Lactose transport in Escherichia coli: effect of transmembrane potential difference on apparent substrate affinity.

The lac carrier of Escherichia coli enables lactose to pass across the cytoplasmic membrane and to accumulate within the cell. The energy cost of concentrative sugar uptake is borne by the transmembrane proton free-energy gradient (A&+) as envisaged by Mitchell (1977) in the form of pH and electrical potential gradients (ApH and Av). Obligate 1 : 1 proton/galactoside symport via the permease couples the lactose and proton free-energy gradients (West, 1970; West & Mitchell, 1973). Although the energetics of substrate accumulation is now much clearer (Ramos et al., 1976; Ramos & Kaback, 1977a,b,c; Booth el al., 1979), the mechanism of carrier function has been elusive in spite of more than 20 years of kinetic studies (Kennedy, 1970; Harold, 1977). In a simple model for galactoside/proton symport, the following elementary steps may be considered: Binding of substrates