Proton/sugar symport

In transportation, leaks can be found in the proton-sugar symport in bacteria where a protein mediates the transport of protons and sugar across the membrane, and adding a protonophore, a parallel pathway occurs, causing a leak in the transport. 

Page, M., et al. (1988). The Effects of pH on Proton Sugar Symport Activity of the Lactose Permease Purified horn Escherichia coli," J. Biol. Chem. 263 15897-15905. 

Transport systems can be described in a functional sense according to the number of molecules moved and the direction of movement (Figure 41-10) or according to whether movement is toward or away from equilibrium. A uniport system moves one type of molecule bidirectionally. In cotransport systems, the transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. A symport moves these solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+ -sugar transporters (for glucose and certain other sugars) and Na -amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions (eg, Na in and Ca out). 

Most transporters are proteins. Small proteins can bind some substance on one side of a membrane, diffuse across the membrane, and then release that substance on the other side. Such mobile carriers may bind a single substance, or they may bind two different substances, like the proton-solute symporter portrayed in Figure 3-l4a. Candidates for transport by a proton symporter in plants include inorganic ions such as Cl- and metabolites such as sugars and amino acids. Many substances apparently move in pores or channels, which can be membrane-spanning proteins. Some channels can have a series of binding sites, where the molecule or molecules transported go from site to site through the membrane (Fig. 3-l4b). As another... 

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. 

The galactose, arabinose and xylose transporters of E. coli The bacterium E. coli possesses at least 7 proton-linked, active transport systems for sugars (for a recent review see [212]). Three of these transporters, which catalyze the uptake of L-arabinose, D-xylose and D-galactose by symport with protons, are related in sequence to the sugar transporters discussed above. They probably represent the best-characterized of the non-mammalian transporters, and so are discussed here in some detail. 

This is a typical case of what we prefer to call a leak . The two pathways (symport and leak) are completely independent and, therefore, can be described as two elemental reactions in the MNET treatment. In fact, the intrinsic passive permeability of the membrane for protons as well as for the sugar constitutes a leak that is always present the importance of that leak can only be assessed by experiment. 

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