Mechanism of porcine pancreatic alpha-amylase. Inhibition of amylose and maltopentaose hydrolysis by alpha-, beta- and gamma-cyclodextrins

Analysis of the mechanism of resistance to enzymatic hydrolysis of RS-5 resistant starch

RS-5 refers to the resistant starch formed by complexation of starch molecules with other molecules. In this study, the molecular mechanism of RS-5 was analysed. First, it was found, when α-amylase acted on the starch-lipid complexes, the glucose residues involved in complexation cannot be hydrolyzed by α-amylase, while the glucose residues not directly involved in complexation can be hydrolyzed. Second, lipid molecules are not necessary for the formation of RS-5 and can be replaced with small peptides or decanal molecules. Considering the multiple health hazards that may result from excessive lipid intake, small peptides composed of essential amino acids may be more desirable materials for RS-5 preparation. Third, starch-lipid complexes had strong interactions with α-amylase, which provides evidence in support of the sliding continuum hydrolysis hypothesis of α-amylase. These results revealed the mechanism of RS-5 at the molecular level, which provides a reference for the production and research of RS-5.

Evaluation of inhibitory activity, purification and X-ray crystallography of Alpha-Amylase inhibitor from Phaseolus vulgaris cultivars of Uttarakhand, India

Abstract The present work is based on analysis of inhibitory activity of alpha-amylase inhibitor in selected cultivars of Phaseolus vulgaris of Uttarakhand. Fifteen samples were assessed for inhibitory activity of alpha-amylase inhibitor. Significant variations were found in different cultivars. Crude extract of alpha-amylase inhibitor from sample PUR (Purola) have shown maximum inhibitory activity (70.2 ± 0.84). Crude extract of all the cultivars have shown considerable variations in inhibitory activity in the temperature ranging from 20ºC to 100ºC. Based on inhibitory activity and heat stability profile, the alpha amylase inhibitor was purified from PUR cultivar. The purified inhibitor was found to be stable even at 90ºC with an inhibitory activity of 97.20 ±0.09. The molecular weight of purified inhibitor on Native PAGE (Polyacrylamide gel electrophoresis) was found to be 31kd, consisting of two subunits of 17kd and 14kd on SDS-PAGE.

Conductometric Studies of Formation the Inclusion Complexes of Phenolic Acids with β-Cyclodextrin and 2-HP-β-Cyclodextrin in Aqueous Solutions

An attempt was made to evaluate the possibility of creating and assessing the stability of inclusion complexes of selected phenolic acids [trans-4-hydroxycinnamic acid (trans-p-coumaric acid), trans-3,4-dihydroxycinnamic acid (trans-caffeic acid), trans-4-hydroxy-3-methoxycinnamic acid, (trans-ferulic acid) and trans-3-phenylacrylic acid (trans-cinnamic acid)] with β-cyclodextrin and 2-HP-β-cyclodextrin in aqueous solutions in a wide temperature range 283.15 K-313.15 K. On the basis of the values of the limiting molar conductivity (Λ_CDNaDod), calculated from the experimental data, the values of the formation constants and the thermodynamic functions of formation (standard enthalpy, entropy, and Gibs standard enthalpy) of the studied complexes were determined. It has been found that the stability of the studied complexes increases with lowering of the molar mass of cyclodextrin and lowering of the temperature.

Lecithin Inclusion by α-Cyclodextrin Activates SREBP2 Signaling in the Gut and Ameliorates Postprandial Hyperglycemia

BACKGROUND

α-cyclodextrin (α-CD) is one of the dietary fibers that may have a beneficial effect on cholesterol and/or glucose metabolism, but its efficacy and mode of action remain unclear.

METHODS

In the present study, we examined the anti-hyperglycemic effect of α-CD after oral loading of glucose and liquid meal in mice.

RESULTS

Administration of 2 g/kg α-CD suppressed hyperglycemia after glucose loading, which was associated with increased glucagon-like peptide 1 (GLP-1) secretion and enhanced hepatic glucose sequestration. By contrast, 1 g/kg α-CD similarly suppressed hyperglycemia, but without increasing secretions of GLP-1 and insulin. Furthermore, oral α-CD administration disrupts lipid micelle formation through its inclusion of lecithin in the gut luminal fluid. Importantly, prior inclusion of α-CD with lecithin in vitro nullified the anti-hyperglycemic effect of α-CD in vivo, which was associated with increased intestinal mRNA expressions of SREBP2-target genes (Ldlr, Hmgcr, Pcsk9, and Srebp2).

