Sugars sugar transporters

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. 

Several unique features distinguish the phosphotransferase. First, phos-phoenolpyruvate is both the phosphoryl donor and the energy source for sugar transport. Second, four different proteins are required for this transport. Two of these proteins (Enzyme I and HPr) are general and are required for the phosphorylation of all PTS-transported sugars. The other two proteins (Enzyme II and Enzyme III) are specific for the particular sugar to be transported. 

Joost H-G, Thorens B (2001) The extended GLUT-family of sugar/polyol transport facilitators - nomenclature, sequence characteristics, and potential function of its novel members. Mol MembrBiol 18 247-256... 

Wood IS, Trayhurn P (2003) Glucose transporters (GLUT and SGLT) expanded families of sugar transport proteins. BrJNutr 89 3-9... 

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). 

Mechanisms of active and passive transport in a family of homologous sugar transporters found in both prokaryotes and eukaryotes... 

Fig. 1. The structures of sugar-transport inhibitors, (a) Phloretin, (b) diethylstilboestrol, (c) 2-A -[4-(l-azi-2,2,2-trifluoroethyl)benzoyl]-l,3-bis-(D-mannos-4-yloxy)-2-propylamine (ATB-BMPA), (d) forskolin, (e) androsten-4-ene-3,17-dione, (0 cytochalasin B.

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. 

Aligned sequences of 16 members of the sugar transporter family. Residues which are identical in 5=50% of the 16 sugar-transporter sequences (excluding the quinate transporter (qa-y), the citrate transporter (CIT), the tetracycline transporter (pBR322) and lac permease (LacY)) are highlighted, and recorded below the sequences as CONSERVED . The locations of predicted membrane-spanning helices are indicated by horizontal bars. The sequences were taken from the references cited in the text. 

The lactose transporter (lac permease) of E. coli is without doubt the most intensively studied and best understood of the bacterial proton-linked sugar transporters. Since its sequence was reported in 1980 [233] prodigious efforts have been made to elucidate its molecular mechanism by site-directed mutagenesis and other means. These studies have recently been reviewed elsewhere [234,235] and so will not be discussed in detail here. The important question for the present Chapter is whether the protein is related to the sugar-transporter family and so has lessons to teach us about their mechanisms. The permease is a 417-residue protein, and, like the other... 

Fig. 5. A speculative model for the arrangement of the helical regions of the sugar transporters in the membrane. The helices are numbered as shown in Fig. 4. The small circle labelled s represents a glucose molecule.

In summary, studies on the human erythrocyte glucose transporter and other members of a large family of prokaryotic and eukaryotic sugar transporters have yielded... 

Tsuchiya W, Okada Y. 1982. Differential effects of cadmium and mercury on amino acid and sugar transport in the bullfrog small intestine. Experientia (Basel) 38 1073-1075. 

Berteloot, A., Common characteristics for Na1 -dependent sugar transport in Caco-2 cells and human fetal colon, J. Membr. Biol 1987, 99, 113-125. 

The sugar transporters are specific for D-forms of glucose, galactose and fine-... 

Doege, H., et al. Activity and genomic organization of human glucose transporter 9 (GLUT9), a novel member of the family of sugar-transport facilitators predominantly expressed in brain and leucocytes. Biochem. J. 2000, 350, 771-776. 

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