The sialic acid binding site and the three pharmacophore filters defined for neuraminidase A. A) Site points, shown as red-dotted spheres, indicate the required positions of hydrogen bond-accepting or negatively charged atoms to interact with arginine residues in the acid binding sub-pocket. B) The required position of an atom forming a hydrogen bond to Arg 152 (red-dotted sphere) along with a hydrophobic interaction (yellow-dotted sphere). C) Blue-dotted spheres indicate the space available for atoms forming hydrogen bonds or salt bridges to three glutamate residues. This figure was generated using PyMOL .