Xylose transporters

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. 

Davis, E.O. (1985) Xylose Transport in Escherichia coli. Ph.D. Thesis, University of Cambridge. 

Physical or chemical modification of a substrate may additionally selectively affect transformation or uptake Keil and Kirchman (1992) compared the degradation of Rubisco uniformly labeled with 3H amino acids produced via in vitro translation to Rubisco that was reductively methylated with 3H-methane. Although both Rubisco preparations were hydrolyzed to lower molecular weights at approximately the same rate, little of the methylated protein was assimilated or respired. The presence of one substrate may also inhibit uptake of another, as has been demonstrated for anaerobic rumen bacteria. Transport and metabolism of the monosaccharides xylose and arabinose were strongly reduced in Ruminococcus albus in the presence of cellobiose (a disaccharide of glucose), likely because of repression of pentose utilization in the presence of the disaccharide. Glucose, in contrast, competitively inhibited xylose transport and showed noncompetitive inhibition of arabinose transport, likely because of inactivation of arabinose permease (Thurston et al., 1994). 

Xylose can be metabolized by bacteria, fungi or yeast. In bacteria, the initial step of xylose metabolism involves inducible enzymes (i.e., xylose transport enzymes, xylose isomerase and xylulokinase). The direct isomerization of xylose... 

Xylose Transport in Natural Xylose-Utilizing Yeasts Engineering Xylose Transport in S. cerevisiae. . . ... 

Xylose Transport in Natural Xylose-Utlllzing Yeasts... 

It is noteworthy that the Michaehs constant for the low-affinity xylose transport in R stipitis is different from that for the SUTl determined in an hxtl-7 deletion strain of S. cerevisiae [124]. This suggests that the apparent low affinity component is in fact the superposition of several individual transporters. In S. cerevisiae three or more hexose transporter genes are transcribed at the same time [ 127], yet only two components of the glucose uptake system can be kinet-ically distinguished [ 128]. It is, therefore, expected that the kinetic constants for xylose transporters from various species will be refined, once the corresponding genes are cloned and expressed in a model system such as the hxtl-7 S. cerevisiae strain. 

PC. (2008) Role of xylose transporters in xylitol production from engineered Escherichia coli. J. Biotechnol, 134, 246-252. 

As sorbitol, xylitol is not naturally synthesized by LAB, its production was obtained by heterologous expression [236] of the D-xylose reductase gene from the yeast Pichia stipitis CBS 5773 and the xylose transporter gene from Lb. brevis ATCC 8287 in the strain Lc. lactis NZ9800. D-xylose was quantitatively converted to D-xylitol, reaching an amount of 54.4 gl similar to that produced by yeast. 

The effect of insulin on the kinetics of glucose transport have been studied by Morgan et al. (1961a). They find that insulin increases the Km and Fm x of transport in the perfused isolated rat heart. Fisher and Zachariah (1961) have concluded that insulin increases the Km of L-arabinose and D-xylose transport. In peripheral tissues or in the heart, in vivo evidence has been obtained that insulin lowers the tissue threshold for glucose (i.e., permits uptake of glucose at lower concentration) (Butterfield et al., 1958 Hackcl, 1960). 

The xylose transport across cell membrane is the initial step of the xylose utilization by tee yeasts. Pentose-fermenting yeasts generally posses multiple xylose uptake systems, which can be classified as low- or hi -affinity systems. The types of transport system vaiy between the yeasts and depend on nutritional conditions (82, i, 4, J). In batch processes, the increase in sugar concentration... 

Ren C, Chen T, Zhang 1, Liang L, Lin Z. (2009). An evolved xylose transporter from Zymomonas mobilis enhances sugar transport in Escherichia coli. Microb Cell Fact, 8, 66. doi 10.1186/1475-2859-8-66... 

Lee WJ, Kim MD, Ryu YW, Bisson L, Seo JH. (2002). Kinetic studies on glucose and xylose transport in Saccharomyces cerevisiae. Appl Microbiol Biotechnol, 60, 186-191. 

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