ADPG synthetase and ADPG- -glucan 4-glucosyl transferase: enzymes involved in bacterial glycogen and plant starch synthesis

Insight into the structure, dynamics and the unfolding property of amylosucrases: implications of rational engineering on thermostability

Amylosucrase (AS) is a kind of glucosyltransferases (E.C. 2.4.1.4) belonging to the Glycoside Hydrolase (GH) Family 13. In the presence of an activator polymer, in vitro, AS is able to catalyze the synthesis of an amylose-like polysaccharide composed of only α-1,4-linkages using sucrose as the only energy source. Unlike AS, other enzymes responsible for the synthesis of such amylose-like polymers require the addition of expensive nucleotide-activated sugars. These properties make AS an interesting enzyme for industrial applications. In this work, the structures and topology of the two AS were thoroughly investigated for the sake of explaining the reason why Deinococcus geothermalis amylosucrase (DgAS) is more stable than Neisseria polysaccharea amylosucrase (NpAS). Based on our results, there are two main factors that contribute to the superior thermostability of DgAS. On the one hand, DgAS holds some good structural features that may make positive contributions to the thermostability. On the other hand, the contacts among residues of DgAS are thought to be topologically more compact than those of NpAS. Furthermore, the dynamics and unfolding properties of the two AS were also explored by the gauss network model (GNM) and the anisotropic network model (ANM). According to the results of GNM and ANM, we have found that the two AS could exhibit a shear-like motion, which is probably associated with their functions. What is more, with the discovery of the unfolding pathway of the two AS, we can focus on the weak regions, and hence designing more appropriate mutations for the sake of thermostability engineering. Taking the results on structure, dynamics and unfolding properties of the two AS into consideration, we have predicted some novel mutants whose thermostability is possibly elevated, and hopefully these discoveries can be used as guides for our future work on rational design.

Optimized and automated protocols for high-throughput screening of amylosucrase libraries

This article describes the design and validation of a general procedure for the high-throughput isolation of amylosucrase variants displaying higher thermostability or increased resistance to organic solvents. This procedure consists of 2 successive steps: an in vivo selection that eliminates inactive variants followed by automated screening of active variants to isolate mutants displaying enhanced features. The authors chose an Escherichia coli expression vector, allowing a high production rate of the recombinant enzyme in miniaturized culture conditions. The screening assay was validated by minimizing variability for various parameters of the protocol, especially bacterial growth and protein production in cultures in 96-well microplates. Recombinant amylosucrase production was normalized by decreasing the coefficient of variance from 27% to 12.5%. Selective screening conditions were defined to select variants displaying higher thermostability or increased resistance to organic solvents. A first-generation amylosucrase variant library, constructed by random mutagenesis, was subjected to this procedure, yielding a mutant displaying a 25-fold increased stability at 50 degrees C compared to the parental wild-type enzyme.

Increased amylosucrase activity and specificity, and identification of regions important for activity, specificity and stability through molecular evolution

Amylosucrase is a transglycosidase which belongs to family 13 of the glycoside hydrolases and transglycosidases, and catalyses the formation of amylose from sucrose. Its potential use as an industrial tool for the synthesis or modification of polysaccharides is hampered by its low catalytic efficiency on sucrose alone, its low stability and the catalysis of side reactions resulting in sucrose isomer formation. Therefore, combinatorial engineering of the enzyme through random mutagenesis, gene shuffling and selective screening (directed evolution) was applied, in order to generate more efficient variants of the enzyme. This resulted in isolation of the most active amylosucrase (Asn387Asp) characterized to date, with a 60% increase in activity and a highly efficient polymerase (Glu227Gly) that produces a longer polymer than the wild-type enzyme. Furthermore, judged from the screening results, several variants are expected to be improved concerning activity and/or thermostability. Most of the amino acid substitutions observed in the totality of these improved variants are clustered around specific regions. The secondary sucrose-binding site and beta strand 7, connected to the important Asp393 residue, are found to be important for amylosucrase activity, whereas a specific loop in the B-domain is involved in amylosucrase specificity and stability.

