THE KINETIC BASIS FOR THE STABILIZATION OF STAPHYLOCOCCAL NUCLEASE BYXYLOSE

The effect of xylose on the rates of folding and unfolding of staphylo coccal nuclease (nuclease) have been investigated using fluorescence-d etected pressure-jump relaxation kinetics in order to establish the ki netic basis for the observed stabilization of nuclease by this sugar ( Frye KJ, Perman CS, Royer CA, 1996, Biochemistry 35:10234-10239). The activation volumes for both folding and unfolding and the equilibrium volume change for folding were all positive. Their values were within experimental error of those reported previously (Vidugiris GJA, Markle y JL, Royer CA, 1995, Biochemistry 34:4909-4912) and were independent of xylose concentration. The major effect of xylose concentration was to increase significantly the rate of folding. The large positive acti vation volume for folding was interpreted previously as indicating tha t the rate-limiting step in nuclease folding involves dehydration of a significant amount of surface area. A large effect of xylose on the r ate constant for folding provides strong support for this interpretati on, because xylose, an osmolyte, stabilizes the folded state of protei ns through surface tension effects. These studies further characterize the transition state in nuclease folding as lying closer to the folde d, rather than the unfolded state along the folding coordinate in term s of the degree of burial of surface area. The image of the transition state that emerges is consistent with a dry molten globule.