Symport processes

An oppositely charged ion is transported in the same direction (known as a symport process) (Figure 9.13). 

Copper-sulfide cluster 884s Coproporphyrin III 843,845s Comified cell envelope 439 Corrin in transmethylation 592 Corrin ring 867, 868 Corrinoid-dependent synthesis of acetyl-CoA 876, 877 Cosmarium 22 COSY-NOESY diagram 143 Cotransport (symport) process 411,416,417 Coulomb 283... 

Fig. 4. Ion-driven cotransport mechanisms, (a) Symport process involving a symporter (e.g. Na+/glucose transporter) (b) antiport process involving an antiporter (e.g. erythrocyte band 3 anion transporter).

Ion pair receptors are useful in mimicking biological symport processes - the simultaneous transport of oppositely charged ions in the same direction. 

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. 

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],... 

HMIT is a H+-coupled myo-inositol symporter. High levels of its expression are observed in neurons and glia of hippocampus, hypothalamus, cerebellum and brainstem. Since myo-inositol is a precursor for phosphatidyl inositol, which itself is a critical regulator of many neuronal processes (Ch. 20), HMIT regulation is possibly involved in various mood and behavior patterns that are affected by inositol metabolism and by pharmacologic agents that modify inositol metabolism (see Chs 54 and 55). 

Figure 1. Solute transfer across an idealised eukaryote epithelium. The solute must move from the bulk solution (e.g. the external environment, or a body fluid) into an unstirred layer comprising water/mucus secretions, prior to binding to membrane-spanning carrier proteins (and the glycocalyx) which enable solute import. Solutes may then move across the cell by diffusion, or via specific cytosolic carriers, prior to export from the cell. Thus the overall process involves 1. Adsorption 2. Import 3. Solute transfer 4. Export. Some electrolytes may move between the cells (paracellular) by diffusion. The driving force for transport is often an energy-requiring pump (primary transport) located on the basolateral or serosal membrane (blood side), such as an ATPase. Outward electrochemical gradients for other solutes (X+) may drive import of the required solute (M+, metal ion) at the mucosal membrane by an antiporter (AP). Alternatively, the movement of X+ down its electrochemical gradient could enable M+ transport in the same direction across the membrane on a symporter (SP). A, diffusive anion such as chloride. Kl-6 refers to the equilibrium constants for each step in the metal transfer process, Kn indicates that there may be more than one intracellular compartment involved in storage. See the text for details...

Amino add reabsorption in the renal tubules Amino acids are small, easily filtered molecules. Efficient reabsorption mechanisms are vital to conserve amino acids which are metabolically valuable resources. Transport of individual amino acids and small peptides is symport carrier mediated mechanisms in which sodium is co-transported. The process is indirectly ATP dependent because Na is returned to the lumen of the nephron by the sodium pump , Na+/K+ dependent ATPase. 

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.

A second membrane transport system essential to oxidative phosphorylation is the phosphate translocase, which promotes symport of one H2PO4 and one H+ into the matrix. This transport process, too, is favored by the transmembrane proton gradient (Fig. 19-26). Notice that the process requires movement of one proton from the P to the N side of the inner membrane, consuming some of the energy of electron transfer. A complex of the ATP synthase and both translocases, the ATP synthasome, can be isolated from... 

Alternatively, both the first and the second solutes may pass through the membrane bound to the same carrier (cotransport or symport). Another form of active transport is group translocation, a process in which the substance to be transported undergoes covalent modification, e.g., by phosphorylation. Tire modified product enters the cell and within the cell may be converted back to the unmodified substance. Transport processes, whether facilitated or active, often require the participation of more than one membrane protein. Sometimes the name permease is used to describe the protein complexes utilized. 

Because of its cyclic nature, this process presents analogies with molecular catalysis it may be considered as physical catalysis operating a change in location, a translocation, on the substrate, like chemical catalysis operates a transformation into products. The carrier is the transport catalyst which strongly increases the rate of passage of the substrate with respect to free diffusion and shows enzyme-like features (saturation kinetics, competition and inhibition phenomena, etc.). The active species is the carrier-substrate supermolecule. The transport of substrate Sj may be coupled to the flow of a second species S2 in the same (symport) or opposite antiport) direction. 

Electron-cation symport has been realized in a double carrier process where the coupled, parallel transport of electrons and metal cations was mediated simultaneously by an electron carrier and by a selective cation carrier [6.47]. The transport of electrons by a nickel complex in a redox gradient was the electron pump for driving the selective transport of K+ ions by a macrocyclic polyether (Fig. 12). The pro-... 

A case of special interest is that of the transport of divalent ions such as calcium versus monovalent ones. The lipophilic carrier 86, containing a single cation receptor site and two ionizable carboxylic acid groups, was found to transport selectively Ca2+ in the dicarboxylate form and K+ when monoionized, thus allowing pH control of the process. This striking change in transport features as a function of pH involves pH regulation of Ca2+-K+ selectivity in a competitive (Ca2+, K+) symport... 

Light-driven (electron, cation) symport occurs when combining this system with the (nickel complex, macrocycle) process described above [6.62]. Photocontrol of ion extraction and transport has been realized with macrocyclic or acyclic ligands (containing, for instance, azo or spirobenzopyran groups) that undergo a reversible... 

Calcium levels are believed to be controlled in part at least by the uptake and release of Ca2+ from mitochondria.172"174 The capacity of mitochondria for Ca2+ seems to be more than sufficient to allow the buffering of Ca2+ at low cytosol levels. Mitochondria take up Ca2+ by an energy-dependent process either by respiration or ATP hydrolysis. It is now agreed that Ca2+ enters in response to the negative-inside membrane potential developed across the inner membrane of the mitochondrion during respiration. The uptake of Ca2+ is compensated for by extrusion of two H+ from the matrix, and is mediated by a transport protein. Previous suggestions for a Ca2+-phosphate symport are now discounted. The possible alkalization of the mitochondrial matrix is normally prevented by the influx of H+ coupled to the influx of phosphate on the H - PCV symporter (Figure 10). This explains why uptake of Ca2+ is stimulated by phosphate. Other cations can also be taken up by the same mechanism. 

The transport process is unclear. In some cases it is inhibited by uncoupling agents, suggesting that uptake is linked to the electrochemical proton gradient, while evidence has also been found for a symport mechanism involving ferrichrome and Mg2+.1201... 

Active transport of a molecule across a membrane against its concentration gradient requires an input of metabolic energy. In the case of ATP-driven active transport, the energy required for the transport of the molecule (Na+, K+, Ca2+ or H+) across the membrane is derived from the coupled hydrolysis of ATP (e.g Na+/K+-ATPase). In ion-driven active transport, the movement of the molecule to be transported across the membrane is coupled to the movement of an ion (either Na+ or H+) down its concentration gradient. If both the molecule to be transported and the ion move in the same direction across the membrane, the process is called symport (e.g. Na+/glucose transporter) if the molecule and the ion move in opposite directions it is called antiport (e.g. erythrocyte band 3 anion transporter). 

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