Pressure-induced unfolding/refolding of ribonuclease A: static and kinetic Fourier transform infrared spectroscopy study

In this paper, we illustrate the use of high-pressure Fourier transform infrared (FT-IR) spectroscopy to study the reversible presssure-induced unfolding and refolding of ribonuclease A (RNase A) and compare it with the results obtained for the temperature-induced transition. FT-IR spectroscopy monitors changes in the secondary structural properties (amide I' band) or tertiary contacts (tyrosine band) of the protein upon pressurization or depressurization. Analysis of the amide I' spectral components reveals that the pressure-induced denaturation process sets in at 5. 5 kbar at 20 degrees C and pH 2.5. It is accompanied by an increase in disordered structures while the content of beta-sheets and alpha-helices drastically decreases. The denatured state above 7 kbar retains nonetheless some degree of beta-like secondary structure and the molecule cannot be described as an extended random coil. Increase of pH from 2.5 to 5.5 has no influence on the structure of the pressure-denatured state; it slightly changes the stability of the protein only. All experimental evidence indicates that the pressure-denatured states of monomeric proteins have more secondary structure than the temperature-denatured states. Different modes of denaturation, including pressure, may correlate differently with the roughness of the energy scale and slope of the folding funnel. For these reasons we have also carried out pressure-jump kinetic studies of the secondary structural evolution in the unfolding/refolding reaction of RNase A. In agreement with the theoretical model presented by Hummer et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 1552-1555], the experimental data show that pressure slows down folding and unfolding kinetics (here 1-2 orders of magnitude), corresponding to an increasingly rough landscape. The kinetics remains non-two-state under pressure. Assuming a two-step folding scenario, the calculated relaxation times for unfolding of RNase A at 20 degrees C and pH 2.5 can be estimated to be tau(1) approximately 0.7 min and tau(2) approximately 17 min. The refolding process is considerably faster (tau(1) approximately 0.3 min, tau(2) approximately 4 min). Our data show that the pressure stability and pressure-induced unfolding/refolding kinetics of monomeric proteins, such as wild-type staphylococcal nuclease (WT SNase) and RNase A, may be significantly different. The differences are largely due to the four disulfide bonds in RNase A, which stabilize adjacent structures. They probably lead to the much higher denaturation pressure compared to SNase, and this might also explain why the volume change of WT SNase upon unfolding is about twice as large.