Novel Disaccharides Containing Sulfur in the Ring and Nitrogen in the Interglycosidic Linkage. Conformation of Methyl 5'-Thio-4-N-.alpha.-Maltoside Bound to Glucoamylase and Its Activity as a Competitive Inhibitor

Thiogalactopyranosides are resistant to hydrolysis by α-galactosidases

Fluorescently tagged glycosides containing terminal α(1→3) and α(1→4)-linked thiogalactopyranosides have been prepared and tested for resistance to hydrolysis by α-galactosidases. Eight fluorescent glycosides containing either galactose or 5-thiogalactose as the terminal sugar were enzymatically synthesized using galactosyltransferases, with lactosyl glycosides as acceptors and UDP-galactose or UDP-5'-thiogalactose, respectively, as donors. The glycosides were incubated with human α-galactosidase A (CAZy family GH27, a retaining glycosidase), Bacteroides fragilis α-1,3-galactosidase (GH110, an inverting glycosidase), or homogenates of MCF-7 human breast cancer cells or NG108-15 rat glioma cells. Substrate hydrolysis was monitored by capillary electrophoresis with fluorescence detection. All compounds containing terminal O-galactose were readily degraded. Their 5-thiogalactose counterparts were resistant to hydrolysis by human α-galactosidase A and the enzymes present in the cell extracts. B. fragilis α-1,3-galactosidase hydrolyzed both thio- and O-galactoside substrates; however, the thiogalactosides were hydrolyzed at only 1-3 % of the rate of O-galactosides. The hydrolytic resistance of 5-thiogalactose was also confirmed by an in vivo study using cells in culture. The results suggest that 5-thiogalactosides may be useful tools for the study of anabolic pathways in cell extracts or in single cells.

5-thiomannosides block the biosynthesis of dolichol-linked oligosaccharides and mimic class I congenital disorders of glycosylation

In a cell-based assay for novel inhibitors, we have discovered that two glycosides of 5-thiomannose, each containing an interglycosidic nitrogen atom, prevented the correct zymogen processing of the prohormone proopiomelanocortinin (POMC) and the transcription factor sterol-regulatory element-binding protein-2 (SREBP-2) in mouse pituitary cells and Chinese hamster ovary (CHO) cells, respectively. In the case of SREBP-2, these effects were correlated with the altered N-linked glycosylation of subtilisin/kexin-like isozyme-1 (SKI-1), the protease responsible for SREBP-2 processing under sterol-limiting conditions. Further examination of the effects of these compounds in CHO cells showed that they cause extensive protein hypoglycosylation in a manner similar to type I congenital disorders of glycosylation (CDGs) since the remaining N-glycans in treated cells were complete (normal) structures. The under-glycosylation of glycoproteins in 5-thiomannoside-treated cells is now shown to be caused by the compromised biosynthesis of the dolichol-linked oligosaccharide (DLO) N-glycosylation donor, although the nucleotide sugars required for the synthesis of DLOs were neither reduced under these conditions, nor were their effects reversed upon the addition of exogenous mannose. Analysis of DLO intermediates by fluorophore-assisted carbohydrate electrophoresis demonstrated that 5-thiomannose-containing glycosides block DLO biosynthesis most likely at a stage prior to the GlcNAc(2) Man(3) intermediate, on the cytosolic face of the endoplasmic reticulum.

N-glycosylation controls trafficking, zymogen activation and substrate processing of proprotein convertases PC1/3 and subtilisin kexin isozyme-1

