Figure 1.

Model for proposed ground-state conformation of lac permease, which binds substrate with high affinity. For clarity, 6 of the 12 helices in the permease are shown. Glu-126 (helix IV) and Arg-144 (helix V) are charge-paired and with Cys-148 (helix V) comprise the major components of the substrate binding site. Arg-302 (helix IX) is charge-paired with Glu-325 (helix X), while protonated His-322 (helix X) interacts with Glu-269 (helix VIII). Thus, the relevant H⁺ is shared by His-322 and Glu-269. Also shown are the charge pair between Asp-240 (helix VII) and Lys-319 (helix X), which are not essential for the mechanism. See text for further details regarding the transport mechanism.