Stability, Flexibility, and Catalytic Activity

In addition to these complications in interpreting H/D exchange data, it must be bom in mind that hydrogen exchange provides a static measure of protein flexibility proteins in solution exist as an ensemble of different conformations. The population of each conformation is determined by its Gibbs free energy according to the standard statistical thermodynamic relationship 

Other measures of protein flexibility have been found to correlate with thermal stability. One is resistance to proteolysis (Daniel et al., 1982 Fontana, 1988). Another is the quenching of buried tryptophan fluorescence by acrylamide, used in a study by Varley and Pain (1991). Both these processes are mediated by the same combination of local and global unfolding events that determine rates of hydrogen exchange. Their rates will depend on the ability of another molecule, acrylamide or a proteolytic enzyme, to penetrate into normally buried regions of the protein in order to either quench fluorescence or cleave peptide bonds. 

We have very briefly surveyed the types of protein motions thought to be associated with substrate recognition and catalysis. There is no a priori reason to expect that any of the structural fluctuations discussed 

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