Subsite profile of the active center of porcine pancreatic alpha-amylase. Kinetic studies using maltooligosaccharides as substrates

Molecular cloning and characterization of an alpha-amylase inhibitor (TkAAI) gene from Trichosanthes kirilowii Maxim

Trichosanthes kirilowii Maxim taxonomically belongs to the Cucurbitaceae family and Trichosanthes genus. Its whole fruit, fruit peel, seed and root are widely used in traditional Chinese medicines. A ribosome-inactivating protein with RNA N-glycosidase activity called Trichosanthrip was isolated and purified from the seeds of T. kirilowii in our recent previous research. To further explore the biological functions of Trichosanthrip, the cDNA of T. kirilowii alpha-amylase inhibitor (TkAAI) was cloned through rapid-amplification of cDNA ends and its sequence was analyzed. Also, the heterologous protein was expressed in Escherichia coli and its alpha-amylase activity was further measured under optimized conditions. The full-length cDNA of TkAAI was 613 bp. The speculated open reading frame sequence encoded 141 amino acids with a molecular weight of 16.14 kDa. Phylogenetic analysis demonstrated that the Alpha-Amylase Inhibitors Seed Storage domain sequence of TkAAI revealed significant evolutionary homology with the 2S albumin derived from the other plants in the Cucurbitaceae group. In addition, TkAAI was assembled into pET28a with eGFP to generate a prokaryotic expression vector and was induced to express in E. coli. The TkAAI-eGFP infusion protein was proven to exhibit alpha-amylase inhibitory activity against porcine pancreatic amylase in a suitable reaction system. Analysis of gene expression patterns proved that the relative expression level of TkAAI in seeds is highest. The results presented here forecasted that the TkAAI might play a crucial role during the development of T. kirilowii seeds and provided fundamental insights into the possibility of T. kirilowii derived medicine to treat diabetes related diseases.

Enzymatic synthesis, structure of isomalto/malto-polysaccharides from linear dextrins prepared by retrogradation

Isomalto/malto-polysaccharides (IMMPs) with degree of polymerization (DP) 10-100 have novel potential applications, including enhanced solubility and anti-inflammatory. However, there are minimal synthetic methods for preparing IMMPs with a relatively higher DP, which is due to the lack of suitable molecular weight linear dextrins (I-LDs). The existing I-LDs preparation methods have disadvantages, such as low yield and uncontrollable molecular weight. Therefore, this study proposes a method for preparing soluble linear dextrins (S-LDs, M_w = 2.1 kDa) by low-temperature retrogradation from debranched waxy corn starch (M_w = 3.0 kDa). S-LDs reacted with 4,6-α-glucanotransferase GtfB-ΔN from Limosilactobacillus reuteri 121 to yield IMMPs with 12.3 kDa M_w and 83.8% α1 → 6 linkages content. Process monitoring revealed the synthesis mechanism and a detailed reaction process. Finally, IMMPs were identified by enzyme fingerprinting as α1 → 6 chains with α1 → 4 fragments inlaid at the reducing, non-reducing end, and middle part. This study provides a new synthesis method and more structural information for IMMPs.

Cloning, expression, and characterization of a porcine pancreatic α-amylase in Pichia pastoris

Pancreatic α-amylase (α-1, 4-glucan-4-glucanohydrolase, EC.3.2.1.1) plays a primary role in the intestinal digestion of feed starch and is often deficient in weanling pigs. The objective of this study was to clone, express, and characterize porcine pancreatic α-amylase (PPA). The full-length cDNA encoding the PPA was isolated from pig pancreas by RT-PCR and cloned into the pPICZαA vector. After the resultant pPICZαΑ-PPA plasmid was transferred into Pichia pastoris, Ni Sepharose affinity column was used to purify the over-expressed extracellular recombinant PPA protein (rePPA) that contains a His-tag to the C terminus and was characterized against the natural enzyme (α-amylase from porcine pancreas). The rePPA exhibited a molecular mass of approximately 58 kDa and showed optimal temperature (50 °C), optimal pH (7.5), K_m (47.8 mg/mL), and V_max (2,783 U/mg) similar to those of the natural enzyme. The recombinant enzyme was stable at 40 °C but lost 60% to 90% (P < 0.05) after exposure to heating at ≥50 °C for 30 min. The enzyme activity was little affected by Cu²⁺ or Fe³⁺, but might be inhibited (40% to 50%) by Zn²⁺ at concentrations in pig digesta. However, Ca²⁺ exhibited a dose-dependent stimulation of the enzyme activity. In conclusion, the present study successfully cloned the porcine pancreatic α-amylase gene and over-expressed the gene in P.pastoris as an extracellular, functional enzyme. The biochemical characterization of the over-produced enzyme depicts its potential and future improvement as an animal feed additive.

