We present here the first report of the pressure dependence of pressure-jump relaxation kinetics for protein folding transitions. We have studied the relaxation kinetics for the unfolding/refolding of wild-type Staphylococcal nuclease and have found that the relaxation kinetics observed at high pressure are much slower than those observed by pH or denaturant jumps at atmospheric pressure. This indicates that these processes have large, positive values for the activation volumes, most likely stemming from exclusion of solvent from a transition state that is less well packed than the native state. We examined the pressure-jump relaxation kinetics of three single-site mutations in nuclease that lead to alterations in the interactions between the two domains of the protein and changes in the equilibrium constant for isomerization of the lysine-116 to proline 117 peptide bond away from the cis form that predominates in the wild-type enzyme. At comparable pressures, the relaxation times for these mutants were significantly shorter than those observed for the wild type, indicating lower values of the activation volumes. We propose that these mutations cause a decrease in the cooperativity of the unfolding of the two domains, leading to a decrease in the degree of solvent exclusion at the rate-limiting step. The mechanism by which a particular amino acid sequence determines the fold and stability of globular proteins remains one of the most interesting and important unresolved issues in biophysical chemistry. The approaches to increasing our understanding of this phenomenon typically have involved perturbation of the proteins by chemical means or by temperature extremes. The equilibrium or time-dependent responses to these perturbations are then monitored (using a spectroscopic signal, activity, or some other observable) to extract the energetic or kinetic aspects of the unfolding or refolding transitions. Another means of perturbing the system is to modify the protein itself, either chemically or by site-directed mutagenesis, and to assess the effects of modification on the equilibrium or kinetic folding or refolding profiles. This approach has generated a great deal of information about small globular proteins that denature reversibly.
Kinetics of the Hydrolysis of Zeolite 4A Surface by the Pressure-jump Relaxation Method