In summary, we successfully developed a new catalysis system, CASL, by exploiting the tight binding of amyloids with the ligands. Amino groups attached to the amyloid-binding motifs were activated upon proximity to amyloid catalysts, promoting amine modifications in acidic buffers. Crystallographic analysis and amide-to-ester substitution of the amyloid catalysts suggested that the carbonyl oxygen of the Phe-Gly amide bond of the catalyst played a key role in activating the substrate amine via hydrogen bonding. Using CASL, we achieved the selective conversion of substrates possessing equivalently reactive functional groups and the introduction of a new reaction scope (various amine modifications) in the catalytic conversion using amyloids. The substrate-selective conversion ability of CASL significantly expands the applicability of catalytic amyloids in diverse research fields, including chemical biology and materials science.