Application of the BPEC pathway for large-scale biotechnological production of poly(3-mercaptopropionate) by recombinant Escherichia coli, including a novel in situ isolation method

Metabolically engineered Escherichia coli JM109 harboring plasmid pBPP1 and expressing the nonnatural BPEC pathway for synthesis of thermoplastic polyhydroxyalkanoates (PHA) and novel polythioesters (PTE) to provide suitable substrates of PHA synthase was investigated with respect to biotechnological production of poly(3-mercaptopropionate) [poly(3MP)]. Fed-batch fermentation processes were established at the 30- and 500-liter scales in stirred tank bioreactors to produce kilogram amounts of poly(3MP). Cultivation was done in a modified M9 mineral salts medium containing glucose or glycerol as the carbon and energy source and with 3-mercaptopropionic acid (3MP) as the precursor substrate for poly(3MP) biosynthesis provided from the late exponential growth phase. Approximately 23 g of cell dry matter (CDM) per liter and poly(3MP) cell contents of up to 45% (wt/wt) were the highest cell densities and polymer contents obtained, respectively. At best, 69.1% (wt/wt) of 3MP was converted into poly(3MP), indicating that 3MP was mostly used for poly(3MP) biosynthesis. Furthermore, a novel in situ process for rapid and convenient isolation of poly(3MP) from the cells in the bioreactor was developed. This was achieved by addition of sodium dodecyl sulfate to the cultivation broth immediately after the fermentation, heating to 90 degrees C for 20 min with intensive stirring, and subsequent washing steps. The purity of such in situ isolated poly(3MP) was more than 98%, as revealed by gas chromatographic and elemental sulfur analyses of the material isolated.

Combinatorial engineering to enhance amylosucrase performance: construction, selection, and screening of variant libraries for increased activity

Amylosucrase is a glucosyltransferase belonging to family 13 of glycoside hydrolases and catalyses the formation of an amylose-type polymer from sucrose. Its potential use as an industrial tool for the synthesis or the modification of polysaccharides, however, is limited by its low catalytic efficiency on sucrose alone, its low stability, and its side reactions resulting in sucrose isomer formation. Therefore, combinatorial engineering of the enzyme through random mutagenesis, gene shuffling, and selective screening (directed evolution) was started, in order to generate more efficient variants of the enzyme. A convenient zero background expression cloning strategy was developed. Mutant gene libraries were generated by error-prone polymerase chain reaction (PCR), using Taq polymerase with unbalanced dNTPs or Mutazyme trade mark, followed by recombination of the PCR products by DNA shuffling. A selection method was developed to allow only the growth of amylosucrase active clones on solid mineral medium containing sucrose as the sole carbon source. Automated protocols were designed to screen amylosucrase activity from mini-cultures using dinitrosalicylic acid staining of reducing sugars and iodine staining of amylose-like polymer. A pilot experiment using the described mutagenesis, selection, and screening methods yielded two variants with significantly increased activity (five-fold under the screening conditions). Sequence analysis of these variants revealed mutations in amino acid residues which would not be considered for rational design of improved amylosucrase variants. A method for the characterisation of amylosucrase action on sucrose, consisting of accurate measurement of glucose and fructose concentrations, was introduced. This allows discrimination between hydrolysis and transglucosylation, enabling a more detailed comparison between wild-type and mutant enzymes.

Molecular Basis of the Amylose-like Polymer Formation Catalyzed by Neisseria polysaccharea Amylosucrase