The limited proteolysis of proteins by the proprotein convertases (PCs) is a common means of producing bioactive proteins or peptides. The PCs are associated with numerous human pathologies and their activity can be reduced through the use of specific inhibitors. Here, we demonstrate an alternative approach to inhibiting PCs by altering their N-glycosylation. Through site-directed mutagenesis, we show that the convertase PC1/3 contains two N-glycans, only one of which is critical for its prosegment cleavage. The exact structure of PC1/3 N-glycans does not significantly affect its zymogen activation within endocrine cells, but glycosylation of Asn(146) is critical. Processing of the PC1/3's substrate proopiomelanocortin (POMC) was used in a cell-based assay to screen a collection of 45 compounds structurally related to known glycosidase inhibitors. Two 5-thiomannose-containing disaccharide derivatives were discovered to block PC1/3 and POMC processing into the analgesic peptide β-endorphin. These compounds also reduced the zymogen activation of the convertase subtilisin kexin isozyme-1 (SKI-1), blocked the processing of its substrate the sterol regulatory element-binding protein SREBP-2 and altered its glycosylation. Thus, modification of PC glycosylation may also be a means of blocking their activity, an effect which, in the case of SKI-1, may be of possible therapeutic use since SREBP-2 regulates sterol levels including cholesterol biosynthesis and its metabolism.

The dramatic effect of NH3 co-ligation on the Fe+-assisted activation of carbon dioxide in the gas phase: from bare metal ions to complexes

The catalytic efficiency of Fe(+) ion over the CO(2) decomposition in the gas phase has been extensively investigated with the help of electronic structure calculation methods. Potential-energy profiles for the activation process Fe(+) + CO(2) --> CO + FeO(+) along two rival potential reaction paths, namely the insertion and addition pathways, originating from the end-on kappa(1)-O and kappa(2)-O,O coordination modes of CO(2) with the metal ion, respectively, have been explored by DFT calculations. For each pathway the potential energy surfaces of the high-spin sextet (S = 5/2) and the intermediate-spin quartet (S = 3/2) spin-states have been explored. The complete energy reaction profile calculated by a combination of ab initio and density functional theory (DFT) computational techniques reveals a two-state reactivity, involving two spin inversions, for the decomposition process and accounts well for the experimentally observed inertness of bare Fe(+) ions towards CO(2) activation. Furthermore, the coordination of up to three extra ancillary NH(3) ligands with the Fe(+) metal ion has been explored and the geometric and energetic reaction profiles of the CO(2) activation processes Fe(+) + n x NH(3) + CO(2) --> [Fe(NH(3))(n)(CO(2))](+) --> [Fe(NH(3))(n)(O)(CO)](+) --> CO + [Fe(O)(NH(3))(n)](+) (n = 1, 2 or 3) have thoroughly been scrutinized for both the insertion and the addition mechanisms. Inter alia, the geometries and energies of the various states of the [Fe(NH(3))(n)(CO(2))](+) and [Fe(NH(3))(n)(O)(CO)](+) complexes are explored and compared. Finally, a detailed analysis of the coordination modes of CO(2) in the cationic [Fe(NH(3))(n)(CO(2))](+) (n = 0, 1, 2 and 3) complexes is presented.

Selection of a high-energy bioactive conformation of a sulfonium-ion glycosidase inhibitor by the enzyme glucoamylase G2

Transferred nuclear Overhauser effect and rotating-frame Overhauser enhancement NMR spectroscopies are used to probe the conformation of a bicyclic sulfonium ion, which is an analogue of the naturally occurring glycosidase inhibitor castanospermine, bound to the enzyme glucoamylase G2. Enzyme inhibition assays indicate that the bicyclic sulfonium ion is a slightly better inhibitor (K(i) = 1.32 mM) of glucoamylase G2 than the naturally occurring sulfonium-ion glycosidase inhibitor, salacinol, with a K(i) value of 1.7 mM. The NMR results are interpreted in terms of the selection by the enzyme of a high-energy conformation of the ligand that is already represented in the ensemble of free-ligand conformations.

Synthesis of nitrogen analogues of salacinol and their evaluation as glycosidase inhibitors

