Active-site water molecules and surface electrostatics. (a) Residues making contact with the four waters present in the active-site cavity. The catalytic residues are in yellow, noncatalytic residues are in green and the mesh shows the surface of the molecule. (b) Electrostatic surface potentials of the catalytic domain. Two views rotated by 180° are shown, with the position of the active-site Cys178 in yellow.