Amylosucrase from Neisseria polysaccharea is a remarkable transglucosidase from family 13 of the glycoside-hydrolases that synthesizes an insoluble amylose-like polymer from sucrose in the absence of any primer. Amylosucrase shares strong structural similarities with alpha-amylases. Exactly how this enzyme catalyzes the formation of alpha-1,4-glucan and which structural features are involved in this unique functionality existing in family 13 are important questions still not fully answered. Here, we provide evidence that amylosucrase initializes polymer formation by releasing, through sucrose hydrolysis, a glucose molecule that is subsequently used as the first acceptor molecule. Maltooligosaccharides of increasing size were produced and successively elongated at their nonreducing ends until they reached a critical size and concentration, causing precipitation. The ability of amylosucrase to bind and to elongate maltooligosaccharides is notably due to the presence of key residues at the OB1 acceptor binding site that contribute strongly to the guidance (Arg415, subsite +4) and the correct positioning (Asp394 and Arg446, subsite +1) of acceptor molecules. On the other hand, Arg226 (subsites +2/+3) limits the binding of maltooligosaccharides, resulting in the accumulation of small products (G to G3) in the medium. A remarkable mutant (R226A), activated by the products it forms, was generated. It yields twice as much insoluble glucan as the wild-type enzyme and leads to the production of lower quantities of by-products.

Maltooligosaccharide disproportionation reaction: an intrinsic property of amylosucrase from Neisseria polysaccharea

Amylosucrase from Neisseria polysaccharea (AS) is a remarkable transglycosidase of family 13 of the glycoside hydrolases that catalyses the synthesis of an amylose-like polymer from sucrose and is always described as a sucrose-specific enzyme. Here, we demonstrate for the first time the ability of pure AS to catalyse the disproportionation of maltooligosaccharides by cleaving the alpha-1,4 linkage at the non-reducing end of a maltooligosaccharide donor and transferring the glucosyl unit to the non-reducing end of another maltooligosaccharide acceptor. Surprisingly, maltose, maltotriose and maltotetraose are very poor glucosyl donors whereas longer maltooligosaccharides are even more efficient glucosyl donors than sucrose. At least five glucose units are required for efficient transglucosylation, suggesting the existence of strong binding subsites, far from the sucrose binding site, at position +4 and above.

Identification of key amino acid residues in Neisseria polysaccharea amylosucrase

Amylosucrase from Neisseria polysaccharea catalyzes the synthesis of an amylose-like polymer from sucrose. Sequence alignment revealed that it belongs to the glycoside hydrolase family 13. Site-directed mutagenesis enabled the identification of functionally important amino acid residues located at the active center. Asp-294 is proposed to act as the catalytic nucleophile and Glu-336 as general acid base catalyst in amylosucrase. The conserved Asp-401, His-195 and His-400 residues are critical for the enzymatic activity. These results provide strong support for the predicted close structural and functional relationship between the sucrose-glucosyltransferases and enzymes of the alpha-amylase family.

Characterisation of the activator effect of glycogen on amylosucrase from Neisseria polysaccharea

Amylosucrase produces an insoluble alpha-1,4-linked glucan from sucrose, releasing fructose. In addition to polymerisation, in the presence of sucrose as sole substrate, amylosucrase catalyses sucrose hydrolysis and oligosaccharide synthesis in significant proportions. The effects of both glycogen acceptor and sucrose concentrations on the reactions catalysed by the highly purified amylosucrase from Neisseria polysaccharea were investigated. Sucrose hydrolysis decreased strongly with the increase of the concentration of glycogen, as did oligosaccharide synthesis, by glucose transfer onto glucose and fructose. The glucosyl units consumed were then preferentially used for elongation of glycogen chains. The study of the kinetic behaviour of amylosucrase revealed a strong, sucrose concentration dependent activator effect of glycogen. This activation was decreased at high sucrose concentration. The optimal sucrose concentrations increased with glycogen concentration, suggesting competition between sucrose and glycogen, and the presence of a second non-catalytic acceptor binding site which could bind various acceptors (glucose, maltose, glycogen) and also sucrose.

Amylosucrase from Neisseria polysaccharea: novel catalytic properties

Amylosucrase is a glucosyltransferase that synthesises an insoluble alpha-glucan from sucrose. The catalytic properties of the highly purified amylosucrase from Neisseria polysaccharea were characterised. Contrary to previously published results, it was demonstrated that in the presence of sucrose alone, several reactions are catalysed, in addition to polymer synthesis: sucrose hydrolysis, maltose and maltotriose synthesis by successive transfers of the glucosyl moiety of sucrose onto the released glucose, and finally turanose and trehalulose synthesis - these two sucrose isomers being obtained by glucosyl transfer onto fructose. The effect of initial sucrose concentration on initial activity demonstrated a non-Michaelian profile never previously described.