The mechanism of interactions between tea polyphenols and porcine pancreatic alpha-amylase: Analysis by inhibition kinetics, fluorescence quenching, differential scanning calorimetry and isothermal titration calorimetry

SCOPE

This study aims to use a combination of biochemical and biophysical methods to derive greater mechanistic understanding of the interactions between tea polyphenols and porcine pancreatic α-amylase (PPA).

METHODS AND RESULTS

The interaction mechanism was studied through fluorescence quenching (FQ), differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC) and compared with inhibition kinetics. The results showed that a higher quenching effect of polyphenols corresponded to a stronger inhibitory activity against PPA. The red-shift of maximum emission wavelength of PPA bound with some polyphenols indicated a potential structural unfolding of PPA. This was also suggested by the decreased thermostability of PPA with these polyphenols in DSC thermograms. Through thermodynamic binding analysis of ITC and inhibition kinetics, the equilibrium of competitive inhibition was shown to result from the binding of particularly galloylated polyphenols with specific sites on PPA. There were positive linear correlations between the reciprocal of competitive inhibition constant (1/Kic ), quenching constant (KFQ ) and binding constant (Kitc ).

CONCLUSION

The combination of inhibition kinetics, FQ, DSC and ITC can reasonably characterize the interactions between tea polyphenols and PPA. The galloyl moiety is an important group in catechins and theaflavins in terms of binding with and inhibiting the activity of PPA.

Mechanisms of starch digestion by α-amylase-Structural basis for kinetic properties

Recent studies of the mechanisms determining the rate and extent of starch digestion by α-amylase are reviewed in the light of current widely-used classifications for (a) the proportions of rapidly-digestible (RDS), slowly-digestible (SDS), and resistant starch (RS) based on in vitro digestibility, and (b) the types of resistant starch (RS 1,2,3,4…) based on physical and/or chemical form. Based on methodological advances and new mechanistic insights, it is proposed that both classification systems should be modified. Kinetic analysis of digestion profiles provides a robust set of parameters that should replace the classification of starch as a combination of RDS, SDS, and RS from a single enzyme digestion experiment. This should involve determination of the minimum number of kinetic processes needed to describe the full digestion profile, together with the proportion of starch involved in each process, and the kinetic properties of each process. The current classification of resistant starch types as RS1,2,3,4 should be replaced by one which recognizes the essential kinetic nature of RS (enzyme digestion rate vs. small intestinal passage rate), and that there are two fundamental origins for resistance based on (i) rate-determining access/binding of enzyme to substrate and (ii) rate-determining conversion of substrate to product once bound.

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

The interplay of α-amylase and amyloglucosidase activities on the digestion of starch in in vitro enzymic systems

In vitro hydrolysis assays are a key tool in understanding differences in rate and extent of digestion of starchy foods. They offer a greater degree of simplicity and flexibility than dynamic in vitro models or in vivo experiments for quantifiable, mechanistic exploration of starch digestion. In the present work the influence of α-amylase and amyloglucosidase activities on the digestion of maize and potato starch granules was measured using both glucose and reducing sugar assays. Data were analysed through initial rates of digestion, and by 1st order kinetics, utilising logarithm of slope (LOS) plots. The rate and extent of starch digestion was dependent on the activities of both enzymes and the type of starch used. Potato required more enzyme than maize to achieve logarithmic reaction curves, and complete digestion. The results allow targeted design of starch digestion experiments through a thorough understanding of the contributions of α-amylase and amyloglucosidase to digestion rates.

The mechanism of action of alpha-amylase from Lactobacillus fermentum on maltooligosaccharides

The action pattern of Lactobacillus fermentum alpha-amylase (FERMENTA) was examined using a series of maltooligosaccharides (G2-G7) as substrates. Structurally, this enzyme has a molecular mass (106 kDa) almost twofold higher than alpha-amylases from mammalians and cereals. The product pattern was investigated through an analysis of products and substrates using HPAEC with pulsed amperometric detection. FERMENTA was consistent with an endo-type of amylase. The bond cleavage frequencies were studied using maltooligosaccharides of various chain lengths as substrate, i.e. maltose up to maltoheptaose and DP 4900-amylose catalyzed by FERMENTA. The catalytic efficiency (k(cat)/K(m)) increased with chain length from maltose (8.7 x 10(4) M(-1) s(-1)) up to amylose (1 x10(9) M(-1) s(-1)). These action pattern results revealed that FERMENTA can readily cleave the third linkage from the reducing end of the maltooligosaccharides (G5-G7).

