PB-3 bound to several PLY mutants as well, binding affinity and stability of the protein-ligand complex were considerably reduced for the PLY-C428A mutant. These findings suggest a noncovalent, rapidly reversible binding mode of PB-3 to the mutant PLY-C428A and the formation of a more stable complex with wildtype PB-3 with slow off-kinetics and a long residence time.