Transporters symporters

Na+ co-transporter (symport) allows uptake of X (e.g. amino acid or glucose)... 

The interplay of H, K, Na", and the transmembrane potential and their effect on membrane transport is a complex phenomenon. Which factors are the independent variables in a transport process and which are the dependent variables is difficult to determine. However, it is well-established that transport of many amino acids, sugars, and other carbon and nitrogen sources requires the co-transport (symport) of an ion along with it and probably the reverse transport (antiport) of a similarly charged ion. 

Cox and Henick-Kling (1989, 1990) reported activity similar to that of a proton pump and ATPase in the formation of ATP during MLF. L-malate enters the cell through the action of a specific transport enzyme and is decarboxylated as described previously. To prevent proton accumulation and, eventually, cell death, it is necessary to export them continually. This is accomplished by transport (symport) of L-lactate along with a a single proton. Repeated proton translocation creates a protonmotive force, or delta-pH (A-pH), across the membrane. Reentry of protons through membrane-associated ATPase generates ATP. Theoretically, one ATP is... 

Figure 2.24 Alkali-metal chloride transport (symport) through a membrane by the receptor 2.108.

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. 

Figure 12-11. Combination of phosphate transporter ( ) with the adenine nucleotide transporter ((2)) in ATP synthesis. The H+ZP, symport shown is equivalent to the P /OH antiport shown in Figure 12-10. Four protons are taken into the mitochondrion for each ATP exported. However, one less proton would be taken in when ATP is used inside the mitochondrion.

Figure 12-13. Malate shuttle for transfer of reducing equivalents from the cytosol into the mitochondrion. Ketoglutarate transporter , glutamate/aspartate transporter (note the proton symport with glutamate).

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). 

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. 

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 transport of toluene-4-sulfonate into Comamonas testosteroni has been examined (Locher et al. 1993), and rapid uptake required growth of the cells with toluene-4-sulfonate or 4-methylbenzoate. From the results of experiments with various inhibitors, it was concluded that a toluenesulfonate anion/proton symport system operates rather than transport driven by a difference in electrical potential (A (/), and uptake could not take place under anaerobic conditions unless an electron acceptor such as nitrate was present. 

While the lactate-H+ symporter and the K+/H+ exchanger are involved in acidification of the cell, the Na+/H+ exchanger present in the basal cells exports protons out of the cell in exchange for Na+ [139]. It was observed that removal of Na+ from the Ringer s solution decreased intracellular pH by 0.5 unit in basal cells, possibly due to inhibition of the Na+/H+ exchanger. As the basal cells are the precursors for the superficial cells of the corneal epithelium, it is quite likely that similar exchange processes are also present in the superficial layer, the principal barrier to ion and drug transport [99,103],... 

Bretschneider, B., M. Brandsch, and R. Neubert. Intestinal transport of beta-lactam antibiotics analysis of the affinity at the H+/peptide symporter (PEPT1), the uptake into Caco-2 cell monolayers and the transepithelial flux. Pharm. Res. 1999, 16, 55-61. 

The recovery of neurotransmitters from synaptic clefts and their storage in cytoplasmic vesicles is accomplished by the tandem actions of the secondary transporters in plasma and vesicular membranes. Sodium-dependent symporters mediate neurotransmitter reuptake from synaptic clefts into neurons and glia, whereas proton-dependent antiporters concentrate neurotransmitters from neuronal cytoplasm into synaptic vesicles (Fig. 5-13). 

Glutamate transporters in brain are coded by five different but closely related genes, SLC1A1-4 and SLC1A6. There are several trivial names for each of the corresponding proteins. The transporters can all symport one Glu, with three Na+ and one H+, and antiport one K+ within each cycle, but they differ in their cellular expression. 

Brain capillary endothelial cells and some neurons also express a Na+-dependent D-glucose symporter, SGLT1. SGLT1 (SLC5A1) was the first characterized of the large SLC5 family of Na-dependent symporters (SSSF) which transport various solutes and ions into cells [77, 78]. SGLT1 is found mainly in the intestine, trachea,... 

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