CONCLUSIONS

α-CD elicits its anti-hyperglycemic effect after glucose loading by inducing lecithin inclusion in the gut lumen and activating SREBP2, which is known to induce cholecystokinin secretion to suppress hepatic glucose production via a gut/brain/liver axis.

Cyclodextrin in starchy foods

Cyclodextrins are widely used in various fields including food industry. In this review, their role in high carbohydrate-containing, starchy foods are reviewed and discussed. Both the effects as functional ingredients affecting the structural properties of starch and as active ingredients slowing down starch digestion and, as a consequence, decreasing the glycaemic index of starchy foods are overviewed without considering the traditional applications as carriers and stabilisers of aroma and flavour, essential oils, polyunsaturated fatty acids, and other bioactive components to enrich foods, even if they are carbohydrate foods. The effect on starch metabolism is explained by the structural transformations caused by cyclodextrins on starch amylose and amylopectin. Several examples are shown how the technological and sensorial properties of bread, rice products, pasta, and other starchy foods are modified by cyclodextrin supplementation, and how the digestibility is changed resulting in reduced glycaemic and insulinaemic effects.

Blackcurrant (Ribes nigrum) lowers sugar-induced postprandial glycaemia independently and in a product with fermented quinoa: a randomised crossover trial

Berries rich in anthocyanins have beneficial effects on postprandial glycaemia. We investigated whether blackcurrant (75 g in a portion) independently and in a product with fermented quinoa induced similar effects on the sugar-induced postprandial glucose metabolism as observed before with 150 g of blackcurrant. Twenty-six healthy subjects (twenty-two females and four males) consumed four test products after fasting overnight in a randomised, controlled crossover design. Each test product portion contained 31 g of available carbohydrates and had similar composition of sugar components: 300 ml water with sucrose, glucose and fructose (SW; reference), blackcurrant purée with added sugars (BC), a product consisting of the blackcurrant purée and a product base with fermented quinoa (BCP) and the product base without blackcurrant (PB). Blood samples were collected at 0, 15, 30, 45, 60, 90, 120 and 180 min after eating each test product to analyse the concentrations of glucose, insulin and NEFA. In comparison with the SW, the intake of both the BC and BCP resulted in reduced glucose and insulin concentrations during the first 30 min, a more balanced decline during the first hour and improved glycaemic profile. The BCP induced more efficient effects than the BC due to the product base with fermented quinoa. A rebound of NEFA after the sugar-induced hypoglycaemic response was attenuated at the late postprandial phase by the BC and BCP. In conclusion, we showed that 75 g of blackcurrant and the product with fermented quinoa were able to lower postprandial glycaemia and insulinaemia.

Alpha-Cyclodextrin Attenuates the Glycemic and Insulinemic Impact of White Bread in Healthy Male Volunteers

The demonstration of a physiological benefit has recently become an indispensible element of the definition of dietary fibers. In the here-reported pilot study, the effect of alpha-cyclodextrin (alpha-CD) on the postprandial glycemic and insulinemic effect of starch was examined. Twelve fasted, healthy male volunteers received, on three subsequent days, a test breakfast consisting of (A) 100 g fresh white bread (providing 50 g starch) and 250 mL drinking water, (B) the same bread with a supplement of 10 g alpha-CD dissolved in the drinking water, and (C) 25 g alpha-CD dissolved in drinking water. Capillary and venous blood was sampled before the breakfast and in regular intervals for a three-hour period thereafter. Glucose was determined in capillary blood and insulin in the plasma of venous blood samples. Breakfast (A) led to a rapid rise in blood glucose and insulin. In breakfast (B), alpha-CD reduced the areas under the curve of blood glucose and insulin significantly by 59% and 57%, respectively, demonstrating that alpha-CD inhibits and thereby delays starch digestion. Treatment (C) was not associated with a rise of blood glucose. Hence, alpha-CD complies with the current definition of dietary fiber in every respect.

