Cytoplasmic view of sugar-binding site in LacY (stereo view). LacY structure with bound sugar (PDBid: 4OAA, blue trace, and colored side chains) is superposed on the LacY structure with covalently bound MTS-Gal (PDBid: 2Y5Y, gray trace and transparent side chains). TDG is shown as green sticks, and side chains forming hydrogen bonds with TDG are in yellow. Broken lines represent likely hydrogen bonds. Ala122 and Cys148, which are close to TDG but do not make direct contact, are shown in light blue. Glu325 and Arg302 are shown in pink color. Roman numbers indicate the helix number.