to-U) transitions of cytochrome c at pressures between 50 and 2200 bar. We have used the pressure dependence of T(M) to evaluate the changes in volume, Delta(v), accompanying each protein transition event as a function of temperature and pressure. From analysis of the temperature and pressure dependences of Delta(v), we have additionally calculated the changes in expansibility, Delta(e), and isothermal compressibility, Delta(k)(T), associated with the thermally induced conformational transitions of cytochrome c. Specifically, if extrapolated to 25 degrees C, the native-to-unfolded (N-to-U) transition is accompanied by changes in volume, Delta(v), expansibility, Delta(e), and isothermal compressibility, Delta(k)(T), of -(5 +/- 3) x 10(-3) cm(3) g(-1), (1.8 +/- 0.3) x 10(-4) cm(3) g(-1) K(-1), and approximately 0 cm(3) g(-1) bar(-1), respectively. The molten globule-to-unfolded (MG-to-U) transition is accompanied by changes in volume, Delta(v), and isothermal compressibility, Delta(k)(T), of -(2.9 +/- 0.3) x 10(-3) cm(3) g(-1) at 40 degrees C and -(1.9 +/- 0.3) x 10(-6) cm(3) g(-1) bar(-1) at 35 degrees C, respectively. By comparing the volumetric properties of the N-to-U and N-to-MG transitions of cytochrome c, we have estimated the properties of the native-to-molten globule (N-to-MG) transition. For the latter transition, the changes in volume, Delta(v), and isothermal compressibility, Delta(k)(T), are approximately 0 cm(3) g(-1) at 40 degrees C and 1.9 cm(3) g(-1) bar(-1) at 35 degrees C, respectively. Our estimate for the change in expansibility, Delta(e), upon the N-to-MG is negative and equal to -(5 +/- 3) x 10(-4) cm(3) g(-1) K(-1). This finding contrasts with the results of previous studies all of which report positive changes in expansibility associated with protein denaturation. In general, our volumetric data permit us to assess the combined effect of temperature and pressure on the stability of various conformational states of cytochrome c.