The EVB reaction profiles for the reference reaction in water and in glucagon fibril enzyme are shown in Figure . The energetics for the concerted PT-NA reference reaction in water was obtained using a distance constraint of 10 kcal/mol/Å 2 at 3.5 Å between the O and N δ atoms of Ser118 and His117 for the PT, and between the O of Ser118 and the carbonyl carbon atom of the ester group for the NA. In contrast, within the fibril, we applied a 2 kcal/mol/Å 2 distance constraint for the same atom pairs.