Human milk oligosaccharides are minimally digested in vitro

In examining the functional aspects of human milk oligosaccharides (HMO), it is not known whether they are digested during the passage through the infant's gastrointestinal tract. HMO were prepared from individual milk samples (n = 6) and separated into neutral and acidic compounds by chromatography. These oligosaccharide fractions were studied for their digestibility by human salivary amylase, porcine pancreatic amylase and brush border membrane vesicles (BBMV) isolated from porcine small intestine; we also examined the effect of low pH on these structures. The characterization of HMO and their digestion products was performed by high-pH anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) as well as TLC. It was shown that neither salivary amylase nor pancreatic amylase cleaved HMO. Only after a 2-h incubation with BBMV were slight modifications of the HMO observed. HPAEC-PAD analysis revealed two new components within the neutral oligosaccharide fractions; these were characterized by mass spectrometric analysis as lacto-N:-triose and galactose. Only lacto-N:-triose was present within digestion assays of oligosaccharides, which did not contain fucosyl or N:-acetylneuraminic acid residues. These results suggest that <5% of the HMO are digested in the intestinal tract. Hence, HMO may play a role as prebiotics or as factors influencing the local immune system of the intestine in breast-fed infants.

Effect of temperature on the saccharide composition obtained after alpha-amylolysis of starch

The hydrolysis of starch to low-molecular-weight products (normally characterised by their dextrose equivalent (DE), which is directly related to the number-average molecular mass) was studied at different temperatures. Amylopectin potato starch, lacking amylose, was selected because of its low tendency towards retrogradation at lower temperatures. Bacillus licheniformis alpha-amylase was added to 10% [w/w] gelatinised starch solutions. The hydrolysis experiments were done at 50, 70, and 90 degrees C. Samples were taken at defined DE values and these were analysed with respect to their saccharide composition. At the same DE the oligosaccharide composition depended on the hydrolysis temperature. This implies that at the same net number of bonds hydrolysed by the enzyme, the saccharide composition was different. The hydrolysis temperature also influenced the initial overall molecular-weight distribution. Higher temperatures led to a more homogenous molecular weight distribution. Similar effects were observed for alpha-amylases from other microbial sources such as Bacillus amyloliquefaciens and Bacillus stearothermophilus. Varying the pH (5.1, 6.2, and 7.6) at 70 degrees C did not significantly influence the saccharide composition obtained during B. licheniformis alpha-amylase hydrolysis. The underlying mechanisms for B. licheniformis alpha-amylase were studied using pure linear oligosaccharides, ranging from maltotriose to maltoheptaose as substrates. Activation energies for the hydrolysis of individual oligosaccharides were calculated from Arrhenius plots at 60, 70, 80, and 90 degrees C. Oligosaccharides with a degree of polymerisation exceeding that of the substrate could be detected. The contribution of these oligosaccharides increased as the degree of polymerisation of the substrate decreased and the temperature of hydrolysis increased. The product specificity decreased with increasing temperature of hydrolysis, which led to a more equal distribution between the possible products formed. Calculations with the subsite map as determined for the closely related alpha-amylase from B. amyloliquefaciens reconfirmed this finding of a decreased substrate specificity with increased temperature of hydrolysis. Copyright 1999 John Wiley & Sons, Inc.

