presents a comparative analysis of PFK and PFR fibrils' catalytic properties towards nitrocefin hydrolysis. Specifically, PFS, PFD, PFDK and ScFK did not exhibit β-lactam hydrolytic activity (V0 values close to zero for all these variants) likely accounting for the absence of organized fibril structure (for ScFK), or lack of / lower abundance of lysine sidechains serving as nucleophilic sites (in the case of PFS, PFD, and PFDK). Furthermore, PFS, PFD, and PFDK fibrils are anionic or zwitterionic (Figure ), thereby exhibiting electrostatic repulsion to the negatively charged nitrocefin molecules (at physiological pH).