Sodium-substrate cotransport

Wilson, T. H. and Ding, P. Z. (2001). Sodium-substrate cotransport in bacteria, Biochim. Biophys. Acta - Bioenerg., 1505, 121-130. 

Kouzuki, H., Suzuki, H., Ito, K., Ohashi, R., Sugiyama, Y., Contribution of sodium taurocholate cotransporting polypeptide to the uptake of its possible substrates into rat hepatocytes, J. Pharmacol. Exp. 

Cotransporters use the sodium or hydrogen ion gradient to drive transport of a substrate. Many cotransporters have been described, cloned, sequenced, and expressed. The sodium-glucose cotransporter just described is one of these. 

The outer membrane, the plasmalemma, efficiently protects the cell from the environment while, at the same time, carrying out functions important for cell metabolism the uptake of substrates and the elimination of toxic compounds. Substrate exchange with the environment is controlled by transport proteins embedded in the membrane (energy-requiring pumps such as Na+,K+-ATPase, or other transport units such as the Na+/glucose cotransporter and sodium and calcium ion channels) [1],... 

The biotin-dependent decarboxylases of anerobic microorganisms are transmembrane proteins. In addition to their roles in the metabolism of ox-aloacetate, methylmalonyl CoA, and glutaconyl CoA, they serve as energy transducers. They tr ansport 2 mol of sodium out of the cell for each mole of substrate decarboxylated. The resultant sodium gradient is then used for active tremsport of substrates by sodium cotransport systems, or maybe used to drive ATP synthesis in a similar manner to the proton gradient in mammalian mitochondria (Buckel, 2001). 

While the characteristics of this type of transport have been studied and identified in a number of cell systems and for a number of substances (Table 6), little is known of the molecular mechanism involved, although kinetic analysis of facilitated diffusion may be found (Stein, 1967 Neame and Richards, 1972). However, because the proposed mechanism involves a reversible reaction of the substrate with a membrane carrier to form a complex which traverses the membrane and releases the substrate at the other side, it may be that facilitated diffusion mechanisms do not differ from those involved in active transport of the same molecule (Csaky, 1965 Wilbrandt, 1972). Specifically, the active transport of D-glucose may simply require the presence and cotransport of sodium ions (Crane, 1962 Stein, 1967). This theory has recently received support from studies of Na+-gradient-dependent uptake of o-glucose by isolated intestinal and renal brush border membranes (Murer and Hopfer, 1974 Kinne et al., 1975). However, the elec-trogenic nature of D-glucose transport is probably more accurately class-... 

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