Mechanism of action and the substrate-dependent pH maximum shift of the alpha-amylase of Bacillus coagulans

The alpha-amylase of Bacillus coagulans is a saccharifying alpha-amylase which hydrolyses the disaccharide maltose [L. Keating, C. Kelly, and W. Fogarty, Biochem. Soc. Trans., 24 (1996) 44S]. The pH maximum for maltose hydrolysis is pH 5.0, differing from the pH maximum for starch hydrolysis which is pH 6.0. Studies using reducing end 14C-labeled maltooligosaccharides revealed a substrate-dependent pH maximum shift; hydrolysis of radiolabeled maltotriose (G3*) was maximal at pH 5.0 while the pH maximum for hydrolysis of radiolabeled maltopentaose (G5*) and maltohexaose (G6*) was pH 6.0. With maltotetraose (G4*) however, the pH maximum was pH 5.0-6.0. In addition, the bond cleavage pattern of G4* was dependent on pH. At pH 5.0, the pH maximum for maltose hydrolysis, the frequency of hydrolysis of the reducing end terminal bond of G4* was maximal. Determination of the pH maximum of the productive binding modes of the cleavage patterns of G3* to G6* illustrated the possible role of the occupation of subsite r + 2 in the pH control mechanism of B. coagulans alpha-amylase.

Effect of pressure on the mechanism of hydrolysis of maltotetraose, maltopentaose, and maltohexose catalyzed by porcine pancreatic alpha-amylase

Pressure effects on the time course of the products' composition accompanying the hydrolysis of maltooligosaccharides [maltotetraose (G4) maltopentaose (G5), and maltohexaose (G6)] catalyzed by porcine pancreatic alpha-amylase (PPA) were measured up to 300MPa at 30 degrees C. The composition of products, glucose (G ), maltose (G2), and maltotriose (G3), for the hydrolysis of G4, and G5 substrates changed a little by compression. But for G6 substrate, pressure induced some changes in the composition of products, G2, G3, and G4, respectively. From the pressure dependence of the observed rate constants on PPA catalyzed hydrolysis of G6, the volume difference between two kinds of Michaelis complexes of alpha-amylase-G6 is about 5.4 cm3/mol. The mechanism of an interesting pressure-induced reaction catalyzed by PPA is discussed in the terms of the reaction volumes.

Structure of a pancreatic alpha-amylase bound to a substrate analogue at 2.03 A resolution

The structure of pig pancreatic alpha-amylase in complex with carbohydrate inhibitor and proteinaceous inhibitors is known but the successive events occurring at the catalytic center still remain to be elucidated. The X-ray structure analysis of a crystal of pig pancreatic alpha-amylase (PPA, EC 3.2.1.1.) soaked with an enzyme-resistant substrate analogue, methyl 4,4'-dithio-alpha-maltotrioside, showed electron density corresponding to the binding of substrate analogue molecules at the active site and at the "second binding site." The electron density observed at the active site was interpreted in terms of overlapping networks of oligosaccharides, which show binding of substrate analogue molecules at subsites prior to and subsequent to the cleavage site. A weaker patch of density observed at subsite -1 (using a nomenclature where the site of hydrolysis is taken to be between subsites -1 and +1) was modeled with water molecules. Conformational changes take place upon substrate analogue binding and the "flexible loop" that constitutes the surface edge of the active site is observed in a specific conformation. This confirms that this loop plays an important role in the recognition and binding of the ligand. The crystal structure was refined at 2.03 A resolution, to an R-factor of 16.0 (Rfree, 18.5).

Crystal structure of pig pancreatic alpha-amylase isoenzyme II, in complex with the carbohydrate inhibitor acarbose

Two different crystal forms of pig pancreatic alpha-amylase isoenzyme II (PPAII), free and complexed to a carbohydrate inhibitor (acarbose), have been compared together and to previously reported structures of PPAI. A crystal form obtained at 4 degrees C, containing nearly 72% solvent, made it possible to obtain a new complex with acarbose, different from a previous one obtained at 20 degrees C [Qian, M., Buisson, G., Duée, E., Haser, H. & Payan, F. (1994) Biochemistry 33, 6284-6294]. In the present form, six contiguous subsites of the enzyme active site are occupied by the carbohydrate ligand; the structural data indicate that the binding site is capable of holding more than the five glucose units of the scheme proposed through kinetic studies. A monosaccharide ring bridging two protein molecules related by the crystal packing is located on the surface, at a distance of 2.0 nm from the reducing end of the inhibitor ligand; the symmetry-related glucose ring in the crystal lattice is found 1.5 nm away from the non-reducing end of the inhibitor ligand.

