Transport system lactose

In bacteria, accumulation of substrates against a concentration gradient can occur through two main classes of transport systems (see [30] for a summary). The prototype of the first class of transporters is the /3-galactoside permease of Escherichia coli (see [31]). It is a relatively simple system involving only a single membrane-bound protein. It catalyzes a lactose-H symport. Other transporters... 

In the induction of enzymes of galactose metabolism in E. coli, three enzymes are involved -galactosidase (which catalyses the hydrolysis of the y -glycosidic bonds of lactose), galactose permease (which is responsible for transport of lactose across the cell membrane) and a third enzyme, A-protein, apparently not directly involved in galactose metabolism. The system has an environmental inducer, galactose, and in its presence the number of /)-galactosidase molecules rises from 5-10 to 10,000 within the cell. The addition of the inducer can increase the protein production in less than five minutes after its addition. Protein synthesis of these enzymes stops almost immediately in the absence of lactose. 

Purification of the membrane-bound lactose carrier protein is a very different problem from the purification of the soluble OMP synthase. Both the approach to purification and the assays for the protein during purification are quite novel. The assay involves reconstituting a transport system with membranes that are free of lactose carrier protein, then adding the partially purified carrier protein and radioactively labeled lactose. The activity in this assay system is proportional to the transport of radioactive lactose across the membrane in the cell-free reconstituted system. 

Active transport of a solute against a concentration gradient also can be driven by a flow of an ion down its concentration gradient. Table 17.6 lists some of the active-transport systems that operate in this way. In some cases, the ion moves across the membrane in the opposite direction to the primary substrate (antiport) in others, the two species move in the same direction (symport). Many eukaryotic cells take up neutral amino acids by coupling this uptake to the inward movement of Na+ (see fig. 17.26c). As we discussed previously, Na+ influx is downhill thermodynamically because the Na+-K+ pump keeps the intracellular concentration of Na+ lower than the extracellular concentration and sets up a favorable electric potential difference across the membrane. Another example is the /3-galactosidc transport system of E. coli, which couples uptake of lactose to the inward flow of protons (see fig. 17.26Proton influx is downhill because electron-transfer reactions (or,... 

Newman, M.J. Wilson, T.H. (1980). Solubilization and reconstitution of the lactose transport system from Escherichia coli. J. Biol. Chem. 255,10583-10586. 

Many active-transport systems couple the uphill flow of one ion or molecule to the downhill flow of another. These membrane proteins, called secondary transporters or cotransporters, can be classified as antiporters, symporters, and uni porters. Antiporters couple the downhill flow of one type of ion in one direction to the uphill flow of another in the opposite direction. Symporters move both ions in the same direction. Uniporters transport a substrate in either direction, determined by the concentration differences. Studies of the lactose permease from E. coli have been a source of insight into both the structures and the mechanisms of secondary transporters. 

A surprisingly high number of carriers are present constitutively in the cytoplasmic membrane. One may speculate about the advantages for bacteria to possess always carriers for several solutes. Possibly the possession of constitute carriers would enable an organism to scavenge intermediates leaked passively out of the cell and/or allow the organism to react rapidly to changes in the external medium. Besides constitutive transport systems inducible transport systems are also found in bacteria such as those for lactose transport in E. coli or for citrate transport in B. subtilis. 

L.A., Nadal, L, Monedero, V. et al (2010) Sorbitol production om lactose by engineered Lactobacillus casei deficient in sorbitol transport system and mannitol-l-phosphate dehydrogenase. Appl. Microbiol Biotechnol, 85, 1915-1922. 

Azido-2-nitrophenyl- l-thio-jS-D-galdcto- pyranoside Lactose transport system in E. coli Analog is transported, competitive with lactose, irradiation inactivates in presence of D-lactate 96... 

Becker et al. specifically inactivated the biotin transport system of E. coli using biotin p-nitrophenyl ester. The lactose transport protein of E. coli was labeled by AT-bromoacetyl-/8-D-galactopyranosylamine. In the only reported attempt to isolate an affinity labeled protein, glucose 6-isothiocyanate, which is an affinity label for the glucose transport system in human erythrocytes, gave enough nonspecific labeling of other membrane proteins to render identification of the transport protein difficult. 

The dietary carbohydrates also include sucrose and lactose. Specific disaccharidases which convert these sugars into their constituent monosaccharides are present in the brush border of the intestinal epithelial cells. Only monosaccharides can be absorbed and an active transport system ensures that glucose, galactose and other sugars having the structural features shown below... 

Kowalczyk, M., Cocaign-Bousquet, M., Loubiere, P., and Bardowski, J. (2008) Identification and functional characterisation of cellobiose and lactose transport systems in Lactococcus lactis IL1403. Arch Microbiol 189,187—196. 

It was therefore of interest to compare the properties of the E. coli transport system for lactose, incorporated in Rps. sphaeroides with those of the endogenous transport system for alanine. The initial rate of uptake of lactose and the transmembrane electrical potential (Aijj) were measured simultaneously at different light intensities at pH 8 (Fig. 2a). The dependence on light intensity of the uptake of alanine and lactose is very similar. 

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. 

The Enzymes II (E-IIs) of the phosphoenolpyruvate (P-enolpyruvate)-dependent phosphotransferase system (PTS) are carbohydrate transporters found only in prokaryotes. They not only transport hexoses and hexitols, but also pentitols and disaccharides. The PTS substrates are listed in Table I. The abbreviations used (as superscripts) throughout the text for these substrates are as follows Bgl, jS-gluco-side Cel, cellobiose Fru, fructose Glc, glucose Gut, glucitol Lac, lactose Man, mannose Mtl, mannitol Nag, iV-acetylglucosamine Scr, sucrose Sor, sorbose Xtl, xylitol. 

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