THE EFFECT OF DENATURANTS ON PROTEIN-STRUCTURE

Virtually all studies of the protein-folding reaction add either heat, acid, or a chemical denaturant to an aqueous protein solution in orde r to perturb the protein structure. When chemical denaturants are used , very high concentrations are usually necessary to observe any change in protein structure. In a solution with such high denaturant concent rations, both the structure of the protein and the structure of the so lvent around the protein can be altered. X-ray crystallography is the obvious experimental technique to probe both types of changes. In this paper, we report the crystal structures of dihydrofolate reductase wi th urea and of ribonuclease A with guanidinium chloride. These two cla ssic denaturants have similar effects on the native structure of the p rotein. The most important change that occurs is a reduction in the ov erall thermal factor. These structures offer a molecular explanation f or the reduction in mobility. Although the reduction is observed only with the native enzyme in the crystal, a similar decrease in mobility has also been observed in the unfolded state in solution (Makhatadze G , Privalov FL. 1992. Protein interactions with urea and guanidinium ch loride: A calorimetric study. J Mol Biol 226:491-505).