Synthesis of novel α-glucans with potential health benefits through controlled glucose release in the human gastrointestinal tract

The glycemic carbohydrates we consume are currently viewed in an unfavorable light in both the consumer and medical research worlds. In significant part, these carbohydrates, mainly starch and sucrose, are looked upon negatively due to their rapid and abrupt glucose delivery to the body which causes a high glycemic response. However, dietary carbohydrates which are digested and release glucose in a slow manner are recognized as providing health benefits. Slow digestion of glycemic carbohydrates can be caused by several factors, including food matrix effect which impedes α-amylase access to substrate, or partial inhibition by plant secondary metabolites such as phenolic compounds. Differences in digestion rate of these carbohydrates may also be due to their specific structures (e.g. variations in degree of branching and/or glycosidic linkages present). In recent years, much has been learned about the synthesis and digestion kinetics of novel α-glucans (i.e. small oligosaccharides or larger polysaccharides based on glucose units linked in different positions by α-bonds). It is the synthesis and digestion of such structures that is the subject of this review.

Characterization of α-Glucosidase Inhibitor/Cyclodextrin Complex Prepared by Freeze-Drying

Miglitol (MT) is an α-glucosidase inhibitor with a postmeal blood glucose level lowering effect that is used to treat type 2 diabetes. In addition, α-cyclodextrin (αCD) has been reported to inhibit increases in postmeal blood glucose. The aim of this study was to prepare a freeze-dried product (FD) composed of MT and αCD or γCD (molar ratio of MT/αCD = 1/1, MT/γCD = 1/1) and to evaluate the physicochemical properties and biological activity of the FD. The PXRD profile of FD exhibited a halo pattern, and characteristic peaks derived from MT, αCD, and γCD were not observed. The TG-DTA results for FD indicated an increased weight loss temperature and the absence of an endothermic peak for MT. The NIR absorption spectrum measurement suggested an intermolecular interaction between MT and αCD or γCD in the FD. ¹H-¹H NOESY NMR spectroscopy (D₂O) revealed an intermolecular interaction in the FD. The results of the α-glucosidase activity inhibition test and the α-amylase activity inhibition test indicated that the FD exhibited the same inhibition rate as MT alone and the effects of MT were not altered by the freeze-drying method.

Cyclodextrins in Food Technology and Human Nutrition: Benefits and Limitations

Cyclodextrins are tasteless, odorless, nondigestible, noncaloric, noncariogenic saccharides, which reduce the digestion of carbohydrates and lipids. They have low glycemic index and decrease the glycemic index of the food. They are either non- or only partly digestible by the enzymes of the human gastrointestinal (GI) tract and fermented by the gut microflora. Based on these properties, cyclodextrins are dietary fibers useful for controlling the body weight and blood lipid profile. They are prebiotics, improve the intestinal microflora by selective proliferation of bifidobacteria. These antiobesity and anti-diabetic effects make them bioactive food supplements and nutraceuticals. In this review, these features are evaluated for α-, β- and γ-cyclodextrins, which are the cyclodextrin variants approved by authorities for food applications. The mechanisms behind these effects are reviewed together with the applications as solubilizers, stabilizers of dietary lipids, such as unsaturated fatty acids, phytosterols, vitamins, flavonoids, carotenoids and other nutraceuticals. The recent applications of cyclodextrins for reducing unwanted components, such as trans-fats, allergens, mycotoxins, acrylamides, bitter compounds, as well as in smart active packaging of foods are also overviewed.

