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. 2000 Jul 4;97(16):8938–8943. doi: 10.1073/pnas.140224497

Figure 1.

Figure 1

Secondary structure of lac permease. The one-letter amino acid code is used, and putative transmembrane helices are shown in boxes. Residues in the central cytoplasmic loop are shown. Residues that are irreplaceable with respect to active transport are enlarged—Glu-126 (helix IV) and Arg-144 (helix V) are critical for substrate binding, and Glu-269 (helix VIII), Arg-302 (helix IX), His-322 (helix X), and Glu-325 (helix X) are essential for H+ translocation and coupling. The charge pairs Asp-237 (helix VII)/Lys-358 (helix XI) and Asp-240 (helix VII)/Lys-319 (helix X) are also shown.