The structure on the right shows the scaffold of the binding complexes used for FEP/λ-REMD simulations. Protein backbones are shown in silver new cartoon mode. In noncovalent complex, the catalytic cysteine is shown in CPK mode with atom color code: yellow sulfur, red oxygen, blue nitrogen, and cyan carbon. Ligand 4 is shown in licorice mode with the same color code. In covalent complex, the catalytic cysteine is covalently linked to the ligand warhead. All hydrogen atoms are omitted for clarity.