The syntheses of two nitrogen analogues (11 and 12) of the naturally occurring sulfonium ion, salacinol (7) are described. The latter compound is one of the active principles in the aqueous extracts of Salacia reticulata that are traditionally used in Sri Lanka and India for the treatment of diabetes. The synthetic strategy relies on the nucleophilic attack of a 1,4-dideoxy-1,4-imino-D- or L-arabinitol at the least hindered carbon of 2,4-O-benzylidene D- or L-erythritol-1,3-cyclic sulfate. The nitrogen analogues bear a permanent positive charge and serve as mimics of the sulfonium ion. We reasoned that these ammonium derivatives should function in a manner similar to that of known glycosidase inhibitors of the alkaloid class such as castanospermine (4) and deoxynojirimycin (5). Enzyme inhibition assays indicate that salacinol (7) is a weak (K(i) = 1.7 mM) inhibitor of glucoamylase, whereas compounds 11 and 12 inhibit glucoamylase with K(i) values in the range approximately 10-fold higher. The nitrogen analogues 11 and 12 showed no significant inhibitory effect of either barley alpha-amylase (AMY1) or porcine pancreatic alpha-amylase (PPA) at concentrations of 5 mM. In contrast, salacinol (7) inhibited AMY1 and PPA in the micromolar range, with K(i) values of 15 +/- 1 and 10 +/- 2 microM, respectively.

Glucoamylase: structure/function relationships, and protein engineering

Glucoamylases are inverting exo-acting starch hydrolases releasing beta-glucose from the non-reducing ends of starch and related substrates. The majority of glucoamylases are multidomain enzymes consisting of a catalytic domain connected to a starch-binding domain by an O-glycosylated linker region. Three-dimensional structures have been determined of free and inhibitor complexed glucoamylases from Aspergillus awamori var. X100, Aspergillus niger, and Saccharomycopsis fibuligera. The catalytic domain folds as a twisted (alpha/alpha)(6)-barrel with a central funnel-shaped active site, while the starch-binding domain folds as an antiparallel beta-barrel and has two binding sites for starch or beta-cyclodextrin. Certain glucoamylases are widely applied industrially in the manufacture of glucose and fructose syrups. For more than a decade mutational investigations of glucoamylase have addressed fundamental structure/function relationships in the binding and catalytic mechanisms. In parallel, issues of relevance for application have been pursued using protein engineering to improve the industrial properties. The present review focuses on recent findings on the catalytic site, mechanism of action, substrate recognition, the linker region, the multidomain architecture, the engineering of specificity and stability, and roles of individual substrate binding subsites.

Novel 4-thiogalactofuranosyl-containing disaccharides with nitrogen in the interglycosidic linkage

The syntheses of three novel disaccharides containing a 4-thiogalactofuranosyl residue as the non-reducing unit and a nitrogen in the interglycosidic linkage are described. Acid-catalyzed condensation reactions of 4-thio-alpha/beta-D-galactofuranose with either methyl 3-amino-3-deoxy-alpha-D-mannopyranoside, methyl 2-amino-2-deoxy-alpha-D-mannopyranoside, or methyl 2-acetamido-6-amino-2,6-dideoxy-beta-D-glucopyranoside gave methyl 3-amino-3-deoxy-3-N-(4-thio-alpha/beta-D-galactofuranosyl)-alpha-D-manno pyranoside, methyl 2-amino-2-deoxy-2-N-(4-thio-alpha/beta-D-galactofuranosyl)-alpha-D-manno pyranoside, or methyl 2-acetamido-6-amino-2,6-dideoxy-6-N-(4-thio-alpha/beta-D-galactofuranosy l)-beta-D-glucopyranoside.

Purification, enzymatic characterization, and nucleotide sequence of a high-isoelectric-point alpha-glucosidase from barley malt

High-isoelectric-point (pI) alpha-glucosidase was purified 7, 300-fold from an extract of barley (Hordeum vulgare) malt by ammonium sulfate fractionation, ion-exchange, and butyl-Sepharose chromatography. The enzyme had high activity toward maltose (k(cat) = 25 s(-1)), with an optimum at pH 4.5, and catalyzed the hydrolysis by a retaining mechanism, as shown by nuclear magnetic resonance. Acarbose was a strong inhibitor (K(i) = 1.5 microM). Molecular recognition revealed that all OH-groups in the non-reducing ring and OH-3 in the reducing ring of maltose formed important hydrogen bonds to the enzyme in the transition state complex. Mass spectrometry of tryptic fragments assigned the 92-kD protein to a barley cDNA (GenBank accession no. U22450) that appears to encode an alpha-glucosidase. A corresponding sequence (HvAgl97; GenBank accession no. AF118226) was isolated from a genomic phage library using a cDNA fragment from a barley cDNA library. HvAgl97 encodes a putative 96.6-kD protein of 879 amino acids with 93.8% identity to the protein deduced from U22450. The sequence contains two active site motifs of glycoside hydrolase family 31. Three introns of 86 to 4,286 bp interrupt the coding region. The four exons vary from 218 to 1,529 bp. Gene expression analysis showed that transcription reached a maximum 48 h after the start of germination.

