The solution conformation of acarbose

Enzymatic synthesis of a new inhibitor of α-amylases: acarviosinyl-isomaltosyl-spiro-thiohydantoin

Synthesis of acarviosinyl-isomaltosyl-spiro-thiohydantoin in yields up to 20%, has been achieved by Bacillus stearothermophilus maltogenic amylase (BSMA). BSMA is capable of transferring the acarviosine-glucose residue from an acarbose donor onto glucopyranosylidene-spiro-thiohydantoin. Reactions were followed using HPLC and MALDI-TOF MS. 1H and 13C NMR studies revealed that the enzyme reserved its stereoselectivity. Glycosylation took place mainly at C-6 resulting in alpha-acarviosinyl-(1-->4)-alpha-D-glucopyranosyl-(1-->6)-D-glucopyranosylidene-spiro-thiohydantoin. This compound was found to be a much more efficient salivary amylase inhibitor than glucopyranosylidene-spiro-thiohydantoin with kinetic constants of K(EI)=0.19 microM and K(ESI)=0.24 microM.

The biosynthesis of acarbose and validamycin

The studies reported here have established the biosynthetic origin of the mC7N units of acarbose and validamycin from sedo-heptulose 7-phosphate, and have identified 2-epi-5-epi-valiolone as the initial cyclization product. The deoxyhexose moiety of acarbose arises from glucose with deoxythymidyl-diphospho-4-keto-6-deoxy-D-glucose (dTDP-4-keto-6-deoxy-D-glucose) as a proximate intermediate. However, despite the identical origin of the aminocyclitol moieties in acarbose and validamycin A, the pathways of their formation seem to be substantially different. Validamycin A formation involves a number of discrete ketocyclitol intermediates, 5-epi-valiolone, valienone, and validone, whereas no free intermediates have been identified on the pathway from 2-epi-5-epi-valiolone to the pseudodisaccharide moiety of acarbose. The stage is now set for unraveling the mechanism or mechanisms by which the two components of the pseudodisaccharide moieties of acarbose and validamycin are uniquely coupled to each other via a nitrogen bridge.

Addition of maltodextrins to the nonreducing-end of acarbose by reaction of acarbose with cyclomaltohexaose and cyclomaltodextrin glucanyltransferase

New kinds of acarbose analogues were synthesized by the reaction of acarbose with cyclomaltohexaose and cyclomaltodextrin glucanyltransferase (CGTase). Three major CGTase coupling products were separated and purified by Bio-Gel P2 gel-permeation chromatography. Digestion of the three products by beta-amylase and glucoamylase showed that they were composed of maltohexaose (G6), maltododecaose (G12), and maltooctadecaose (G18), respectively, attached to the nonreducing-end of acarbose. 13C NMR of the glucoamylase product (D-glucopyranosyl-acarbose) showed that the D-glucose moiety was attached alpha- to the C-4-OH group of the nonreducing-end cyclohexene ring of acarbose, indicating that the maltodextrins were attached alpha-(1-->4) to the nonreducing-end cyclohexene of acarbose.

Two butylated aminooligosaccharides isolated from the culture filtrate of Streptomyces luteogriseus

Two novel aminooligosaccharides, butytatins M03 and M13 were isolated and purified from the culture filtrate of Streptomyces luteogriseus. Analysis by liquid chromatography coupled to electrospray ionization mass spectrometry indicated their resemblance to isovalertatin, with a four-carbon acyl group. Their structures were established by NMR as aminooligosaccharide derivatives possessing a butylated side chain.

Structural revision of isovalertatins M03, M13, and M23 isolated from the culture of Streptomyces luteogriseus

Three aminooligosaccharides, isovalertatins M03 (1), M13 (2), and M23 (3) were isolated and purified from the culture filtrate of Streptomyces luteogriseus. Their physicochemical properties, liquid chromatographic behavior, and spectroscopic data were in full accordance with the reported compounds [Xu, Q.; Wang, Q.; Lu, D. CN Patent 1100756, 1995; Chem. Abstr. 1995, 123, 110278n], but their structures were reinvestigated and revised by spectroscopic methods, including ESI multistage mass spectrometry and 2-dimensional NMR techniques.

X-ray structure of acarbose bound to amylomaltase from Thermus aquaticus. Implications for the synthesis of large cyclic glucans

As a member of the alpha-amylase superfamily of enzymes, amylomaltase catalyzes either the transglycosylation from one alpha-1,4 glucan to another or an intramolecular cyclization. The latter reaction is typical for cyclodextrin glucanotransferases. In contrast to these enzymes, amylomaltase catalyzes the formation of cyclic glucans with a degree of polymerization larger than 22. To characterize the factors that determine the size of the synthesized cycloamyloses, we have analyzed the X-ray structure of amylomaltase from Thermus aquaticus in complex with the inhibitor acarbose, a maltotetraose derivative, at 1.9 A resolution. Two acarbose molecules are bound to the enzyme, one in the active site groove at subsite -3 to +1 and a second one approximately 14 A away from the nonreducing end of the acarbose bound to the catalytic site. The inhibitor bound to the catalytic site occupies subsites -3 to +1. Unlike the situation in other enzymes of the alpha-amylase family, the inhibitor is not processed and the inhibitory cyclitol ring of acarbose, which mimicks the half chair conformation of the transition state, does not bind to catalytic subsite -1. The minimum ring size of cycloamyloses produced by this enzyme is proposed to be determined by the distance of the specific substrate binding sites at the active site and near Tyr54 and by the size of the 460s loop. The 250s loop might be involved in binding of the substrate at the reducing end of the scissile bond.