Property of midgut α-amylase from Mythimna separata (Lepidoptera: Noctuidae) larvae and its responses to potential inhibitors in vitro

Midgut α-amylase is an important digestive enzyme involved in larval energy metabolism and carbohydrate assimilation. In this article, the properties of midgut α-amylase from the Oriental armyworm, Mythimna separata (Lepidoptera: Noctuidae), larvae were characterized, and its in vitro responses to chemical inhibitors were also determined. The kinetic parameters Km and Vmax of midgut α-amylase were 0.064 M, 4.81 U mg pro(-1) in phosphate buffer, and 0.128 M, 1.96 U mg pro(-1) in barbiturate-acetate buffer; α-amylase activity linearly increased as starch concentration increased. α-Amylase activity was not influenced by amino acids such as Pro, Met, Try, His, Ala, and Phe but was strongly activated by antioxidants. Reduced glutathione, 1,4-dithiothreitol, β-mercaptoethanol, and ascorbic acid improved the activity of α-amylase about 2.06, 3.46, 3.37, and 6.38 times, respectively, relative to the control. Ethylenediaminetetraacetic acid, sodium dodecyl sulfonate, and N-bromosuccinimide (NBS) strongly inhibited α-amylase. α-, β-, and γ-cyclodextrin were not the preferred substrates for α-amylase. Kinetic analysis showed that IC50 value of NBS against α-amylase was 1.52 (±0.26) µM, and the mode of action of NBS with Ki as 2.53 (0.35) µM was a mixed-type inhibition that indicated a combination of partial competitive and pure noncompetitive inhibition. The midgut α-amylase of armyworm larvae may be a potential target for novel insecticide development and pest control.

Enzymatic conversions of starch

This article surveys methods for the enzymatic conversion of starch, involving hydrolases and nonhydrolyzing enzymes, as well as the role of microorganisms producing such enzymes. The sources of the most common enzymes are listed. These starch conversions are also presented in relation to their applications in the food, pharmaceutical, pulp, textile, and other branches of industry. Some sections are devoted to the fermentation of starch to ethanol and other products, and to the production of cyclodextrins, along with the properties of these products. Light is also shed on the enzymes involved in the digestion of starch in human and animal organisms. Enzymatic processes acting on starch are useful in structural studies of the substrates and in understanding the characteristics of digesting enzymes. One section presents the application of enzymes to these problems. The information that is included covers the period from the early 19th century up to 2009.

X-ray crystallographic analyses of pig pancreatic alpha-amylase with limit dextrin, oligosaccharide, and alpha-cyclodextrin

Further refinement of the model using maximum likelihood procedures and reevaluation of the native electron density map has shown that crystals of pig pancreatic alpha-amylase, whose structure we reported more than 15 years ago, in fact contain a substantial amount of carbohydrate. The carbohydrate fragments are the products of glycogen digestion carried out as an essential step of the protein's purification procedure. In particular, the substrate-binding cleft contains a limit dextrin of six glucose residues, one of which contains both alpha-(1,4) and alpha-(1,6) linkages to contiguous residues. The disaccharide in the original model, shared between two amylase molecules in the crystal lattice, but also occupying a portion of the substrate-binding cleft, is now seen to be a tetrasaccharide. There are, in addition, several other probable monosaccharide binding sites. Furthermore, we have further reviewed our X-ray diffraction analysis of alpha-amylase complexed with alpha-cyclodextrin. alpha-Amylase binds three cyclodextrin molecules. Glucose residues of two of the rings superimpose upon the limit dextrin and the tetrasaccharide. The limit dextrin superimposes in large part upon linear oligosaccharide inhibitors visualized by other investigators. By comprehensive integration of these complexes we have constructed a model for the binding of polysaccharides having the helical character known to be present in natural substrates such as starch and glycogen.

Mucoadhesive microspheres for nasal administration of cyclodextrins

The aim of this study was to examine the in vitro capacity of cyclodextrins to interfere on the beta-amyloid fibril formation; then, mucoadhesive microspheres containing cyclodextrins were prepared and characterised as nasal delivery system for brain targeting. Eight batches of microspheres containing chitosan or alginate loaded with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin in two different cyclodextrin to polymer ratios were produced by spray drying. The results show that none of the tested CDs has direct cellular toxicity and they protect the cell viability from beta-peptide. The microspheres prepared are characterised by small particle sizes, ability to absorb water and to delay the in vitro dissolution rate of the CDs; good ex vivo mucoadhesive properties of the formulations are assessed. The microsphere properties are influenced by the kind of polymer, of cyclodextrin and by cyclodextrin to polymer ratio used. In particular, the alginate formulation containing the higher cyclodextrin content shows the best performance.

