Na+ symport

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. 

Neutral amino acids Na+ (symport) Eukaryotic cells and bacteria... 

Predict the effects of the following on the initial rate of glucose transport into vesicles derived from animal cells that accumulate this sugar by means of Na+ symport. Assume that initially A P = 0, ApH = 0 (pH = 7), and the outside medium contains 0.2MNa+, whereas the vesicle interior contains an equivalent amount of K+. 

In E. coli, lactose is taken up by means of proton symport, maltose by means of a binding (ABC-type) protein-dependent system, melibiose by means of Na+ symport, and glucose by means of the phosphotransferase system (PTS). Although this bacterium normally does not transport sucrose, suppose that you isolated a strain that does. How do you determine whether one of the four mechanisms just listed is responsible for sucrose transport in this mutant strain ... 

Sucrose uptake should be inhibited by a proton iono-phore if uptake is by a proton symport. If a proteinbinding system was operational, membrane vesicles or cells subjected to osmotic shock would be defective in uptake. If a Na+ symport was involved, uptake would be dependent on extracellular Na+. If a PTS was operational, sucrose phosphorylation would be dependent on PEP and not ATP in a crude cell extract. 

Amino acids, dipeptides, and some tripeptides are transported from the lumen of the intestine through the membrane of the brush border of the mucosal cells and into the cytoplasm, where the peptides are hydrolyzed to amino acids. Transport of peptides and amino acids is active and analogous to glucose transport i.e., they are transported, together with Na+, across the gut-cell membrane by specific transport proteins called Na+ symports. Between the gut lumen and the cytoplasm of the cell there is a concentration gradient of Na+ that is maintained by Na+/K+ ATPase at the base of the cell adjacent to the blood capillaries this Na+/K ATPase pumps Na+ from the cell into the blood. 

Neurotransmitter/Na+ symporter family Serotonin/Na+ symporter Paroxetine (inhibitor)... 

With the exception of acetylcholine, all the neurotransmitters shown in Figure 7-41 are removed from the synaptic cleft by transport into the axon terminals that released them. Thus these transmitters are recycled Intact, as depicted in Figure 7-42 (step 5]). Transporters for GABA, norepinephrine, dopamine, and serotonin were the first to be cloned and studied. These four transport proteins are all Na -linked symporters. They are 60-70 percent Identical In their amino acid sequences, and each is thought to contain 12 transmembrane a helices. As with other Na symporters, the movement of Na into the cell down Its electrochemical gradient provides the energy for uptake of the neurotransmlt-ter. To maintain electroneutrality, CM often Is transported via an ion channel along with the Na and neurotransmitter. 

Briefly, neutral and anionic amino acids are transferred by Na" " symporters during the secondary active transport from the lumen into... 

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