2,5-dideoxy-2,5-imino-d-mannitol and -d-glucitol. Two-step bio-organic syntheses from 5-azido-5-deoxy-d-glucofuranose and -l-idofuranose; evaluation as glucosidase inhibitors and application in affinity purification and characterisation of invertase from yeast

A Convenient Approach towards the Synthesis of ADMDP Type Iminosugars and Nojirimycin Derivatives from Sugar-Derived Lactams

An efficient method for the synthesis of nojirimycin- and pyrrolidine-based iminosugar derivatives has been developed. The strategy is based on the partial reduction in sugar-derived lactams by Schwartz’s reagent and tandem stereoselective nucleophilic addition of cyanide or a silyl enol ether dictated by Woerpel’s or diffusion control models, which affords amino-modified iminosugars, such as ADMDP or higher nojirimycin derivatives.

Stereoselective synthesis of (2S,3S,4R,5S)-3,4-dihydroxy-2,5-dihydroxymethyl pyrrolidine from L-sorbose

One of the most frequently synthesized iminosugar derivatives is DMDP. Starting from L-sorbose, a practical method for the synthesis of derivatives of this five-membered iminocyclitol has been developed, involving straightforward steps and a convenient selective reduction of a ketoxime intermediate.

C-branched iminosugars: α-glucosidase inhibition by enantiomers of isoDMDP, isoDGDP, and isoDAB-L-isoDMDP compared to miglitol and miglustat

The Ho crossed aldol condensation provides access to a series of carbon branched iminosugars as exemplified by the synthesis of enantiomeric pairs of isoDMDP, isoDGDP, and isoDAB, allowing comparison of their biological activities with three linear isomeric natural products DMDP, DGDP, and DAB and their enantiomers. L-IsoDMDP [(2S,3S,4R)-2,4-bis(hydroxymethyl)pyrrolidine-3,4-diol], prepared in 11 steps in an overall yield of 45% from d-lyxonolactone, is a potent specific competitive inhibitor of gut disaccharidases [K(i) 0.081 μM for rat intestinal maltase] and is more effective in the suppression of hyperglycaemia in a maltose loading test than miglitol, a drug presently used in the treatment of late onset diabetes. The partial rescue of the defective F508del-CFTR function in CF-KM4 cells by L-isoDMDP is compared with miglustat and isoLAB in an approach to the treatment of cystic fibrosis.

Synthesis of (2R,5S)-dihydroxymethyl-(3R,4R)-dihydroxypyrrolidine (DGDP) via stereoselective amination using chlorosulfonyl isocyanate

A stereoselective approach for synthesizing (2R,5S)-dihydroxymethyl-(3R,4R)-dihydroxypyrrolidine 1 (2,5-dideoxy-2,5-imino-d-glucitol, DGDP) was achieved using a seven-step approach starting from 2,3,4,6-tetra-O-benzyl-d-mannose (7). Key steps for the preparation of the title compound 1 involved the regioselective and diastereoselective amination of the cinnamyl anti-1,2-polybenzyl ethers 5 and 6 using chlorosulfonyl isocyanate (CSI) and ring cyclization to form the pyrrolidine ring. The reaction between anti-1,2-polybenzyl ether 5 and CSI in toluene at 0 degrees C afforded the corresponding anti-1,2-amino alcohol 4 as a major product with a diastereoselectivity of 16:1 in 76% yield. The mechanism underlying these reactions may be explained by the neighboring-group effect leading to the retention of stereochemistry.

Insights into the fine architecture of the active site of chicory fructan 1-exohydrolase: 1-kestose as substrate vs sucrose as inhibitor

* Invertases and fructan exohydrolases (FEHs) fulfil important physiological functions in plants. Sucrose is the typical substrate for invertases and bacterial levansucrases but not for plant FEHs, which are usually inhibited by sucrose. * Here we report on complexes between chicory (Cichorium intybus) 1-FEH IIa with the substrate 1-kestose and the inhibitors sucrose, fructose and 2,5 dideoxy-2,5-imino-D-mannitol. Comparisons with other family GH32 and 68 enzyme-substrate complexes revealed that sucrose can bind as a substrate (invertase/levansucrase) or as an inhibitor (1-FEH IIa). * Sucrose acts as inhibitor because the O2 of the glucose moiety forms an H-linkage with the acid-base catalyst E201, inhibiting catalysis. By contrast, the homologous O3 of the internal fructose in the substrate 1-kestose forms an intramolecular H-linkage and does not interfere with the catalytic process. Mutagenesis showed that W82 and S101 are important for binding sucrose as inhibitor. * The physiological implications of the essential differences in the active sites of FEHs and invertases/levansucrases are discussed. Sucrose-inhibited FEHs show a K(i) (inhibition constant) well below physiological sucrose concentrations and could be rapidly activated under carbon deprivation.

