D-Fructose-L-sorbose interconversions. Access to 5-thio-D-fructose and interaction with the D-fructose transporter, GLUT5

A comprehensive overview of substrate specificity of glycoside hydrolases and transporters in the small intestine : "A gut feeling"

The human body is able to process and transport a complex variety of carbohydrates, unlocking their nutritional value as energy source or as important building block. The endogenous glycosyl hydrolases (glycosidases) and glycosyl transporter proteins located in the enterocytes of the small intestine play a crucial role in this process and digest and/or transport nutritional sugars based on their structural features. It is for these reasons that glycosidases and glycosyl transporters are interesting therapeutic targets to combat sugar related diseases (such as diabetes) or to improve drug delivery. In this review we provide a detailed overview focused on the molecular structure of the substrates involved as a solid base to start from and to fuel research in the area of therapeutics and diagnostics.

Molecular Tools for Facilitative Carbohydrate Transporters (Gluts)

Facilitative carbohydrate transporters-Gluts-have received wide attention over decades due to their essential role in nutrient uptake and links with various metabolic disorders, including diabetes, obesity, and cancer. Endeavors directed towards understanding the mechanisms of Glut-mediated nutrient uptake have resulted in a multidisciplinary research field spanning protein chemistry, chemical biology, organic synthesis, crystallography, and biomolecular modeling. Gluts became attractive targets for cancer research and medicinal chemistry, leading to the development of new approaches to cancer diagnostics and providing avenues for cancer-targeting therapeutics. In this review, the current state of knowledge of the molecular interactions behind Glut-mediated sugar uptake, Glut-targeting probes, therapeutics, and inhibitors are discussed.

Flow synthesis of a versatile fructosamine mimic and quenching studies of a fructose transport probe

We describe the synthesis of 1-amino-2,5-anhydro-D-mannose ("mannitolamine"), a key intermediate to the 7-nitro-1,2,3-benzadiazole conjugate (NBDM), using commercially available fluidic devices to increase the throughput. The approach is the first example of a flow-based Tiffeneau-Demjanov rearrangement. Performing this step in flow enables a ~64-fold throughput enhancement relative to batch. The flow process enables the synthesis to be accomplished three times faster than the comparable batch route. The high throughput enabled the production of larger quantities of the fluorescent fructose transport probe NBDM, enabling us to measure key photophysical properties that will facilitate future uptake studies.

Amine-linked diglycosides: Synthesis facilitated by the enhanced reactivity of allylic electrophiles, and glycosidase inhibition assays

Diglycose derivatives, consisting of two monosaccharides linked at non-anomeric positions by a bridging nitrogen atom, have been synthesised. Conversion of one of the precursor monosaccharide coupling components into an unsaturated derivative enhances its electrophilicity at the allylic position, facilitating coupling reactions. Mitsunobu coupling between nosylamides and 2,3-unsaturated-4-alcohols gave the 4-amino-pseudodisaccharides with inversion of configuration as single regio- and diastereoisomers. A palladium-catalysed coupling between an amine and a 2,3-unsaturated 4-trichloroacetimidate gave a 2-amino-pseudodisaccharide as the major product, along with other minor products. Derivatisation of the C=C double bond in pseudodisaccharides allowed the formation of Man(N4–6)Glc and Man(N4–6)Man diglycosides. The amine-linked diglycosides were found to show weak glycosidase inhibitory activity.

Intramolecular Benzyl Protection Delivery: A Practical Synthesis of DMDP and DGDP from d-Fructose

[reaction: see text] A two-step protection of 1,2-diols as the corresponding o-xylylene cyclic ethers, involving an intramolecular ring-closing O-benzylation reaction, has been developed to overcome the problems associated to regioselective benzylation reactions. The strategy has been applied to the high-yielding synthesis of the pyrrolidine glycosidase inhibitors DMDP and DGDP.

Pseudoamide-Type Pyrrolidine and Pyrrolizidine Glycomimetics and Their Inhibitory Activities against Glycosidases

Coupling reaction of (2R,3R,4R,5R)-2,5-hydroxymethyl-3,4-dihydroxypyrrolidine (DMDP) with isothiocyanates afforded the corresponding thiourea adducts, which were transformed into isourea-type bicyclic oxapyrrolizidine glycomimetics by mercury(II) oxide-assisted intramolecular sulfur displacement. Cyclic carbamate and thiocarbamate analogues were also prepared by direct carbonylation or thiocarbonylation of DMDP. Evaluation of the glycosidase inhibitory properties demonstrated that remarkable specificities in enzyme inhibition can be achieved upon modifications on the pseudoaglyconic side chain and on the nature of the sp(2)-hybridized endocyclic ring nitrogen.

Inhibition of the D-fructose transporter protein GLUT5 by fused-ring glyco-1,3-oxazolidin-2-thiones and -oxazolidin-2-ones

The glucose transporter 5 (GLUT5)-a specific D-fructose transporter-belongs to a family of facilitating sugar transporters recently enlarged by the human genome sequencing. Prompted by the need to develop specific photolabels of these isoforms, we have studied the interaction of conformationally locked D-fructose and L-sorbose derived 1,3-oxazolidin-2-thiones and 1,3-oxazolidin-2-ones to provide a rational basis for an interaction model. The inhibition properties of the D-fructose transporter GLUT5 by glyco-1,3-oxazolidin-2-thiones and glyco-1,3-oxazolidin-2-ones is now reported. In vitro, the fused-rings systems tested showed an efficient inhibition of GLUT5, thus bringing new insights on the interaction of D-fructose with GLUT5.

Development of high-affinity ligands and photoaffinity labels for the D-fructose transporter GLUT5

The GLUT5 transporter catalyses the specific uptake of D-fructose and can accept this hexose in its furanose and pyranose ring forms. The transporter does not accept fructose epimers and has very limited tolerance of bulky groups substituted at the 2-, 3-, 4- and 5-OH positions [Tatibouët, Yang, Morin and Holman (2000) Bioorg. Med. Chem. 8, 1825-1833]. To further explore whether bulky groups can be tolerated at the primary OH positions, a D-fructose analogue with an allylamine group substitution to replace the 1-OH group was synthesized and was found to be quite well tolerated ( K (i)=27.1 mM). However, this analogue occurs in multiple ring forms. By contrast, 2,5-anhydro-D-mannitol is a symmetrical molecule that occurs only in a furanose ring form in which C-1 and C-6 are equivalent. We have therefore synthesized new 2,5-anhydro-D-mannitol analogues (substituted at the equivalent of the 6-OH of D-fructose) and from studies in Chinese hamster ovary cells expressing GLUT5 cells report that (i) the allylamine derivative of 2,5-anhydro-D-mannitol is well tolerated ( K (i)=2.66 mM); (ii) introduction of a di-nitrophenyl-substituted secondary amine group enhances affinity ( K (i)=0.56 mM); (iii) introduction of amide-linked biotinylated photolabel moieties is possible without loss of affinity relative to 2,5-anhydro-D-mannitol but a small secondary amine spacer between the biotinylated photolabelling moiety and the fructofuranose ring increases affinity (fructose photolabel 2; K (i)=1.16 mM); (iv) introduction of a hydrophilic tartarate spacer between biotin and the diazirine photoreactive groups can be accomplished without reduction in affinity and (v) photoactivation of biotinylated fructose photolabels leads to specific biotin tagging of GLUT5. These data suggest that substitution of a secondary amine group (-NH) to replace the C-6 (or C-1) -OH of 2,5-anhydro-D-mannitol results in compounds of high affinity; the affinity is enhanced over 10-fold compared with D-fructose.