Transport system galactose

The galactose, arabinose and xylose transporters of E. coli The bacterium E. coli possesses at least 7 proton-linked, active transport systems for sugars (for a recent review see [212]). Three of these transporters, which catalyze the uptake of L-arabinose, D-xylose and D-galactose by symport with protons, are related in sequence to the sugar transporters discussed above. They probably represent the best-characterized of the non-mammalian transporters, and so are discussed here in some detail. 

Carbohydrates mainly occur in food in the form of polymers (starches and glycogen). They are cleaved by pancreatic amylase into oligosaccharides and are then hydrolyzed by glycosidases, which are located on the surface of the intestinal epithelium, to yield monosaccharides. Glucose and galactose are taken up into the enterocytes by secondary active cotransport with Na"" ions (see p. 220). In addition, monosaccharides also have passive transport systems in the intestine. 

Thompson, J. 1978. In vivo regulation of glycolysis and characterization of suger phos-photransferase systems in Streptococcus lactis. J. Bacteriol 136, 465-476. Thompson, J. 1980. Galactose transport systems in Streptococcus lactis. J. Bacteriol 144, 683-691. 

Glucose and galactose compete for a common transport system. This system is an active transport system i.e., the monosaccharides are absorbed against a concentration gradient, it is saturable and obeys Michaelis-Menten... 

Existence of an active-transport system for D-glucose in mammalian intestine has been recognized for some time, but the mechanism of D-fructose transport is still controversial. Part of the controversy can be attributed to species differences in transport systems. Although the rate of uptake of D-fructose is lower than that observed for actively transported sugars, such as D-glucose and D-galactose (see Table I),... 

The D-fructose transport-system seems to be highly sugar-specific, although a decrease in uptake occurs in the presence of D-sorbose, D-glucose, D-galactose, 3-O-methyl-D-glucose, and sucrose. Little effect on uptake is shown by the presence of D-arabinose, D-fucose, D-man-nose, L-sorbose, D-tagatose, and D-xylose.17... 

The host enzymes that cleave various dietary substances to allow absorption by the host are not uniformly distributed along the Intestinal tract. Some are located In the proximal portion of the small Intestine (e.g., the lactase enzymes) others are located In the distal portion of the small Intestine and In the colon Itself (e.g., the maltase enzymes). This localization alone allows one to make certain Inferences about the presence or absence of the substrates for these cleavage and absorption enzymes. Furthermore, the different absorption systems within the same general region often exhibit very different activities. For example, the host s transport system for the absorption of galactose Is very active and nearly Impossible to saturate under physiological conditions whereas that for the absorption of arablnose Is relatively Ineffective. Thus, from Indirect Information concerning the relative abundance of various substances In the diet, the localization of host enzyme systems that permit absorption of these, and the rates at which absorption... 

Figure 9.2 Glucose transport system and carbon catabolite repression reiated with giucose-specific phosphotransferase system in Escherichia coli. MgIBAC the gaiactose ABC transporter GaiP the galactose H+ symporter CRP the cAMP receptor protein PEP phosphoenoipyruvate.

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... 

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