Transport systems, sugar

FIGURE 10.26 Glucose transport in E. coli is mediated by the PEP-dependent phosphotransferase system. Enzyme I is phosphorylated in the first step by PEP. Successive phosphoryl transfers to HPr and Enzyme III in Steps 2 and 3 are followed by transport and phosphorylation of glucose. Enzyme II is the sugar transport channel. 

Transport systems can be described in a functional sense according to the number of molecules moved and the direction of movement (Figure 41-10) or according to whether movement is toward or away from equilibrium. A uniport system moves one type of molecule bidirectionally. In cotransport systems, the transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. A symport moves these solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+ -sugar transporters (for glucose and certain other sugars) and Na -amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions (eg, Na in and Ca out). 

Yeasts contain a large number of different active and passive sugar-transport systems. The first of these to be cloned was the glucose-repressible, high-affinity passive glucose transporter of Saccharomyces cerevisiae, which is encoded by the SNF3 gene... 

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. 

The system can be applied for examination of control mechanisms of metabolic coupled enzyme systems, such as the sugar transport system in bacteria. 

M. Okamoto and K. Hayashi, Control mechanism for a bacterial sugar-transport system theoretical hypothesis, J. Theor. Biol, 113, 785-790 (1985). 

Chauvin, R Brand, L. Roseman, S. Sugar transport by the bacterial phosphotransferase system. Characterization of the Escherichia coli enzyme I monomer/dimer transition kinetics by fluorescence anisotropy. J. Biol. Chem., 269, 20270-20274 (1994)... 

Roseman, S. 1972. A bacterial phosphotransferase system and its role in sugar transport. In The Molecular Basis of Biological Transport. J. F. Woissner, Jr. and J. Huijing (Editors). Academic Press, New York, pp. 181-218. 

Figure 1. The bacterial phosphotransferase sugar transport system...

Depending upon the loss of one or more components, the system can be shown to mimic the four principal types of sugar transport observed in various mutants of E. coli. 

The definition on a molecular basis of these different sugar transport systems has not progressed to the same extent as with bacteria, partly because of the greater convenience of bacteria as experimental organisms, and also because of the greater complexity of the mammalian systems themselves. 

The predominant active sugar transport systems of intestine and kidney are specific for sugars which have the pyranose structure with a... 

The significance, if any, of these complexes in sugar transport is not yet understood. The specificity pattern however has some suggestive correlations with those observed for transport, and the complexes may have some secondary role in determining the overall specificity (similar to that perhaps played by the hypothetical transporter or T substance of Figure 2) in the overall proposed scheme for the permease system. Considered in this sense the primary specificity of the system would be determined by the permease protein (P) in accelerating the formation of the substrate-transporter complex, but the overall specificity of the system would reflect the properties of all components. 

Both approaches have been used for mutarotase and have given a considerable amount of information on the phylogenetic distribution of the enzyme and its relationship to developing sugar transport systems. 

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