Carbohydrate binding sites in a pancreatic alpha-amylase-substrate complex, derived from X-ray structure analysis at 2.1 A resolution

The X-ray structure analysis of a crystal of pig pancreatic alpha-amylase (PPA, EC 3.2.1.1.) that was soaked with the substrate maltopentaose showed electron density corresponding to two independent carbohydrate recognition sites on the surface of the molecule. Both binding sites are distinct from the active site described in detail in our previous high-resolution study of a complex between PPA and a carbohydrate inhibitor (Qian M, Buisson G, Duée E, Haser H, Payan F, 1994, Biochemistry 33:6284-6294). One of the binding sites previously identified in a 5-A-resolution electron density map, lies at a distance of 20 A from the active site cleft and can accommodate two glucose units. The second affinity site for sugar units is located close to the calcium binding site. The crystal structure of the maltopentaose complex was refined at 2.1 A resolution, to an R-factor of 17.5%, with an RMS deviation in bond distances of 0.007 A. The model includes all 496 residues of the enzyme, 1 calcium ion, 1 chloride ion, 425 water molecules, and 3 bound sugar rings. The binding sites are characterized and described in detail. The present complex structure provides the evidence of an increased stability of the structure upon interaction with the substrate and allows identification of an N-terminal pyrrolidonecarboxylic acid in PPA.

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.

The pH dependence of the action pattern in porcine pancreatic alpha-amylase-catalyzed reaction for maltooligosaccharide substrates

Porcine pancreatic alpha-amylase (EC 3.2.1.1; abbreviated PPA), which hydrolyzes alpha-D-(1,4) glucosidic bonds in starch and amylose, displays an optimum at pH 6.9 for the majority of substrates. The optimum pH, however, shifted to 5.2 for the hydrolysis of some low molecular substrates (Ishikawa, K., et al., 1990, Biochemistry 29, 7119-7123). Details of the substrate-dependent shift of the optimum pH in PPA were studied by use of a series of maltooligosaccharides with 14C-labeled reducing end glucose as substrates. The optimum pH for maltotriose was 5.2, whereas that for maltopentaose and maltohexaose was unchanged at pH 6.9. The pH profile for the intermediate size substrate maltotetraose showed abnormality; the apparent optimum pH was broadened between 5.5 and 6.5 and the bond cleavage pattern depended on pH, unlike that for the other substrates examined. These results were independent of either buffer systems or substrate concentration. Analyses of the hydrolysates of the maltooligosaccharides revealed that the shift of the optimum pH to the neutral region occurred only when the fifth subsite of PPA in the productive binding modes was occupied by a glucosyl residue of a substrate. The three-catalytic residue model of PPA deduced from the analysis of the hydrolysis of some modified maltooligosaccharides (p-nitrophenyl-alpha-D-maltoside, gamma-cyclodextrin, maltopentaitol, and maltohexaitol) (Ishikawa, K., et al., 1990, Biochemistry 29, 7119-7123) was successfully adapted to the linear maltooligosaccharides used in this work. These results indicate that the different productive binding modes of the linear oligosaccharide substrates affect directly the catalytic power and the optimum pH of PPA.

Synthesis of O-alpha-D-glucopyranosyl-(1----4)-O-alpha -D-xylopyranosyl-(1----4)-O-alpha -D-xylopyranosyl-(1----4)-D-glucopyranose as a substrate analogue of alpha amylase

The tetrasaccharide a-D-Glcp-(1----4)-a-D-Xylp-(1----4)-a-D-Xylp-(1----4)-D- Glcp (1) has been synthesized, as a substrate analogue of alpha amylase, by silver perchlorate-catalyzed glycosylation of benzyl 2,3,6-tri-O-benzyl-4-O-(2,3-di-O-benzyl-a-D-xylopyranosyl)-beta-D- glucopyranoside (30) with 2,3-di-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-a-D- glucopyranosyl)-a-D-xylopyranosyl chloride or by methyl triflate-promoted condensation of 30 with methyl 2,3-di-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl)-1-thio- beta-D-xylopyranoside, followed by removal of protecting groups of the resulting tetrasaccharide derivative 40.

