The catalytic activity induced by PFK amyloid fibrils, i.e. initial hydrolysis reaction rates extracted from the absorbance curves versus initial substrate concentrations, is presented in Figure (KCl concentrations were 0.2 M in all samples, assuring fibril formation). Importantly, the experimental data can be fitted according to two distinct catalytic models (fitted purple and orange curves, Figure ). If we consider all datapoints, a sigmoidal curve is apparent upon fitting to the Hill equation (purple curve in Figure ), generally employed for analysis of interactions between ligands and biological macromolecules . The sigmoidal curve indicated an allosteric, cooperative behavior of the amyloid fibril catalyst and nitrocefin substrates, which is also manifested in the Hill coefficient value of 7 . Indeed, sigmoidal-type activities are the hallmark of allosteric enzymes such as proteases . Allosteric behavior, however, has not been previously reported for catalytic peptides.