The nutraceutical role of thePhaseolus vulgarisα-amylase inhibitor

The present review assesses the potential of thePhaseolus vulgarisα-amylase inhibitor isoform 1 (α-AI1) starch blockers as a widely used remedy against obesity and diabetes. Consumption of the α-amylase inhibitor causes marginal intraluminal α-amylase activity facilitated by the inhibitor's appropriate structural, physico-chemical and functional properties. As a result there is decreased postprandial plasma hyperglycaemia and insulin levels, increased resistance of starch to digestion and increased activity of colorectal bacteria. The efficacy and safety of the amylase inhibitor extracts, however, depend on the processing and extraction techniques used. The extracts are potential ingredients in foods for increased carbohydrate tolerance in diabetics, decreased energy intake for reducing obesity and for increased resistant starch. Research developments in the distribution and biosynthesis of the α-amylase inhibitor, relevant physico-chemical properties, the molecular starch-blocking mechanism, anti-obesity and anti-diabetes effects, safety of extracts and the need for research into their potential anti-colorectal cancer effect are discussed.

The emulsion flocculation stability of protein–carbohydrate diblock copolymers

The effect of the steric layer thickness on the flocculation stability of beta-lactoglobulin-carbohydrate diblock copolymers was assessed. The diblock copolymers were created by conjugating beta-lactoglobulin to maltose or a series of different M(n) maltodextrins using the Maillard reaction. The thickness and spatial arrangement of the interfacial layers were assessed via latex adsorption and selective enzymatic digestion studies. An increase in the molecular weight of the maltodextrin (900, 1900 and 3800 Da) increased the interfacial thickness (1.1, 2.5 and 7.3 nm, respectively). No detectable change to interfacial thickness was observed upon the attachment of maltose. The increase in the interfacial layer thickness scaled with the hydrodynamic size of the carbohydrate. The beta-lactoglobulin-maltodextrin conjugates were found to have a diblock architecture, with the protein anchored at the surface and the carbohydrate protruding into the aqueous continuous phase. The stability of oil-in-water emulsions formed using the conjugates was assessed by exposing them to salt (150 mM NaCl or 0-20 mM CaCl(2)), heat alone or heat in the presence of 150 mM NaCl. Conjugation of a 900 Da maltodextrin provided sufficient steric stabilization to prevent flocculation in high salt environments. The effect of the (number) density of the steric layer was also assessed by controlling the average number of maltodextrins attached per beta-lactoglobulin molecule. The steric layer density at which emulsions became unstable was a function of carbohydrate M(n). Emulsions made from the 900 Da maltodextrin conjugate became unstable below a steric layer density of one tail per 7.5 nm(2), whilst emulsions made from the 1900 Da maltodextrin were unstable below a steric layer density of one tail per 9.5 nm(2). This trend was expected and can be explained by the stronger van der Waals attraction that arises from the closer interdroplet separations that are permissible with the shorter maltodextrins. The excellent flocculation stability of Maillard conjugate emulsions is thought to arise from the combined effects of weak electrostatic repulsion from the screened protein surface charge and steric repulsion from the attached carbohydrate layer. This means that attachment of a relatively thin steric layer is enough to stabilize the emulsions against flocculation. These findings have important implications for the development of commercial processes to manufacture protein-carbohydrate Maillard conjugate emulsifiers. Furthermore the work provides a greater empirical understanding of the relationship between interfacial architecture and colloidal stability, and may provide the means for greater theoretical understanding of biopolymer stabilization of interfaces.

The interplay of processivity, substrate inhibition and a secondary substrate binding site of an insect exo-β-1,3-glucanase

Abracris flavolineata midgut contains a processive exo-beta-glucanase (ALAM) with lytic activity against Saccharomyces cerevisiae, which was purified (yield, 18%; enrichment, 37 fold; specific activity, 1.89 U/mg). ALAM hydrolyses fungal cells or callose from the diet. ALAM (45 kDa; pI 5.5; pH optimum 6) major products with 0.6 mM laminarin as substrate are beta-glucose (61%) and laminaribiose (39%). Kinetic data obtained with laminaridextrins and methylumbelliferyl glucoside suggest that ALAM has an active site with at least six subsites. The best fitting of kinetic data to theoretical curves is obtained using a model where one laminarin molecule binds first to a high-affinity accessory site, causing active site exposure, followed by the transference of the substrate to the active site. The two-binding-site model is supported by results from chemical modifications of amino acid residues and by ALAM action in MUbetaGlu plus laminarin. Low laminarin concentrations increase the modification of His, Tyr and Asp or Glu residues and MUbetaGlu hydrolysis, whereas high concentrations abolish modification and inhibit MUbetaGlu hydrolysis. Our data indicate that processivity results from consecutive transferences of substrate between accessory and active site and that substrate inhibition arises when both sites are occupied by substrate molecules abolishing processivity.