Determination of the solution conformation of d-gluco-dihydroacarbose, a high-affinity inhibitor bound to glucoamylase by transferred NOE NMR spectroscopy

The determination of the bound solution conformation of D-gluco-dihydroacarbose (GAC), a tight-binding inhibitor of several glycosidase and amylase enzymes, by glucoamylase is described. Transferred NOE NMR experiments and line-broadening effects indicate that GAC is bound in a conformation resembling that observed in the crystal structure. This contrasts with the predominant conformation of GAC when free in solution. The NMR results also suggest regions on the carbohydrate that are in close contact with the protein. The determination of the bound solution conformation of GAC by glucoamylase using transferred NOE (trNOE) measurements is a significant achievement given the high affinity constant (Ka = 3 x 10(7) M(-1)) for this receptor-ligand pair. It is striking that the off-rate for complexation is still sufficiently high to permit observation of trNOEs.

Complexes of glucoamylase with maltoside heteroanalogues: bound ligand conformations by use of transferred NOE experiments and molecular modeling

Transferred nuclear Overhauser effect (trNOE) experiments have been performed to investigate the conformations of the competitive inhibitors, methyl 5'-thio-4-N-alpha-maltoside 3a and methyl 5'-thio-4-S-alpha-maltoside 4 when bound to the catalytic subunit of the enzyme glucoamylase. These NMR data suggest that, although each of the free ligands populates two conformational families, both heteroanalogues are bound by the enzyme in conformations in the area of the global energy minimum. These conformations have been used as initial points for docking into the active site of the enzyme taken from a X-ray crystal structure of the related glucoamylase-D-gluco-dihydroacarbose 2 complex. Minimization of the resulting complexes has yielded structures for the bound complexes. Corroboration of the structures is provided by fast T(1)(rho)-relaxation effects for certain ligand protons as a result of close contacts with protons in the enzyme active site. The results auger well for the combined use of transferred NOE spectroscopy and molecular modeling based on X-ray crystal structures of complexes of suitable congeners for the rapid analysis of ligand-receptor interactions.

Is there a generalized reverse anomeric Effect? substituent and solvent effects on the configurational equilibria of neutral and protonated N-arylglucopyranosylamines and N-aryl-5-thioglucopyranosylamines

The effects of substitution and solvent on the configurational equilibria of neutral and protonated N-(4-Y-substituted-phenyl) peracetylated 5-thioglucopyranosylamines (Y = OMe, H, CF(3), NO(2)) 1-4 and N-(4-Y-substituted-phenyl) peracetylated glucopyranosylamines (Y = OMe, H, NO(2)) 9-11 are described. The configurational equilibria were determined by direct integration of the resonances of the individual isomers in the (1)H NMR spectra after equilibration of both alpha- and beta-isomers. The equilibrations of the neutral compounds 1-4 in CD(3)OD, CD(3)NO(2), and (CD(3))(2)CO were achieved by HgCl(2) catalysis and those of the neutral compounds 9-11 in CD(2)Cl(2) and CD(3)OD by triflic acid catalysis. The equilibrations of the protonated compounds in both the sulfur series (solvents, CD(3)OD, CD(3)NO(2), (CD(3))(2)CO, CDCl(3), and CD(2)Cl(2)) and oxygen series (solvents, CD(2)Cl(2) and CD(3)OD) were achieved with triflic acid. The substituent and solvent effects on the equilibria are discussed in terms of steric and electrostatic effects and orbital interactions associated with the endo-anomeric effect. A generalized reverse anomeric effect does not exist in neutral or protonated N-aryl-5-thioglucopyranosylamines and N-arylglucopyranosylamines. The anomeric effect ranges from 0.85 kcal mol(-)(1) in 2 to 1.54 kcal mol(-)(1) in 10. The compounds 1-4 and 9-11 show an enhanced endo-anomeric effect upon protonation, ranging from 1.73 kcal mol(-)(1) in 2 to 2.57 kcal mol(-)(1) in 10. We estimate the increase in the anomeric effect upon protonation of 10 to be approximately 1.0 kcal mol(-)(1). However, this effect is offset by steric effects due to the associated counterion which we estimate to be approximately 1.2 kcal mol(-)(1). The values of K(eq)(axial-equatorial) in protonated 1-4 increase in the order OMe < H < CF(3) < NO(2), in agreement with the dominance of steric effects (due to the counterion) over the endo-anomeric effect. The values of K(eq)(axial-equatorial) in protonated 9-11 show the trend OMe > H < NO(2) that is explained by the balance of the endo-anomeric effect and steric effects in the individual compounds. The trends in the values of the C(1)-H(1) coupling constants for 1-4 and the corresponding deacetylated compounds 5-8 as a function of substituent and alpha- or beta-configuration are discussed in terms of the Perlin effect and the interplay of the endo- and exo-anomeric effects.

