Fig. 1.

Transport reactions of LacY. (A) $\mathrm{∆}{\stackrel{\sim }{\mu }}_H+$-driven influx (i.e., active transport): free energy released from the downhill translocation of H⁺ in response to $\mathrm{∆}{\stackrel{\sim }{\mu }}_{\text{H}}+$ (interior negative and/or alkaline) generated by the respiratory chain or F₁/F_o ATPase drives energetically uphill translocation of galactoside. (B) Influx: galactoside influx down a concentration gradient drives uphill H⁺ translocation with generation of $\mathrm{∆}{\stackrel{\sim }{\mu }}_{\text{H}}+$ (interior positive and acidic). (C) Efflux: galactoside efflux down a concentration gradient with generation of $\mathrm{∆}{\stackrel{\sim }{\mu }}_{\text{H}}+$ (interior negative and alkaline). (D) Imposition of ΔΨ (interior positive) during galactoside efflux: generated by influx of K⁺ down a concentration gradient in the presence of valinomycin.