New substrate specificity of modified porcine pancreatic alpha-amylase

Conversion of the substrate specificity of porcine pancreatic alpha-amylase (PPA) was studied using chemical modification of His residues. Diethyl pyrocarbonate modified His residues in PPA and the activity of the modified PPA for the hydrolysis of the alpha-D-(1,4)glucoside bond in starch or oligosaccharides decreased to less than 1% of that of the native enzyme. However, the activity for the hydrolysis of the bond between p-nitrophenol and oligosaccharides in p-nitrophenyl oligosaccharides was increased by chemical modification. When the modified PPA was incubated with a proteinaceous alpha-amylase inhibitor (Mr 60,000) purified from white kidney bean (Phaseolus vulgaris), it bound to the inhibitor. As a result, the remaining less than 1% hydrolytic activity of the modified PPA for starch disappeared completely but that for p-nitrophenyl oligosaccharides remained unaltered. The hydrolytic activity of the native PPA for the alpha-D-(1,4)glucoside bond in oligosaccharides was stronger than that between p-nitrophenyl and oligosaccharides in p-nitrophenyl oligosaccharides. Therefore, when p-nitrophenyl oligosaccharides (three to five glucose residues) were used as substrates for the native PPA, the alpha-D-(1,4)glucoside bonds in the oligosaccharides were hydrolyzed. However, the modified PPA-inhibitor complex hydrolyzed only the bond between p-nitrophenol and oligosaccharides in p-nitrophenyl oligosaccharides. The above results reveal that, by chemical modification with diethyl pyrocarbonate and biochemical modification with an amylase inhibitor, amylase can be converted to a new exo-type enzyme which hydrolyzes only the bond between p-nitrophenol and oligosaccharides in p-nitrophenyl oligosaccharides.

Selective inhibition of histidine-modified pancreatic alpha-amylase by proteinaceous inhibitor from Phaseolus vulgaris

Chemical modification of two histidine residues of porcine pancreatic alpha-amylase (EC 3.2.1.1) by diethyl pyrocarbonate in the presence of a high concentration of maltotriose caused a decrease of amylase activity and an increase of maltosidase activity (hydrolysis of p-nitrophenyl-alpha-maltoside). By binding a proteinaceous inhibitor from Phaseolus vulgaris (white kidney bean) with the modified enzyme, the amylase activity was further decreased but the maltosidase activity was retained to about 100% that of the native enzyme. Both amylase and maltosidase activities of the native enzyme were almost completely inhibited by the proteinaceous inhibitor. The increase of maltosidase activity by histidine modification was due to an increase of kcat, whereas the Km value was not changed; but binding of the proteinous inhibitor affected mainly the Km value of the modified enzyme.

Kinetics and subsite mapping of a D-xylobiose- and D-xylose-producing Aspergillus niger endo-(1----4)-beta-D-xylanase

A previously described endo-(1----4)-beta-D-xylanase produced by Aspergillus niger was allowed to react with linear unlabeled and labeled D-xylo-oligosaccharides ranging from D-xylotriose to D-xylo-octaose. No evidence of multiple attack or of condensation and trans-D-xylosylation reactions was found. Maximum rates and Michaelis constants were measured at 40 degrees and pH 4.85. The former increased with increasing chain-length from D-xylotriose through D-xylohexaose to approximately 70% of that on soluble larchwood D-xylan, and then decreased slightly for D-xyloheptaose and D-xylo-octaose. Michaelis constants decreased monotonically with increasing chain-length. Bond-cleavage frequencies were highest near the reducing end of short substrates, with the locus of highest frequencies moving towards the middle of larger substrates. These data indicated that the endo-D-xylanase has five main subsites, with the catalytic site located between the third and fourth subsites, counting from the nonreducing end of the bound substrate. The subsite to the nonreducing side of the catalytic site strongly repels its corresponding D-xylosyl residue, while the two subsites farther towards the nonreducing end of the substrate strongly attract their corresponding residues. The subsite to the reducing side of the catalytic site moderately attracts D-xylosyl residues, while the next one towards the reducing end has a high affinity for them. The residual error of the numerical estimation was allocated largely to the Michaelis constants of the different D-xylo-oligosaccharides, whose calculated values were appreciably smaller than measured values, especially for shorter substrates. This suggests that the subsite model cannot fully account for the experimental data. Estimated and measured values of maximum rates, bond-cleavage frequencies, and dissociation constant when the active site is fully occupied by substrate agreed more closely with each other.

