LacY structure. (a) Side view of the inward-facing conformation of LacY (PDB ID code: 2V8N). Helices are shown as rods and are colored according to the helix triplets (helices I–III: blue, IV–VI: green, VII–IX: orange, and X–XII: yellow). Critical residues are indicated (green and orange, residues involved in sugar binding; magenta, residues involved in translocation exclusively; gray, salt-bridged residues; and yellow, weakly salt-bridged residue; details are provided in the main text). The water-accessible surface of the cavity is shown as a light blue surface [calculated using the Computed Atlas of Surface Topography of proteins (CASTp) Web tool with a probe size of 1.4 Å]. (b) Cytoplasmic view; the color coding is the same as in A. (c) Topology of the internal structural repeats in the LacY; the helices are colored the same as in A and B blue. Inverted-topology repeats are indicated as triangles: blue: repeat unit A, green: repeat unit B, orange: repeat unit C, and yellow: repeat unit D.