Anion-cation symport

Cation-anion cotransport was effected by an optically active macrotricyclic cryp-tand that carried simultaneously an alkali cation and a mandelate anion and displayed weak chiroselectivity [4.23a], as did also the transport of mandelate by an optically active acyclic ammonium cation [6.39]. Employing together a cation and an anion carrier should give rise to synergetic transport with double selection by facilitating the flow of both components of a salt (see the electron-cation symport below). Selective transport of amino acids is effected by a convergent dicarboxylic acid receptor [4.24b]. 

Fig. 9. Schematic presentation of four secondary transport processes. (1) Passive transport of a neutral solute (2) passive transport of a cation (3) facilitated transport of an anion in symport with one proton (4) facilitated transport of a neutral solute in symport with one proton. For each process the driving force and the steady state accumulation level of the solute are indicated.

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. 

One of the many families included in the MFS is the SLC (solute carrier) superfamiiy. The SLC superfamiiy represents approximately 300 genes in the human genome that encode for either facilitated transporters or secondary active symporters or antiporters. Members of the SLC superfamiiy transport various ionic and nonionic endogenous compounds and xenobiotics. The SLC22 family includes anion and cation transporters (Organic Anion Transporters, OATs Organic Cation Transporters, OCTs). 

HPTS is a pH-sensitive fluorophore (pk, 7.3) [6]. The opposite pH sensitivity of the two excitation maxima permits the ratiometric (i.e. unambiguous) detection of pH changes in double-channel fluorescence measurements. The activity of synthetic ion channels is determined in the HPTS assay by following the collapse of an applied pH gradient. In response to an external base pulse, a synthetic ion channel can accelerate intravesicular pH increase by facilitating either proton efflux or OH influx (Fig. 11.5c). These transmembrane charge translocations require compensation by either cation influx for proton efflux or anion efflux for OH influx, i.e. cation or anion antiport (Fig. 11.5a). Unidirectional ion parr movement is osmotically disfavored (i.e. OH /M or X /H symport). HPTS efflux is possible with pores only (compare Fig. 11.5b/c). Modified HPTS assays to detect endovesiculation (Fig. 11.1c) [16], artificial photosynthesis [17] and catalysis by pores [18] exist. 

When the transported molecule and cotransported Ion move in the same direction, the process Is called symport when they move in opposite directions, the process is called antiport (see Figure 7-2). Some cotransporters transport only positive ions (cations), while others transport only negative ions (anions). An important example of a cation cotransporter is the Na /H antiporter, which exports from cells coupled to the energetically favorable Import of Na. An example of an anion cotransporter is the AEl anion antiporter protein, which catalyzes the one-for-one exchange of Cl and HCOs across the plasma membrane. Yet other cotransporters mediate movement of both cations and anions together. In this section, we describe the operation and physiological role of several widely distributed symporters and antiporters. 

In this way, a proton gradient arises which drives the ion uptake into the cell by means of the following mechanism. The ion to be taken up is loaded outside with H and is thereby drawn inwardly by the OH . In the case of anions, this is only possible if more protons are transported than negative charges. The cation uptake does not suffer from such problems, but there is sometimes a symport together with H" from outside to inside. So-called transporters assist in this process. These are incorpo-... 

Picrate salts are most convenient for the detection of cation/anion symport, which can be readily extended to the detection of cation antiport. The yellow picrate salts are added to the cis buffer, whereas a colorless salt is placed in the trans buffer. During cation antiport, picrate is transported as hydrophobic counteranion of the carrier-cation complex across the bulk membrane, and the increase in absorption of picrate in trans buffer readily indicates the velocity of the process. Caution Picrates are explosives they should be handled in small portions only.)... 

Coulombic considerations dictate that transport processes needs to be electroneutral, or nearly so. Therefore, in the case of carriers the diffusion of irais across the membrane is expected to occur via two limiting mechanism referred to as symport or co-transport, and antiport or exchange. Symport/co-transport occurs when both cations and anions (with the exact relative stoichiometry determined by charge considerations) are transported concurrently and in the same direction. In contrast, antiport/exchange processes involve the simultaneous transport of two ions in opposite directions (cf. Fig. 12.19). To date, most work has focused on anion transporters that do not allow for the concurrent complexation of a countercation. However, recently a few reports have appeared wherein ditopic receptors were employed as transporters. 

The use of an external cationic transporter to assist in the transport of anions by certain carriers has been explored by Gale and others recently. This was demonstrated by Gale in the case the triazole-strapped calix[4]pyrrole 36, which was combined with the potassium carrier valinomycin. The combination led to enhanced transport of chloride anions across POPC membranes (cf. Fig. 12.23) [71]. The overall transport process was consistent with a KVC1 symport mechanism. This dual host approach was employed by Sessler, Gale, Shin, and coworkers in the case of the pyridine-diamide strapped caUx[4]pyrrole Cl transporter 37 (cf. Fig. 12.23) [72]. It was found that the combination of 37 and... 

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