The proton electrochemical gradient induces a kinetic asymmetry in the symport cycle of LacY.

The findings reveal that ∆ μ ∼ H + induces an asymmetry in the transport cycle based on the following observations: 1) the efflux rate of WT LacY exhibits a pK app of ∼7.2 that is unaffected by the imposed ΔΨ; 2) ΔΨ increases the rate of efflux at all tested pH values, but enhancement is almost 2 orders of magnitude less than observed for influx; 3) mutant Glu325 ˗ Ala does little or no efflux in the absence or presence of ΔΨ, and ambient pH has no effect; and 4) the effect of ΔΨ (interior positive) on the K m for efflux is almost insignificant relative to the 50- to 100-fold decrease in the K m for influx driven by ΔΨ (interior negative). membranes transport permease membrane proteins efflux HHS | National Institutes of Health (NIH) 100000002 GM120043 H.Ronald Kaback NSF | BIO | Division of Molecular and Cellular Biosciences (MCB) 100000

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Therefore, ΔΨ (interior positive) has only a borderline significant effect on the K m for lactose efflux.

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