Synthesis of ethyl 2-acetamido-6-S-(5-amino-5-deoxy-beta-D- arabinopyranosyl)-2-deoxy-1,6-dithio-beta-D-glucopyranoside: a sulfur-linked 5-amino-5-deoxyglycopyranosyl disaccharide

A novel pseudo-disaccharide having an imino sugar residue at the non-reducing end, namely, a sulfur-linked 5-amino-5-deoxyglycopyranosyl disaccharide, which is a potential specific inhibitor for glycosidases that recognize not only the glycosidic linkage but also the aglycone moiety, was synthesized. Glycosidation of N-Boc-5-amino-5-deoxy-D-arabinose with ethyl 2-acetamido-3,4-di-O-acetyl-2-deoxy-1,6-dithio-beta-D- glucopyranoside in the presence of TsOH gave exclusively the corresponding 1,2-cis-linked thioglycoside. The interglycosidic linkage proved stable enough under conditions for the deprotection of the N-Boc group with TFA. This pseudodisaccharide was unstable at pH > 5, but stable at lower pH. The sulfur-linked 5-amino-5-deoxyglycopyranosyl disaccharide was shown to be formed from 5-amino-5-deoxy-D-arabinose and ethyl 2-acetamido-2-deoxy-1,6-dithio-beta-D-glucopyranoside in an acidic buffer solution.

Synthesis and glycosidase inhibitory activity of 5-thioglucopyranosylamines. Molecular modeling of complexes with glucoamylase

The synthesis of a series of 5-thio-D-glucopyranosylarylamines by reaction of 5-thio-D-glucopyranose pentaacetate with the corresponding arylamine and mercuric chloride catalyst is reported. The products were obtained as anomeric mixtures of the tetraacetates which can be separated and crystallized. The tetraacetates were deprotected to give alpha/beta mixtures of the parent compounds which were evaluated as inhibitors of the hydrolysis of maltose by glucoamylase G2 (GA). A transferred NOE NMR experiment with an alpha/beta mixture of 7 in the presence of GA showed that only the alpha isomer is bound by the enzyme. The Ki values, calculated on the basis of specific binding of the alpha isomers, are 0.47 mM for p-methoxy-N-phenyl-5-thio-D-glucopyranosylamine (7), 0.78 mM for N-phenyl-5-thio-D-glucopyranosylamine (8), 0.27 mM for p-nitro-N-phenyl-5-thio-D-glucopyranosylamine (9) and 0.87 mM for p-trifluoromethyl-N-phenyl-5-thio-D-glucopyranosylamine (10), and the K(m) values for the substrates maltose and p-nitrophenyl alpha-D-glucopyranoside are 1.2 and 3.7 mM, respectively. Methyl 4-amino-4-deoxy-4-N-(5'-thio-alpha-D-glucopyranosyl)-alpha-D-glucopyrano side (11) is a competitive inhibitor of GA wild-type (Ki 4 microM) and the active site mutant Trp120-->Phe GA (Ki 0.12 mM). Compounds 7, 8, and 11 are also competitive inhibitors of alpha-glucosidase from brewer's yeast, with Ki values of 1.05 mM, > 10 mM, and 0.5 mM, respectively. Molecular modeling of the inhibitors in the catalytic site of GA was used to probe the ligand-enzyme complementary interactions and to offer insight into the differences in inhibitory potencies of the ligands.