Dose-Dependent Inhibition of the Post-Prandial Glycaemic Response to a Standard Carbohydrate Meal following Incorporation of Alpha-Cyclodextrin

BACKGROUND

This study evaluated the dose-response effects of alpha-cyclodextrin, a cyclic oligosaccharide, on the glycaemic and insulinaemic responses to the consumption of a standard carbohydrate meal.

METHODS

In a double-blind, randomised, cross-over design, 10 healthy subjects consumed boiled white rice containing 50 g of digestible carbohydrate to which 0 (control), 2, 5 or 10 g of alpha-cyclodextrin was added. Plasma glucose and insulin concentrations were determined prior to and for 2 h after consumption of each meal.

RESULTS

The area under the plasma glucose curve was negatively related to the dose of alpha-cyclodextrin (r(2)=0.97, p=0.02), with the areas being significantly reduced at the 5- and 10-gram doses compared with the control (p<0.05). alpha-Cyclodextrin did not affect the area under the plasma insulin curve (p=0.39). Higher doses of alpha-cyclodextrin resulted in greater satiety, but were associated with reduced palatability and an increased incidence of minor gastrointestinal complaints (stomach ache, nausea, bloating).

CONCLUSION

alpha-Cyclodextrin reduces the glycaemic response to a standard carbohydrate meal in a dose-dependent manner and may be useful as an ingredient for reducing the glycaemic impact of such foods.

Involvement of Individual Subsites and Secondary Substrate Binding Sites in Multiple Attack on Amylose by Barley α-Amylase

Barley alpha-amylase 1 (AMY1) hydrolyzed amylose with a degree of multiple attack (DMA) of 1.9; that is, on average, 2.9 glycoside bonds are cleaved per productive enzyme-substrate encounter. Six AMY1 mutants, spanning the substrate binding cleft from subsites -6 to +4, and a fusion protein, AMY1-SBD, of AMY1 and the starch binding domain (SBD) of Aspergillus niger glucoamylase were also analyzed. DMA of the subsite -6 mutant Y105A and AMY1-SBD increased to 3.3 and 3.0, respectively. M53E, M298S, and T212W at subsites -2, +1/+2, and +4, respectively, and the double mutant Y105A/T212W had decreased DMA of 1.0-1.4. C95A (subsite -5) had a DMA similar to that of wild type. Maltoheptaose (G7) was always the major initial oligosaccharide product. Wild-type and the subsite mutants released G6 at 27-40%, G8 at 60-70%, G9 at 39-48%, and G10 at 33-44% of the G7 rate, whereas AMY1-SBD more efficiently produced G8, G9, and G10 at rates similar to, 66%, and 60% of G7, respectively. In contrast, the shorter products appeared with large individual differences: G1, 0-15%; G2, 8-43%; G3, 0-22%; and G4, 0-11% of the G7 rate. G5 was always a minor product. Multiple attack thus involves both longer translocation of substrate in the binding cleft upon the initial cleavage to produce G6-G10, essentially independent of subsite mutations, and short-distance moves resulting in individually very different rates of release of G1-G4. Accordingly, the degree of multiple attack as well as the profile of products can be manipulated by structural changes in the active site or by introduction of extra substrate binding sites.

