Ion antiport

The difference in the hydrogen ion electrochemical potential, formed in bacteria similarly as in mitochondria, can be used not only for synthesis of ATP but also for the electrogenic (connected with net charge transfer) symport of sugars and amino acids, for the electroneutral symport of some anions and for the sodium ion/hydrogen ion antiport, which, for example, maintains a low Na+ activity in the cells of the bacterium Escherichia coli. 

Cl cotransporter), (5) ion antiports (Na /H exchange), (6) facilitated diffusion (glucose via CLUT-1), (7) active transport (P-gp), (8) active antiport transport (Na /K ATPase), and (9) endocytosis (receptor as insulin or transferrin or adsorption mediated). Adapted from Huber etal. [47]. 

Transporters genes encode proteins, generally constituted by 12 transmembrane spanning regions. These mediated Na+ or H+ dependent accumulation of small molecules such as neurotransmitters, antibiotics, ions and cationic amino-acid transporters into the cells or organelles. The transport is performed by different mechanisms uniport, substrate-ion sym-port, substrate-ion antiport, substrate-substrate or ion-ion antiport, and ATP-dependent translocation. 

Two genes, rebU and rebT, could participate in rebeccamycin resistance and/or secretion. The deduced product of rebU is similar to a family of Na /H+ exchange membrane proteins which function as antiporters of Na" " or K" " and H+ and play a key role in maintaining cellular pH and other processes. RebU was found to be most similar to a putative integral membrane ion antiporter and antibiotic transporter. The second candidate for rebeccamycin resistance and/or secretion, the rebT gene... 

Jezek, R Mahdi, E GarUd, K. D. Reconstitution of the beef heart and rat liver mitochondrial potassium/ hydrogen ion (sodium/hydrogen ion) antiporter quantitation of potassium transport with the novel... 

Garlid, K. D. Shariat-Madar, Z. Nath, S. Jezek, P. Reconstitution and partial puriflcation of the sodium-selective sodium/hydrogen ion antiporter of beef heart mitochondria. J. Biol. Chem. 1991, 266, 6518-6523. 

Figure 16.S illustrates the reactions and the com-partmentalization of the enzymes of the urea cycle. The first reaction in urea biosynthesis is the mitochondrial formation of carbamoyl phosphate, the substrate of the urea cycle. The reaction utilizes an ammonium (NH4 ) ion, delivered into the mitochondrion as glutamate by the action of both the glutamate-aspartate (Section 11.3) and the glutamate-hydroxyl ion antiport carriers. Oxidative deamination of glutamate by glutamate dehydrogenase releases an NH4 ion.

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. 

In erythrocytes and most other cells, the major structural link of plasma membranes to the cytoskeleton is mediated by interactions between ankyrin and various integral membrane proteins, including Cf/HCOj antiporters, sodium ion pumps and voltage-dependent sodium ion channels. Ankyrin also binds to the =100 nm, rod-shaped, antiparallel a(3 heterodimers of spectrin and thus secures the cytoskeleton to the plasma membrane. Spectrin dimers self-associate to form tetramers and further to form a polygonal network parallel to the plasma membrane (Fig. 2-9D). Neurons contain both spectrin I, also termed erythroid spectrin, and spectrin II, also termed fodrin. Spectrin II is found throughout neurons, including axons, and binds to microtubules, whereas spectrin I occurs only in the soma and dendrites. 

ATP-dependent Ca2+ pumps and Na+,Ca2+ antiporters act in concert to maintain a low concentration of free cytosolic Ca2+. The concentration of cytosolic free calcium ion, [Ca2+] , in unstimulated cells is between 10 8 and 1 O 7 mol/1, which is more than 10,000-fold lower than extracellular free Ca2+. Most intracellular Ca2+ is stored in... 

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 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...

These methods of solute transfer usually rely on a relatively low intracellular concentration of the solute of interest, so that it will readily diffuse into the cell down the electrochemical gradient (as in the case of ion channels). Alternatively, the solute may be moved into the cell using chemical energy derived from another solute moved in the same direction (co-transport) or opposite direction (countertransport) on the carrier protein (symporters and antiporters respectively). The transfer of the second solute is in turn dependent on an inward electrochemical gradient. Ultimately, these gradients are established by primary, energy-requiring solute pumps (e.g. ATPases), which, on most epithelia, are located on the basolateral/serosal membrane (see Section 5.2 for discussion of ATPases). 

EXPORT FROM THE CELL TO THE BLOOD VIA ION CHANNELS AND ANTIPORTERS... 

The methods of solute transfer across the serosal/basolateral membrane can include ion channels and antiporters similar to those described earlier. In the case of serosally located cation channels, these primarily work because the intracellular electrolyte concentration is high enough to overcome the electrical gradient (e.g. some K+ channels). For anion channels, the negative charge inside the cell compared with the blood will help drive (repel) anions from the cell (e.g. CL efflux on voltage-sensitive channels in the intestine [58]). In the case of antiporters, the operation is fundamentally the same as that used in the mucosal membrane, except that the driving force is derived from an ion... 

Antiporter, a secondary ion transporter that moves a solute against its electrochemical gradient by using energy derived from the movement of another solute in the opposite direction down its electrochemical gradient. Antiporters are also called exchangers, and the exchange process is sometimes referred to as counter transport. 

An antiport simultaneously transports two molecules (or in this case, ions) simultaneously in opposite directions. 

The two ammonium ions produced from glutamine as illustrated in Figures 8.4 to 8.6 are secreted into the PCT lumen the by a Na+/H+ antiport (the NH4+ substitutes for H+). Subsequent metabolism of 2-oxoglutarate has the potential to generate two bicarbonate ions from the hydration of carbon dioxide by carbonic anhydrase ... 

Bicarbonate ions secreted into the blood stream help maintain the normal plasma bicarbonate concentration of approximately 25 mmol/1, whilst the two protons are secreted into the lumen of the proximal tubule in exchange for sodium via a Na+/H+ antiport. 

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