Synthesis of 1,2- and 1,3-N-Linked Disaccharides of 5-Thio-alpha-D-mannopyranose as Potential Inhibitors of the Processing Mannosidase Class I and Mannosidase II Enzymes

The syntheses of S/N acetal heteroatom analogues of 1,2- and 1,3-linked mannopyranose disaccharides are described. The compounds are analogues of the Man-alpha-(1-->2)-Man and Man-alpha-(1-->3)-Man disaccharide components of oligosaccharides found in N-glycoproteins that are cleaved by trimming mannosidases during glycoprotein processing. Glycosylamine formation, without the necessity of hydroxyl group protection, proceeded through acid-catalyzed condensation reactions of 5-thio-D-mannose with either methyl 2-amino-2-deoxy- or 3-amino-3-deoxy-alpha-D-mannopyranoside to give methyl 2-amino-2-deoxy-2-N-(5-thio-alpha/beta-D-mannopyranosyl)-alpha-D-mannopyranoside (2) or methyl 3-amino-3-deoxy-3-N-(5-thio-alpha/beta-D-mannopyranosyl)-alpha-D-mannopyranoside (3), respectively. The superiority of mercuric chloride over acetic acid as a catalyst for this reaction is reported. Acetylation of the anomeric mixtures gave the heptaacetates from which the major beta'-isomers could be separated by chromatography and/or crystallization.

Automated docking of monosaccharide substrates and analogues and methyl alpha-acarviosinide in the glucoamylase active site

Glucoamylase is an important industrial glucohydrolase with a large specificity range. To investigate its interaction with the monosaccharides D-glucose, D-mannose, and D-galactose and with the substrate analogues 1-deoxynojirimycin, D-glucono-1,5-lactone, and methyl alpha-acarviosinide, MM3(92)-optimized structures were docked into its active site using AutoDock 2.1. The results were compared to structures of glucoamylase complexes obtained by protein crystallography. Charged forms of some substrate analogues were also docked to assess the degree of protonation possessed by glucoamylase inhibitors. Many forms of methyl alpha-acarviosinide were conformationally mapped by using MM3(92), characterizing the conformational pH dependence found for the acarbose family of glucosidase inhibitors. Their significant conformers, representing the most common states of the inhibitor, were used as initial structures for docking. This constitutes a new approach for the exploration of binding modes of carbohydrate chains. Docking results differ slightly from x-ray crystallographic data, the difference being of the order of the crystallographic error. The estimated energetic interactions, even though agreeing in some cases with experimental binding kinetics, are only qualitative due to the large approximations made by AutoDock force field.

Applications of nuclear magnetic resonance spectroscopy and molecular modeling to the study of protein-carbohydrate interactions

This work provides an overview of the applications of NMR to the study of protein-carbohydrate interactions. The use of TR-NOE experiments in this context is given. In particular, the study of Ricin/lactose and Hevein/chitobiose complexes is detailed.

Structure and dynamics of oligosaccharides: NMR and modeling studies

Recent advances in the conformational analysis of oligosaccharides have focused on protein-bound oligosaccharides, glycopeptides, and glycoproteins, as well as on the conformational dynamics about glycosidic linkages. Significant progress has been made possible by dramatic improvements in NMR techniques and advances in computational chemistry and technology. Transferred nuclear Overhauser effects have been used to infer the conformations of carbohydrate ligands bound to protein receptors such as antibodies, lectins and enzymes. The increased use of combined NMR spectroscopic and computational protocols has resulted in insights into the dynamics of glycan chains.