Symport systems

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 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. 

The melibiose carrier MelB of E. coli is a well-studied sodium symport system. This carrier is of special interest, because it can also use protons or lithium ions for cotransport. The projection structure of MelB has been solved at 8 A resolution [107]. The 12 TM helices are arranged in an asymmetrical pattern similar to the previously solved structure of NhaA, which, however, follows an antiport mechanism (Na+ ions out of the cell and H+ into the cell). 

Figure 9.13 Examples of mitochondrial transport systems for anions. 0 The anb port system transfers malate into but oxo-glutarate out of the mitochondrion. The symport system transfers both pyruvate and protons into the mitochondrion across the inner membrane. Both transport processes are electroneutral.

Pyruvate is transported by an anion symport system (Figure 9.13). 

Figure 36. Uphill transport of K ions as a function of time with (1) double carrier membrane system and (2) a symport system (reprinted with permission from Anal. Chem. 1988, 60, 2302. Copyright 1993 American Chemical Society).

On the other hand, amino acids are transported inside the cell by different transporters. To date, 15 transport systems have been identified for amino acids in Sac-charomyces cerevisiae (Barre et al. 1998) and all of them are symport systems coupled to the entry of a proton. This proton must also be sent outside the cell in order to maintain the cellular homeostasis. Therefore, the uptake of ammonium and amino acids must be considered as active transport because it consumes ATP via H+-ATPase. 

An example of leaks can be found in the case of a solute symport system, for instance the proton-sugar symport in bacteria. In that case a protein specifically catalyzes the transport of protons and sugar across the membrane, by being able to... 

Figure 10.27 A schematic model for the sodium-glucose cotransport (symport) system. 

A carrier which transports a single molecule in one direction is called a uniport system. Alternatively, a carrier may carry two molecules simultaneously in the same direction, i.e. a symport system. TTiirdly, a carrier may exchange one molecule for another and therefore transport them in opposite directions, i.e an antiport system (Figure 9.4). 

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