The determination of subsite binding energies of porcine pancreatic alpha-amylase by comparing hydrolytic activity towards substrates

The active centre of porcine pancreatic alpha-amylase contains five subsites. Their occupancy has been studied using as a substrate maltooligosaccharide of various chain lengths (maltose up to maltoheptaose), some of their p- and o-nitrophenylated derivatives, and 412-residue amylose. Quantitative analysis of the digestion products allowed the determination of the subsite occupancy for the various productive complexes, the bond cleavage frequency and respective kcati (where i is the binding mode). The catalytic efficiency (kcat/Km) increases with chain length from maltose (2 M-1 X S-1) up to amylose (1.06 X 10(7) M-1 X S-1). The kinetic parameters of p-nitrophenylmaltoside hydrolysis are quite close to those of maltose, and the ortho compound behaves as maltotriose. Determination of binding energy of glucose residue at the various subsites calculated according to the method of Hiromi et al. (Hiromi, K., Nitta, Y., Numata, C. and Ono, S. (1973) Biochim. Biophys. Acta 302, 362-375) did not give consistent results. A method is proposed based on certain properties of porcine pancreatic alpha-amylase, especially the non-interaction of the p-nitrophenyl moiety of the maltose derivative with subsites 1 and 2, and the o-nitrophenyl group which interacts in a similar way to a glucose residue at the reducing end, and on the grounds that the amylase-amylose complexes are of the productive type. In addition, binding energy differences were calculated from substrates with the same chain length. The subsite energy profile is characterized by a low value at subsite 3 which confirms this subsite as the catalytic one. Another consequence is that the hydrolysis rate constant of productive complexes (kintn) (where n is the number of glucose or glucose equivalent residues for a given substrate) varies with chain length which is in conflict with the hypothesis of Hiromi et al.

The effect of substrate modification on binding of porcine pancreatic alpha amylase: hydrolysis of modified amylose containing D-allose residues

A modified amylose containing 10% of tritiated D-allose residues has been hydrolyzed by porcine pancreatic alpha amylase (PPA). This reaction produced a number of radioactive oligosaccharides of low molecular weight, including modified mono-, di-, and tri-saccharides, as well as larger products. Analysis of these products by chemical and enzymic methods identified D-allose, two isomers of modified maltose, and isomers of modified maltotriose. These results may be interpreted in terms of current PPA models to indicate that D-allose residues may be productively bound at all five subsites of the active site of the enzyme. The distribution of modified residues in these products, however, further suggests that productive binding of D-allose at the subsite where catalytic attack occurs (subsite 3) is less favorable than binding of D-glucose. These results are compared with results of a series of PPA substrates having modifications at C-3 and at other positions. Trends observed in enzyme hydrolysis of these modified substrates reflect factors that contribute to PPA catalysis, with respect to steric, electronic, and hydrogen-bonding interactions between enzyme and substrate.

On porcine pancreatic alpha-amylase action: kinetic evidence for the binding of two maltooligosaccharide molecules (maltose, maltotriose and o-nitrophenylmaltoside) by inhibition studies. Correlation with the five-subsite energy profile

Hydrolysis of small substrates (maltose, maltotriose and o-nitrophenylmaltoside) catalysed by porcine pancreatic alpha-amylase was studied from a kinetic viewpoint over a wide range of substrate concentrations. Non-linear double-reciprocal plots are obtained at high maltose, maltotriose and o-nitrophenylmaltoside concentrations indicating typical substrate inhibition. These results are consistent with the successive binding of two molecules of substrate per enzyme molecule with dissociation constants Ks1 and Ks2. The Hill plot, log [v/(V-v)] versus log [S], is clearly biphasic and allows the dissociation constants of the ES1 and ES2 complexes to be calculated. Maltose and maltotriose are inhibitors of the amylase-catalysed amylose and o-nitrophenylmaltoside hydrolysis. The inhibition is of the competitive type. The (apparent) inhibition constant Kiapp varies with the inhibitor concentration. These results are also consistent with the successive binding of at least two molecules of maltose or maltotriose per amylase molecule with the dissociation constants Ki1 and Ki2. These inhibition studies show that small substrates and large polymeric ones are hydrolysed at the same catalytic site(s). The values of the dissociation constants Ks1 and Ki1 of the maltose-amylase complexes are identical. According to the five-subsite energy profile previously determined, at low concentration, maltose (as substrate and as inhibitor) binds to the same two sites (4,5) or (3,4), maltotriose (as substrate and as inhibitor) and o-nitrophenyl-maltoside (as substrate) bind to the same three subsites (3,4,5). The dissociation constants Ks2 and Ki2 determined at high substrate and inhibitor concentration are consistent with the binding of the second ligand molecule at a single subsite. The binding mode of the second molecule of maltose (substrate) and o-nitrophenylmaltoside remains uncertain, very likely because of the inaccuracy due to simplifications in the calculations of the subsite binding energies. No binding site(s) outside the catalytic one has been taken into account in this model.