Lactose-H + symport

In bacteria, accumulation of substrates against a concentration gradient can occur through two main classes of transport systems (see [30] for a summary). The prototype of the first class of transporters is the /3-galactoside permease of Escherichia coli (see [31]). It is a relatively simple system involving only a single membrane-bound protein. It catalyzes a lactose-H symport. Other transporters... 

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

Guan, L., Sahin-Toth, M. and Kaback, H. R. (2002). Changing the lactose permease of Escherichia coli into a galactose-specific symporter, Proc. Natl Acad. Sci. USA, 99, 6613-6618. 

Secondary transporters are ancient molecular machines, common today in bacteria and archaea as well as in eukaryotes. For example, approximately 160 (of approximately 4000) proteins encoded by the E. coli genome appear to be secondary transporters. Sequence comparison and hydropathy analysis suggest that members of the largest family have 12 transmemhrane helices that appear to have arisen by duplication and fusion of a membrane protein with 6 transmemhrane helices. Included in this family is the lactose permease of E. coli. This symporter uses the H+ gradient... 

Figure 13.11. Action of Lactose Permease. Lactose permease pumps lactose into bacterial cells by drawing on the proton-motive force. The binding sites evert when a lactose molecule (L) and a proton (H+) are bound to external sites. After these species are released inside the cell, the binding sites again evert to complete the transport cycle. Lactose permease is an example of a symporter.

Cotransport of sugars with H+ is especially common in bacteria but also occurs in eukaryotes. For example, the alga chlorella employs hexose / symporters. 2 xhe most investigated cotransporter is probably the lactose (lac) permease from E. coU which enables E. coli to take up lactose and other P-galactosides from very dilute solutions. 23-436 a... 

FIGURE 11-42 Lactose uptake in E. coli. (a) The primary transport of out of the cell, driven by the oxidation of a variety of fuels, establishes both a proton gradient and an electrical potential (inside negative) across the membrane. Secondary active transport of lactose into the cell involves symport of and lactose by the lactose transporter. The uptake of lactose against its concentration gradient is entirely dependent on this inflow of H", driven by the electrochemical gradient. 

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