Antiport systems

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). 

In Oxalobacter formigenes, oxalate and its decarboxylation product formate form a one-to-one antiport system, which involves the consumption of an internal proton during decarboxylation, and serves as a proton pump to generate ATP by decarboxylative phosphorylation (Anantharam et al. 1989). 

The Na ion concentration within the cell is maintained low, due to activity of an enzyme known as the Na+/K+ ATPase. This enzyme/carrier is present in the plasma membrane. It is an antiport system that transports three Na+ ions out of the cell and two K+ ions into the cell, for each molecule of ATP that is hydrolysed (Figure 5.10). It is responsible for maintaining a low Na+ ion concentration but a high K+ ion concentration within the ceU. Its constant activity in many it not all cells requires constant ATP hydrolysis, which accounts for more than 10% of the resting energy expenditure of an adult. 

That is, the cotransporter can pump glucose inward until its concentration within the epithelial cell is about 9,000 times that in the intestine. As glucose is pumped from the intestine into the epithelial cell at the apical surface, it is simultaneously moved from the cell into the blood by passive transport through a glucose transporter (GLUT2) in the basal surface (Fig. 11-44). The crucial role of Na+ in symport and antiport systems such as these requires the continued outward pumping of Na+ to maintain the transmembrane Na+ gradient. 

FIGURE 20-15 The Pi-triose phosphate antiport system of the inner chloroplast membrane. This transporter facilitates the exchange of cytosolic P for stromal dihydroxyacetone phosphate. The products of photosynthetic carbon assimilation are thus moved into the cytosol... 

The term ion pump, synonymous with active ion-transport system, is used to refer to a protein that translocates ions across a membrane, uphill against an electrochemical potential gradient. The primary pumps do so by utilization of energy derived from various types of chemical reactions such as ATP hydrolysis, electron transfers (redox processes), and decarboxylations, or from the absorption of light (Table 1). Secondary pumps are symport and antiport systems that derive the energy for uphill movement of one species from a coupled downhill movement of another species. The electrochemical gradient driving the latter movement is often created by a primary pump. 

Viniegra, S., Cragoe, EJ. Jr., Rabito, C.A. (1992). Heterogeneity of the Na+/H+ antiport systems in renal cells. Biochim. Biophys. Acta 1106,99-109. 

Figure 6.8. Digitalis (foxglove) glycosides, a Mode of action. The drags inhibit the Na /K -ATP ase. The increased intracellular sodium concentration will reduce the activity of the Na /Ca antiport system and therefore lead to an increase of intracellular Ca and augment myofilament contraction, b Structures.

H. Maegawa, M. Kato, K.-I. Inui, and R, Hori, pH sensitivity of H+/organic cation antiport system in rat renal brush-border membranes, /. Bin/. Chem., 263 11,150-11,154 (1988). 

ATP is transported from the mitochondrial matrix to the cytosol in exchange for ADP (the ATP-ADP antiport system). 

Figure 9.4 Transport proteins acting as uniport, symport, or antiport systems.

Figure 9.7 Intestinal peptide transport. Peptides are taken upinto enterocytes together with H+ ions. The proton gradient is maintained via an Na + /H+ antiport system in the apical cell membrane. The Na+ gradient is guaranteed by the Na + /I<+-ATPase in the basolateral cell membrane.

Certain glycolytic enzymes appear to be target proteins for protein-tyrosine kinases. They may be responsible for the increased rate of glycolysis in transformed cells. The protein component of ion pumps may also be involved activation of Na+/H+ antiport systems causing mild alkalinization of the cells may play a role in stimulating mitosis. 

FIGURE 20-15 The P -triose phosphate antiport system of the inner... 

A common product resulting from glutathione conjugation is a mercapturic acid as shown in Figure 8.39. Mercapturic acids are excreted by the renal organic anion antiport system in the proximal tubular epithe-lia of the kidney. 

Chanson A (1991) A Ca /H antiport system driven by the tonoplast pyrophosphate-dependent proton pump from maize roots. J Plant Physiol 137 471-476. 

Symport and antiport systems can move one of the transported solutes against a gradient. 

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