Studies of Hydrogen Exchange in Proteins

The hydrogen exchange kinetics of chymotrypsinogen and α-chymotrypsin have been compared under conditions in which the observed exchange rate is known to be a function of thermal unfolding and refolding rates. The results are best explained by a model which (a) does not include any major refolding of the polypeptide chain after the conversion of zymogen to active enzyme and assumes that the dynamic behaviour of the native states is, as far as hydrogen exchange can see, very little influenced by the conversion, (b) includes a difference in the nature of the unfolded states, and (c) is consistent with the fact that the number of hydrogens exchanging through thermal unfolding is 25% larger for chymotrypsin. The last point indicates that the size of the structural unit involved in reversible thermal unfolding is larger for chymotrypsin than for chymotrypsinogen.