Fine tuning of β-glucosidase inhibitory activity in the 2,5-dideoxy-2,5-imino-d-mannitol (DMDP) system

Based on our extensive studies of D-glucosidase inhibiting 2,5-dideoxy-2,5-imino-D-mannitol derivatives, we have been trying to create a series of fluorescent derivatives with a view to an 'inhibitory activity ruler' based on competitive displacement reactions of non-fluorescent inhibitors by fluorescent ones and vice versa, which can be performed and followed in microtiter plates or on-chips. Thus, a set of compounds was assembled with Ki values between 2 nM and 1 microM against Agrobacterium sp. beta-glucosidase.

Iminosugar-producing Thai medicinal plants

Alpha-1-C-hydroxymethylfagomine (7), 3-O-beta-D-glucopyranosyl-DMDP (12), and 2,5-dideoxy-2,5-imino-D-glucitol (13) were isolated from the Thai traditional crude drug "Non tai yak" (Stemona tuberosa), which also contains a high concentration level of alpha-homonojirimycin (0.1% dry weight). "Thopthaep" (Connarus ferrugineus) and "Cha em thai" (Albizia myriophylla) contained 1-deoxymannojirimycin (DMJ) (10) at levels of 0.083% (dry weight) and 0.17% (dry weight), respectively. 2-O-alpha-D-Galactopyranosyl-DMJ (20), 3-O-beta-D-glucopyranosyl-DMJ (21), 1,4-dideoxymannojirimycin (17), 1,4-dideoxyallonojirimycin (18), and 1,4-dideoxyaltronojirimycin (19) from C. ferrugineus and 2-O-beta-D-glucopyranosyl-DMJ (22) and 4-O-beta-D-glucopyranosyl-DMJ (23) from A. myriophylla were isolated as new compounds.

Probing the aglycon binding site of a β-glucosidase: a collection of C-1-modified 2,5-dideoxy-2,5-imino-d-mannitol derivatives and their structure–activity relationships as competitive inhibitors

A range of new C-1 modified derivatives of the powerful glucosidase inhibitor 2,5-dideoxy-2,5-imino-D-mannitol has been synthesised and their biological activities probed with the beta-glucosidase from Agrobacterium sp. Ki values are compared with those of previously prepared close relatives. Findings suggest dramatic effects exerted by the aglycon binding site on substrate/inhibitor binding.

A practical route to partially protected pyrrolidines as precursors for the stereoselective synthesis of alexines

Either 3-O-benzoyl- (2a) or 3-O-benzyl-1,2-O-isopropylidene-beta-D-fructopyranose (2b) were regioselectively O-benzylated at C-4 to give 4a and 4b, respectively, which were transformed into 5-azido-3-O-benzoyl-4-O-benzyl- (6a) and 5-azido-3,4-di-O-benzyl-5-deoxy-1,2-O-isopropylidene-alpha-L-sorbopyranose (6b) by nucleophilic displacement of the corresponding 5-O-mesyl derivatives 5a and 5b by sodium azide in DMF, respectively. Compound 6b was also prepared from 4b in one step by the Mitsunobu methodology. Deacetonation of 6a and 6b gave the partially protected free azidouloses 8a and 8b, respectively, that were protected as their 1-O-TBDPS derivatives 9a and 9b. Hydrogenation of 9b over Raney nickel gave stereoselectively (2R,3R,4R,5S)-3,4-dibenzyloxy-2'-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (12) which was identified by transformation into the well known (2R,3R,4R,5S)-3,4-dihydroxy-2,5-bis(hydroxymethyl)pyrrolidine (1, DGDP).

Disaccharide-mediated regulation of sucrose:fructan-6-fructosyltransferase, a key enzyme of fructan synthesis in barley leaves

