Fig. 4.

Connection between the ligand-binding site and residues known to be involved in protonation/deprotonation during the transport cycle are indicated. The figure is in crossed-eyed stereo. The cytoplasmic surface is upward, and the normal to the membrane plane is approximately vertical. The figure shows the hydrophobic contact of the galactoside ring with the indole ring of Trp-151. O2 and O3 of the galactose are hydrogen-bonded to the imidazole of His-322 that in turn forms hydrogen bonds with Glu-325 and Tyr-236. The hydroxyl of Tyr-236 is also acceptor to a hydrogen bond from Arg-302.