Biotechnological applications of cyclodextrins

Cyclodextrins (CDs) are a family of cyclic oligosaccharides that are composed of alpha-1,4-linked glucopyranose subunits. Cyclodextrins are produced from starch by enzymatic degradation. These macrocyclic carbohydrates with apolar internal cavities can form complexes with and solubilize many normally water-insoluble compounds. This review describes recent applications of CDs in pharmaceuticals with a major emphasis on drug delivery systems. The utility of these water-soluble cyclic glucans in a variety of foods, flavors cosmetics, packaging and textiles is elaborated. The role of these compounds in biocatalysis is also discussed. Cyclodextrins are used in separation science because they have been shown to discriminate between positional isomers, functional groups, homologues and enantiomers. This property makes them a useful agent for a wide variety of separations.

On the mechanism of α-amylase

Two inhibitors, acarbose and cyclodextrins (CD), were used to investigate the active site structure and function of barley alpha-amylase isozymes, AMY1 and AMY2. The hydrolysis of DP 4900-amylose, reduced (r) DP18-maltodextrin and maltoheptaose (catalysed by AMY1 and AMY2) was followed in the absence and in the presence of inhibitor. Without inhibitor, the highest activity was obtained with amylose, kcat/Km decreased 103-fold using rDP18-maltodextrin and 10(5) to 10(6)-fold using maltoheptaose as substrate. Acarbose is an uncompetitive inhibitor with inhibition constant (L1i) for amylose and maltodextrin in the micromolar range. Acarbose did not bind to the active site of the enzyme, but to a secondary site to give an abortive ESI complex. Only AMY2 has a second secondary binding site corresponding to an ESI2 complex. In contrast, acarbose is a mixed noncompetitive inhibitor of maltoheptaose hydrolysis. Consequently, in the presence of this oligosaccharide substrate, acarbose bound both to the active site and to a secondary binding site. alpha-CD inhibited the AMY1 and AMY2 catalysed hydrolysis of amylose, but was a very weak inhibitor compared to acarbose.beta- and gamma-CD are not inhibitors. These results are different from those obtained previously with PPA. However in AMY1, as already shown for amylases of animal and bacterial origin, in addition to the active site, one secondary carbohydrate binding site (s1) was necessary for activity whereas two secondary sites (s1 and s2) were required for the AMY2 activity. The first secondary site in both AMY1 and AMY2 was only functional when substrate was bound in the active site. This appears to be a general feature of the alpha-amylase family.

Thermoanaerobacterium thermosulfurigenes cyclodextrin glycosyltransferase

Cyclodextrin glycosyltransferase (CGTase) uses an alpha-retaining double displacement mechanism to catalyze three distinct transglycosylation reactions. To investigate these reactions as catalyzed by the CGTase from Thermoanaerobacterium thermosulfurigenes the enzyme was overproduced (8 mg.L(-1) culture) using Bacillus subtilis as a host. Detailed analysis revealed that the three reactions proceed via different kinetic mechanisms. The cyclization reaction (cyclodextrin formation from starch) is a one-substrate reaction, whereas the other two transglycosylation reactions are two-substrate reactions, which obey substituted enzyme mechanism kinetics (disproportionation reaction) or ternary complex mechanism kinetics (coupling reaction). Analysis of the effects of acarbose and cyclodextrins on the disproportionation reaction revealed that cyclodextrins are competitive inhibitors, whereas acarbose is a mixed type of inhibitor. Our results show that one molecule of acarbose binds either in the active site of the free enzyme, or at a secondary site of the enzyme-substrate complex. The mixed inhibition thus indicates the existence of a secondary sugar binding site near the active site of T. thermosulfurigenes CGTase.

The effect of guar galactomannan and water availability during hydrothermal processing on the hydrolysis of starch catalysed by pancreatic alpha-amylase