Previous work has indicated that sugar sensing may be important in the regulation of fructan biosynthesis in grasses. We used primary leaves of barley (Hordeum vulgare cv Baraka) to study the mechanisms involved. Excised leaf blades were supplied in the dark with various carbohydrates. Fructan pool sizes and two key enzymes of fructan biosynthesis, sucrose (Suc):Suc-1-fructosyltransferase (1-SST; EC 2. 4.1.99) and Suc:fructan-6-fructosyltransferase (6-SFT; EC 2.4.1.10) were analyzed. Upon supply of Suc, fructan pool sizes increased markedly. Within 24 h, 1-SST activity was stimulated by a factor of three and 6-SFT-activity by a factor of more than 20, compared with control leaves supplemented with mannitol (Mit). At the same time, the level of mRNA encoding 6-SFT increased conspicuously. These effects were increased in the presence of the invertase inhibitor 2, 5-dideoxy-2,5-imino-D-mannitol. Compared with equimolar solutions of Suc, glucose (Glu) and fructose stimulated 6-SFT activity to a lesser extent. Remarkably, trehalose (Tre; Glc-alpha-1 and 1-alpha-Glc) had stimulatory effects on 6-SFT activity and, to a somewhat lesser extent, on 6-SFT mRNA, even in the presence of validoxylamine A, a potent trehalase inhibitor. Tre by itself, however, in the presence or absence of validoxylamine A, did not stimulate fructan accumulation. Monosaccharides phosphorylated by hexokinase but not or weakly metabolized, such as mannose (Man) or 2-deoxy-Glc, had no stimulatory effects on fructan synthesis. When fructose or Man were supplied together with Tre, fructan and starch biosynthesis were strongly stimulated. Concomitantly, phospho-Man isomerase (EC 5.3.1.8) activity was detected. These results indicate that the regulation of fructan synthesis in barley leaves occurs independently of hexokinase and is probably based on the sensing of Suc, and also that the structurally related disaccharide Tre can replace Suc as a regulatory compound.

N1-alkyl-D-gluconamidines: are they 'perfect' mimics of the first transition state of glucosidase action?

The inhibition of four beta-glucosidases of plant, fungal, and mammalian origin by N1-butyl- and N1-dodecyl-D-gluconamidine was determined. Comparison with the inhibition by the corresponding N-alkyl-D-glucosylamines revealed that the strongly basic amidines (pKa 10.8) were at the most 10-times more inhibitory than the weakly basic glucosylamines (pKa 6.5). The small enhancement of inhibitory potency, resulting from transforming the tetrahedral C-1 geometry of the glucosylamines to the planar sp2-geometry of the amidines, was ascribed to the inability of the fully protonated amidines to function as hydrogen bond acceptors with the catalytic acid of the enzyme. Additional evidence for the importance of a hydrogen bond for strong inhibition came from the comparison of K1-values of the weakly basic 5-amino-5-deoxyhexopyranoses and 1,5-iminohexitols with those of the corresponding glyconamidrazones (pKa 8.4), which also have a planar C-1 geometry but are largely protonated under the assay conditions and which had similar or up to 10(4)-times larger K1-values than the former. Transition state resemblance was judged from the ratio KS(alkyl beta-glucoside)/K1(alkyl gluconamidine) relative to the rate acceleration factor kcat/kuncat (Wolfenden, Acc, Chem. Res., 5 (1972) 10-16). Compared to ratios of kcat/kuncat from > or = 10(11) to > or = 10(13), the ratios for KS/K1 were only from 10(3) to 2 x 10(4) except for beta-glucosidase A3 from Asp. wentii which had KS/K1 2.8 x 10(6). This enzyme differs from the others by being strongly inhibited by cationic glycon and substrate analogues rather than by basic ones. The pH-dependence of 1/K1 and the 'slow' approach to the inhibition is discussed with respect to transition state resemblance.

A tetrad of ionizable amino acids is important for catalysis in barley beta-glucanases

Determination of the crystal structures of a 1,3-beta-D-glucanase (E.C. 3.2.1.39) and a 1,3-1,4-beta-D-glucanase (E.C. 3.2.1.73) from barley (Hordeum vulgare) (Varghese, J.N, Garrett, T. P. J., Colman, P. M., Chen, L., Høj, P. B., and Fincher, G. B. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 2785-2789) showed the spatial positions of the catalytic residues in the substrate-binding clefts of the enzymes and also identified highly conserved neighboring amino acid residues. Site-directed mutagenesis of the 1,3-beta-glucanase has now been used to investigate the importance of these residues. Substitution of glutamine for the catalytic nucleophile Glu231 (mutant E231Q) reduced the specific activity about 20,000-fold. In contrast, substitution of glutamine for the catalytic acid Glu288 (mutant E288Q) had less severe consequences, reducing kcat approximately 350-fold with little effect on Km. Substitution of two neighboring and strictly conserved active site-located residues Glu279 (mutant E279Q) and Lys282 (mutant K282M) led to 240- and 2500-fold reductions of Kcat, respectively, with small increases in Km. Thus, a tetrad of ionizable amino acids is required for efficient catalysis in barley beta-glucanases. The active site-directed inhibitor 2,3-epoxypropyl beta-laminaribioside was soaked into native crystals. Crystallographic refinement revealed all four residues (Glu231, Glu279, Lys282, and Glu288) to be in contact with the bound inhibitor, and the orientation of bound substrate in the active site of the glucanase was deduced.