Molecular structure of a barley alpha-amylase-inhibitor complex: implications for starch binding and catalysis

alpha-Amylases are widely occurring, multidomain proteins with a catalytic (beta/alpha)8-barrel. In barley alpha-amylase, insight into the catalytic mechanism is gained from the X-ray crystal structure of its molecular complex with acarbose, a pseudotetrasaccharide that acts like a transition-state analogue and which is shown to bind at two specific regions of the enzyme. The structure of the complex has been refined to an R-factor of 15.1% for all observations with Fo>sigma(Fo) between 10 and 2.8 A resolution. A difference Fourier map produced after refinement of the native structure against the data of the acarbose complex clearly revealed density corresponding to two oligosaccharide-binding sites. One of these is defined as the surface-located starch granule-binding site characteristic of cereal alpha-amylases. It involves stacking of two acarbose rings on Trp276 and Trp277. The other binding region is the active site covering subsites -1, +1 and +2. Here, Glu204 is positioned to act in general acid/base catalysis protonating the glucosidic oxygen atom assisted by Asp289. A water molecule that bridges Glu204 and Asp289 is found at the entrance cavity containing a total of five water molecules. This water molecule is proposed to reprotonate Glu204 and supply the hydroxyl ion for nucleophilic attack on the glucosyl C1 atom. Asp 179 acts as the nucleophile that can bind covalently to the substrate intermediate after bond cleavage. The present complex structure together with the conservation of active-site residues among alpha-amylases and related enzymes, are consistent with a common catalytic mechanism for this class of retaining carbohydrases.

Analysis of the antidiabetic drug acarbose by capillary electrophoresis

This study describes the derivatization of the pseudooligosaccharide acarbose and its main metabolite, component 2, with 7-aminonaphthalene-1,3-disulfonic acid (ANDS) in human urine. Their efficient separation was possible by means of capillary zone electrophoresis, using a capillary tube of fused-silica containing 100 mM triethylammonium phosphate buffer, pH 1.5. On column laser-induced fluorescence allowed the detection of the pseudooligosaccharides in human urine in the nanomolar range. With this method, acarbose and component 2 were quantified in human urine after application of 300 mg of acarbose.

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.

Thermodynamics of inhibitor binding to the catalytic site of glucoamylase from Aspergillus niger determined by displacement titration calorimetry

The binding of different inhibitors to glucoamylase G2 from Aspergillus niger and its temperature and pH dependencies have been studied by titration calorimetry. The enzyme binds the inhibitors 1-deoxynojirimycin and the pseudo-tetrasaccharide acarbose with association constants of 3 x 10(4) and 9 x 10(11) M-1, respectively, at 27 degrees C. The binding free energy for both ligands is remarkably temperature-invariant in the interval from 9 to 54 degrees C as the result of large compensating changes in enthalpy and entropy. Acarbose and 1-deoxynojirimycin bound with slightly different free energy-pH profiles, with optima at 5.5 and 5.5-7.0, respectively. Variations in delta H degrees and T delta S degrees as a function of pH were substantially larger than variations in delta G degrees in a partly compensatory manner. Two titratable groups at or near subsite 1 of the catalytic site were found to change their pKa slightly upon binding. The hydrogenated forms of acarbose, D-gluco- and L-ido-dihydroacarbose, bind with greatly reduced association constants of 3 x 10(7) and 2 x 10(5) M-1, respectively, and the pseudo-disaccharide methyl acarviosinide, lacking the two glucose units at the reducing end compared to acarbose, has a binding constant of 8 x 10(6) M-1; these values all result from losses in both enthalpy and entropy compared to acarbose. Three thio analogues of the substrate maltose, methyl alpha- and beta-4-thiomaltoside and methyl alpha-4,5'-dithiomaltoside, bind with affinities from 3 x 10(3) to 6 x 10(4) M-1.(ABSTRACT TRUNCATED AT 250 WORDS)

The active center of a mammalian alpha-amylase. Structure of the complex of a pancreatic alpha-amylase with a carbohydrate inhibitor refined to 2.2-A resolution