The effects of water-soluble nonstarch polysaccharides (sNSP) on human metabolism are considered to be beneficial because they decrease postprandial glycaemia and insulinaemia following ingestion of starch-rich foods. The mechanisms by which sNSP attenuate the postprandial rise in blood glucose are not well understood but their presence increases the viscosity of gastrointestinal contents, which affects physiological functions, e.g. gastric emptying and peristalsis. Increased viscosity and decreased water activity during hydrothermal treatment of starch could influence alpha-amylase action. Using guar galactomannan as a representative of sNSP, we found that galactomannan has a direct noncompetitive inhibitory effect on alpha-amylase with a K(i) value of approximately 0.5% (3.3 microM). The inhibition is not time dependent and studies suggest direct binding of the enzyme to galactomannan; the resulting galactomannan-amylase complex being inactive. Processing of starch at low water levels greatly affects the catalytic efficiency of alpha-amylase. The Km value for starch heat treated in limited water is raised and kcat is lowered relative to starch gelatinised in excess water. Since galactomannan has no effect on the Km of alpha-amylase, we conclude that the inhibitory action of the polymer is not secondary to a decrease in available water. Neither does it seem to be a consequence of impaired diffusion of enzyme, substrate and products because of an increase in viscosity of the medium.Thus, the effects of sNSP in lowering postprandial glycaemia not only involve modifications of gut physiology, but also include direct inhibition of the first stage in the biochemical degradation of starch.

Plant alpha-amylase inhibitors and their interaction with insect alpha-amylases

Insect pests and pathogens (fungi, bacteria and viruses) are responsible for severe crop losses. Insects feed directly on the plant tissues, while the pathogens lead to damage or death of the plant. Plants have evolved a certain degree of resistance through the production of defence compounds, which may be aproteic, e.g. antibiotics, alkaloids, terpenes, cyanogenic glucosides or proteic, e.g. chitinases, beta-1,3-glucanases, lectins, arcelins, vicilins, systemins and enzyme inhibitors. The enzyme inhibitors impede digestion through their action on insect gut digestive alpha-amylases and proteinases, which play a key role in the digestion of plant starch and proteins. The natural defences of crop plants may be improved through the use of transgenic technology. Current research in the area focuses particularly on weevils as these are highly dependent on starch for their energy supply. Six different alpha-amylase inhibitor classes, lectin-like, knottin-like, cereal-type, Kunitz-like, gamma-purothionin-like and thaumatin-like could be used in pest control. These classes of inhibitors show remarkable structural variety leading to different modes of inhibition and different specificity profiles against diverse alpha-amylases. Specificity of inhibition is an important issue as the introduced inhibitor must not adversely affect the plant's own alpha-amylases, nor the nutritional value of the crop. Of particular interest are some bifunctional inhibitors with additional favourable properties, such as proteinase inhibitory activity or chitinase activity. The area has benefited from the recent determination of many structures of alpha-amylases, inhibitors and complexes. These structures highlight the remarkable variety in structural modes of alpha-amylase inhibition. The continuing discovery of new classes of alpha-amylase inhibitor ensures that exciting discoveries remain to be made. In this review, we summarize existing knowledge of insect alpha-amylases, plant alpha-amylase inhibitors and their interaction. Positive results recently obtained for transgenic plants and future prospects in the area are reviewed.

Starch digestion in tropical fishes: isolation, structural studies and inhibition kinetics of alpha-amylases from two tilapias Oreochromis niloticus and Sarotherodon melanotheron

alpha-Amylases from the intestinal cavity of two tilapia species, Oreochromis niloticus (ONI-AMY) and Sarotherodon melanotheron (SME-AMY), were purified using ammonium sulfate precipitation, affinity chromatography and chromatofocusing procedures. The purification was approximately 100-fold. The amylolytic activity, specific activity, product distribution, pH and temperature profile of ONI-AMY and SME-AMY are quite similar. The molecular mass differs slightly: 56600 (ONI-AMY) vs. 55500 (SME-AMY). As shown by isoelectric focusing analysis, both amylases contain two isoforms A and B with distinct pI: 7.2 (A) and 7.8 (B), vs. 8.3 (A) and 8.8 (B), respectively. It was not possible to isolate B, since B converts into A with time. The kinetics of the inhibition of ONI-AMY and SME-AMY activity by alpha-, beta- and gamma-cyclodextrin (alpha-, beta- and gamma-CD) were investigated using amylose as the substrate. Statistical analysis of the kinetic data expressed using a general velocity equation and assuming rapid equilibrium showed that the inhibition is of the mixed noncompetitive type. Similar results were obtained with ONI-AMY and SME-AMY. beta- and gamma-CD are stronger inhibitors than alpha-CD. ONI-AMY and SME-AMY are then closely related and show the general features common to the members of the alpha-amylase class (family 13). They enable ONI and SME tilapias to digest starch in food.