The structural analysis and enzymic synthesis of a pentasaccharide alpha-limit dextrin formed from amylopectin by Bacillus subtilis alpha-amylase

Relationship between branching density and crystalline structure of A- and B-type maize mutant starches

Amylopectin from two double maize mutant starches of A-crystalline (wxdu) and B-crystalline type (aewx) was subjected successively to hydrolysis involving alpha and beta amylases, which isolated clusters and all branching zones of clusters (BZC). Enzymatic analysis together with ionic and size-exclusion chromatography revealed the structural features of the clusters and BZC and their role in starch crystallization. A-type clusters were larger (dp(n) > 80) and contained more (but shorter) chains than B-type clusters. The BZC of A-type starch was also larger, but with a shorter distance between the branching points than in B-type BZC. A-type clusters had a densely packed structure and B-type a poorly branched structure. Models for the structure of A- and B-type clusters are presented, and a hypothesis for the influence of cluster geometry on crystallization is proposed.

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.

Investigation of the fine structure of alpha-dextrins derived from amylopectin and their relation to the structure of waxy-maize starch

Alpha-dextrins, obtained by fractional precipitation with methanol of the products of the action of Bacillus subtilis alpha-amylase on waxy-maize amylopectin, were debranched with isoamylase and the distributions of the unit chains were analysed by gel-permeation chromatography. The large alpha-dextrins still contained long B-chains after hydrolysis for 60 min, but these were absent from the small dextrins with chain numbers of approximately 11 or less. The small dextrins contained increased amounts of chains with lengths intermediate of those of the long B-chains and the main part of the short chains. After hydrolysis for 210 min, almost all of the long B-chains had disappeared and the chains with intermediate lengths had been shortened further. The distributions of the unit chains of the internal chains, obtained by debranching of the phosphorolysis (phi)-limit dextrins, gave similar results and showed that the ratio of A- to B-chains was unchanged during the alpha-amylolysis. Models for the fine structure of the intermediate alpha-dextrins are proposed.

Alpha-amylase structure and activity

Two computerized methods of predicting protein secondary structure from amino acid sequences are evaluated by using them on the alpha-amylase of Aspergillus oryzae, for which the three-dimensional structure has been determined. The methods are then used, with amino acid alignments, to predict the structures of other alpha-amylases. It is found that all alpha-amylases of known amino acid sequence have the same basic structure, a barrel of eight parallel stretches of extended chain surrounded by eight helices. Strong similarities are found in those areas of the proteins believed to bind an essential calcium ion and at that part of the active site that catalyzes bond hydrolysis in the substrates. The active site, as a whole, is formed mainly of amino acids situated on loops joining extended chain to the adjacent helix. Variations in the length and amino acid sequence of these loops, from one alpha-amylase to another, provide the differences in binding the substrates believed to account for the known variations in action pattern of alpha-amylases of different biological origins.

Actions of Aspergillus oryzae alpha-amylase, potato phosphorylase, and rabbit muscle phosphorylase a and b on phosphorylated (1----4)-alpha-D-glucan

Aspergillus oryzae alpha-amylase [(1----4)-alpha-D-glucan glucanohydrolase, EC 3.2.1.1] produced O-(6-phosphoryl-alpha-D-glucopyranosyl)-(1----4)-O-alpha-D-glucopyran osy l-(1----4)-D-glucopyranose (6(3)-phosphorylmaltotriose) and O-alpha-D-glucopyranosyl-(1----4)-O-(3-phosphoryl-alpha-D-glucopyranosyl )- (1----4)-O-alpha-D-glucopyranosyl-(1----4)-D-glucopyranose (3(3)-phosphorylmaltotetraose) from potato starch upon exhaustive hydrolysis. These products indicate that the enzyme hydrolyses the same linkages in the vicinity of the 6-phosphorylated residue as porcine-pancreatic alpha-amylase, but hydrolyses different linkages in the vicinity of the 3-phosphorylated residue when compared with B. subtilis and pancreatic alpha-amylases. Potato phosphorylase [(1----4)-alpha-D-glucan:orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1] and rabbit muscle phosphorylase a and b were unable to by-pass the phosphorylated D-glucosyl residue of 6-phosphorylated (1----4)-alpha-D-glucan, leaving three D-glucosyl residues attached to the 6-phosphorylated residue on the non-reducing side.

Actions of porcine pancreatic and Bacillus subtilis alpha-amylases and Aspergillus niger glucoamylase on phosphorylated (1--4)-alpha-D-glucan

Porcine pancreatic alpha-amylase (1,4-alpha-D-glucan glucanohydrolase, EC 3.2.1.1) produced O-6-phosphoryl-alpha-D-glucopyranosyl-(1--4)-O-alpha-D-glucopyranosyl -(1--4)-D-glucopyranose (6(3)-phosphoryl maltotriose) and O-alpha-D-glucopyranosyl-(1--4)-O-alpha-D-glucopyranosyl- O-3-phosphoryl-alpha-D-glucopyranosyl-(1--4)-D-glucopyranose (3(2)-phosphoryl maltotetraose) from potato starch upon exhaustive hydrolysis, while Bacillus subtilis alpha-amylase (liquefying type) yielded O-alpha-D-glucopyranosyl-(1--4)-O-6-phosphoryl-alpha-D- -glucopyranosyl-(1--4)-D-glucopyranose (6(2)-phosphoryl maltotriose) and 3(2)-phosphoryl maltotetraose. Thus, the two alpha-amylases cleave different sites in the vicinity of the phosphate group at C-6 of the glucosyl residue but the same site in the vicinity of that at C-3. Aspergillus niger glucoamylase (1,4-alpha-D-glucan glucohydrolase, EC 3.2.1.3) hydrolyzed the (1--4)-alpha-linkage of the 6-phosphoryl residue at the non-reducing site (Abe, J., Takeda, Y. and Hizukuri, S. (1982) Biochim. Biophys. Acta 703, 26-33) but not that of the 3-phosphoryl residue, leaving a glucosyl residue attached to the 3-phosphoryl residue. These results indicate that the phosphate group at C-3 is more obstructive than that at C-6 for the actions of these three amylases.

Amylases: enzymatic mechanisms

Many types of amylases are found throughout the animal, vegetable and microbial kingdoms. They have evolved along different pathways to enable the organism to convert insoluble starch (or glycogen) into low molecular weight, water soluble dextrins and sugars. Alpha amylases are dextrinogenic and can attack the interior of starch molecules. The products retain the alpha anomeric configuration. Beta amylases act only at the non-reducing chain ends and liberate only beta maltose. Both alpha and beta amylases exhibit multiple (repetitive) attack, that is, after the initial catalytic cleavage, the enzyme may remain attached to the substrate and lead to several more cleavages before dissociation of the enzyme-substrate complex. Amylases have extended substrate binding sites, in the range 4-9 glucose units. This enables the enzyme to stress the substrate and lower the activation energy for hydrolysis. Similarly the enzyme exerts a torsion on the glucose unit at the catalytic site, inducing a transition state conformation (oxycarbonium ion). Alpha and beta amylases differ in the stereospecific hydration of the oxycarbonium ion, in the sequence of liberation of the right-hand vs the left-hand product, and the direction of motion of the retained substrate to give multiple attack.