An X-ray structure analysis of a crystal of pig pancreatic alpha-amylase (EC 3.2.1.1) that was soaked with acarbose (a pseudotetrasaccharide alpha-amylase inhibitor) showed electron density corresponding to five fully occupied subsites in the active site. The crystal structure was refined to an R-factor of 15.3%, with a root mean square deviation in bond distances of 0.015 A. The model includes all 496 residues of the enzyme, one calcium ion, one chloride ion, 393 water molecules, and five bound sugar rings. The pseudodisaccharide acarviosine that is the essential structural unit responsible for the activity of all inhibitors of the acarbose type was located at the catalytic center. The carboxylic oxygens of the catalytically competent residues Glu233 and Asp300 form hydrogen bonds with the "glycosidic" NH group of the acarviosine group. The third residue of the catalytic triad Asp197 is located on the opposite side of the inhibitor binding cleft with one of its carbonyl oxygens at a 3.3-A distance from the anomeric carbon C-1 of the inhibitor center. Binding of inhibitor induces structural changes at the active site of the enzyme. A loop region between residues 304 and 309 moves in toward the bound saccharide, the resulting maximal mainchain movement being 5 A for His305. The side chain of residue Asp300 rotates upon inhibitor binding and makes strong van der Waals contacts with the imidazole ring of His299. Four histidine residues (His101, His201, His299, and His305) are found to be hydrogen-bonded with the inhibitor. Many protein-inhibitor hydrogen bond interactions are observed in the complex structure, as is clear hydrophobic stacking of aromatic residues with the inhibitor surface. The chloride activator ion and structural calcium ion are hydrogen-bonded via their ligands and water molecules to the catalytic residues.

Reaction mechanisms of Trp120-->Phe and wild-type glucoamylases from Aspergillus niger. Interactions with maltooligodextrins and acarbose

Interactions of wild-type and Trp120-->Phe glucoamylase with maltooligodextrin (Gx) substrates and the tight-binding inhibitor acarbose (A) were investigated here using stopped-flow fluorescence spectroscopy and steady-state kinetic measurements. All wild-type and Trp120-->Phe glucoamylase reactions followed the three-step model E + Gx(or A) (k1) <==> (k-1) EGx (or A) (k2) <==> (k-2) E*Gx(or A) (k3) --> E + P or E-A, previously shown to account for the glucoamylase-maltose system [Olsen, K., Svensson, B., & Christensen, U. (1992) Eur. J. Biochem. 209, 777-784]. K1 = k-1/k1, k2, and k-2, and the catalytic constant, k3, are determined. Binding of maltooligodextrins in the first reaction step is weak, with little difference between wild-type and Trp120-->Phe glucoamylase. The second step, involving a conformational change, in contrast, is strongly influenced by the mutation and by the substrate length. Here wild-type glucoamylase reacts faster and forms more stable intermediates the longer the substrate. In contrast, Trp120-->Phe reacts slower the longer the substrate. The effect of the mutation is thus smallest on maltose. The Trp120-->Phe substitution reduces the fluorescence signal only by 12-20%, indicating that other tryptophanyl residues are important in reporting the conformational change. Trp120 also strongly influences the actual catalytic step, since the mutation decreases the kc values 30-80-fold. Acarbose behaves similar to maltotetraose in the first and the second steps with wild-type but not the Trp120-->Phe glucoamylase. Also, a third step in the acarbose reaction which parallels the catalytic step is strongly affected by the mutation. The rate constant k3 increases 200-fold.

Molecular modelling of acarviosine, the pseudo-disaccharide moiety of acarbose, and other inhibitors of alpha-amylases

Acarbose and its homologs inhibit alpha-D-glucosidases, particularly alpha-amylases. These homologs have the same core, the pseudo-disaccharide acarviosine, linked to various numbers of glucose residues. The conformations of (R)- and (S)-acarviosine have been analysed. The potential energy maps, obtained by molecular mechanics calculations, show that acarviosine is flexible and has several important minima. One low-energy form is close to the shape assumed by the acarviosine moiety when acarbose is adsorbed on the surface of glycogen phosphorylase. Another likely conformation is the same as that inferred from n.m.r. data and HSEA calculations. The results reconcile those conflicting reports. Molecular modelling of other inhibitors of alpha-amylases, such as 4-thiomaltose and moranoline, shows that these pseudo-disaccharides can fill similar volumes of conformational space.

Controlled reduction of acarbose: conformational analysis of acarbose and the resulting saturated products

Saturation of the double bond in the non-reducing terminal unit of the tetrasaccharide amylase inhibitor, acarbose (1), with Raney nickel as the catalyst and at pH 8, gave 57% of a approximately 1:1 mixture of the 5a-carba-gluco (2) and -ido (3) isomers together with cleavage products including 26% of the trisaccharide 6-deoxy-alpha-D-Glcp4N-(1----4)-alpha-D-Glcp-(1----4)-D-Glc (4). The saturated compounds were isolated and characterised using 1H- and 13C-n.m.r. spectroscopy. The preferred conformations of 1 and 2 were dependent on the state of ionisation of the bridging nitrogen atom. The inhibition by 1-4 of the hydrolysis of methyl beta-maltoside by glucoamylase has been investigated; 1 and 2 were strong inhibitors.