Abstract
Crystals of alpha-chymotrypsin (CHT) at equilibrium in solutions of 2.0 M guanidine hydrochloride and 3.0 M urea at pH 3.6 were prepared, three-dimensional X-ray intensities were measured, and difference electron-density maps were calculated and examined. The guanidine hydrochloride derivative displayed changes occurring exclusively on the surface of the protein. The difference peaks represented mostly small changes in various protein surface groups and in the adjacent solvent regions, and some displayed convincing evidence of binding of the guanidinium ion to the protein. The urea difference map likewise showed that changes had occurred on the surface of the protein, but also that numerous changes in the structure occurred in the hydrophobic interior of the CHT molecule. Further, the urea difference map contained evidence for two kinds of interactions of urea with protein groups. There are examples of bound urea either causing or accompanying structural changes and examples of urea binding with no accompanying changes to the protein. Examples of both kinds of binding were observed in both the surface regions and in the hydrophobic interior of the molecule. From an examination of these two derivatives, it is clear that guanidine hydrochloride and urea unfold proteins by different mechanisms.
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