Lactose transport in Escherichia coli cells Dependence of kinetic parameters on the transmembrane electrical potential difference |
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Authors: | Alexandre Ghazi Emanuel Shechter |
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Institution: | Laboratoire des Biomembranes (LA 04 272), Université de Paris-Sud, Département de Biochimie et de Biophysique, 91405 Orsay France |
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Abstract: | We determine the kinetic parameters and of lactose transport in Escherichia coli cells as a function of the electrical potential difference (Δψ) at pH 7.3 and . We report that transport occurs simultaneously via two components: a component which exhibits a high (larger than 10 mM) and whose contribution is independent of Δψ, a component which exhibits a low independent of Δψ (0.5 mM) but whose increases drastically with increasing Δψ. We associate these components of lactose transport with facilitated diffusion and active transport, respectively. We analyze the dependence upon Δψ of and of the active transport component in terms of a mathematical kinetic model developed by Geck and Heinz (Geck, P. and Heinz, E. (1976) Biochim. Biophys. Acta 443, 49–63). We show that within the framework of this model, the analysis of our data indicates that active transport of lactose takes place with a H+/lactose stoichiometry greater than 1, and that the lac carrier in the absence of bound solutes (lactose and proton(s)) is electrically neutral. On the other hand, our data relative to facilitated diffusion tend to indicate that lactose transport via this mechanism is accompanied by a H+/lactose stoichiometry smaller than that of active transport. We discuss various implications which result from the existence of H+/lactose stoichiometry different for active transport and facilitated diffusion. |
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Keywords: | Active transport Facilitated diffusion Lac carrier (E coli) triphenylmethyl phosphonium ion (bromide salt) CCCP PCMBS Mes |
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