title: Mechanism of the melibiose porter in membrane vesicles of Escherichia coli.
abstract: The melibiose transport system of Escherichia coli catalyzes sodium--methyl 1-thio-beta-D-galactopyranoside (TMG) symport, and the cation is required not only for respiration-driven active transport but also for binding of substrate to the carrier in the absence of energy and for carrier-mediated TMG efflux. As opposed to the proton--beta-galactoside symport system [Kaczorowski, G. J., & Kaback, H. R. (1979) Biochemistry 18, 3691], efflux and exchange of TMG occur at the same rate, implying that the rates of the two processes are limited by a common step, most likely the translocation of substrate across the membrane. Furthermore, the rate of exchange, as well as efflux, is influenced by imposition of a membrane potential (delta psi; interior negative), suggesting that the ternary complex between sodium, TMG, and the porter may bear a net positive charge. Consistently, energization of the vesicles leads to a large increase in the Vmax for TMG influx, with little or no change in the apparent Km of the process. It is proposed that the sodium gradient (Na+out < Na+in) and the delta psi (interior negative) may affect different steps in the overall mechanism of active TMG accumulation in the following manner: the sodium gradient causes an increased affinity for TMG on the outer surface of the membrane relative to the inside and the delta psi facilitates a reaction involved with the translocation of the positively charged ternary complex to the inner surface of the membrane.
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004187:Disaccharides / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D007700:Kinetics; D008553:Melibiose / Q000378:metabolism*; D008564:Membrane Potentials; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008756:Methylgalactosides / Q000378:metabolism; D011485:Protein Binding; D012964:Sodium / Q000494:pharmacology; D027981:Symporters; D013862:Thiogalactosides / Q000378:metabolism
chemical_list: D004187:Disaccharides; D026901:Membrane Transport Proteins; D008756:Methylgalactosides; D027981:Symporters; D013862:Thiogalactosides; C009468:thiomethylgalactoside; C021548:melibiose permease; D008553:Melibiose; D012964:Sodium