MEASUREMENT OF THE BETA-SHEET-FORMING PROPENSITIES OF AMINO-ACIDS

SEVERAL model systems have been used to evaluate the alpha-helical pro pensities of different amino acids(1-7). In contrast, experimental qua ntitation of beta-sheet preferences has been addressed in only one mod el system, a zinc-finger peptide(8). Here we measure the relative prop ensity for beta-sheet formation of the twenty naturally occurring amin o acids in a variant of the small, monomeric, beta-sheet-rich, IgG-bin ding domain from protein G. Amino-acid substitutions were made at a gu est site on the solvent-exposed surface of the beta-sheet. Several cri teria were used to establish that the mutations did not cause signific ant structural changes: binding to the Fc domain of IgG, calorimetric unfolding and NMR spectroscopy. Characterization of the thermal stabil ities of these proteins leads to a thermodynamic scale for beta-sheet propensities that spans a range of similar to 2 kcal mol(-1) for the n aturally occurring amino acids, excluding proline. The magnitude of th e differences suggests that beta-sheet preferences can be important de terminants of protein stability.