Site-directed insertion and insertion-deletion mutations in the Escherichia coli chromosome simplified

A procedure to produce an exact chromosomal replica of an insertion or insertion-deletion mutation produced in vitro in a plasmid with a ColE 1 origin of replication is presented. This procedure uses a previously described property of recD mutations (Biek DP, Cohen SN. J. Bacteriol. 1986;167:594-603) and is limited by (1) the compatibility of the new mutation with recD; and (2) the presence of some Escherichia coli DNA flanking the mutation.

Sequence analysis of the gene encoding amylosucrase from Neisseria polysaccharea and characterization of the recombinant enzyme

The Neisseria polysaccharea gene encoding amylosucrase was subcloned and expressed in Escherichia coli. Sequencing revealed that the deduced amino acid sequence differs significantly from that previously published. Comparison of the sequence with that of enzymes of the alpha-amylase family predicted a (beta/alpha)8-barrel domain. Six of the eight highly conserved regions in amylolytic enzymes are present in amylosucrase. Among them, four constitute the active site in alpha-amylases. These sites were also conserved in the sequence of glucosyltransferases and dextransucrases. Nevertheless, the evolutionary tree does not show strong homology between them. The amylosucrase was purified by affinity chromatography between fusion protein glutathione S-transferase-amylosucrase and glutathione-Sepharose 4B. The pure enzyme linearly elongated some branched chains of glycogen, to an average degree of polymerization of 75.

Biosynthesis of bacterial glycogen: genetic and allosteric regulation of glycogen biosynthesis in Salmonella typhimurium LT-2

Structural gene mutants of the glycogen biosynthetic enzymes adenosine diphosphate glucose pyrophosphorylase (glgC) and glycogen synthase (glgA) were isolated and partially characterized. The cotransduction frequencies of these genes with the aspartic semialdehyde dehydrogenase (asd) and glycerol-3-phosphate dehydrogenase (glpD) genes suggested the unambiguous gene order of glpD glgA glgC asd. The results of the three-factor cross glpD- glgA- glgC+ X glpD+ glgA+ glgC- were consistent with the proposed order. A simultaneous and approximately equivalent derepression of the glgC, glgA, and glgB (branching enzyme) gene products was observed in the late logarithmic-early stationary phase of growth on enriched media. These results are consistent with the coordinately regulated synthesis of the three glycogen biosynthetic enzymes in Salmonella typhimurium.

The biosynthesis of granulose by Clostridium pasteurianum

1. Mutant strains of Clostridium pasteurianum were obtained, which are unable to synthesize granulose (an intracellularly accumulated amylopectin-like alpha-polyglucan). 2. These mutants lacked either (a) ADP-glucose pyrophosphorylase (EC 2.7.7.27), or (b) granulose synthase (i.e. ADP-glucose-alpha-1,4-glucan glucosyltransferase, EC 2.4.1.21). 3. Although both of these enzymes were constitutively synthesized by the wild-type organism, massive deposition of granulose in a sporulating culture coincided with a threefold increase in the specific activity of ADP-glucose pyrophosphorylase. 4. The soluble ADP-glucose pyrophosphorylase was partially purified (33-fold). Its ATP-saturation curve was not sigmoidal and its activity was not enhanced by phosphorylated intermediates of glycolysis, pyruvate, NAD(P)H or pyridoxal 5'-phosphate. ADP at relatively high concentrations acted as a competitive inhibitor (K(i)=19mm). 5. The dependence of granulose synthase on a suitable polyglucan primer was demonstrated by using enzyme obtained from a granulose-free mutant strain (lacking ADP-glucose pyrophosphorylase). 6. Partial purification of granulose synthase from wild-type strains was facilitated by its being bound to the native particles of granulose. No activator was discovered, but ADP, AMP and pyridoxal 5'-phosphate were competitive inhibitors, ADP being most effective (K(i) about 0.2mm). 7. It would appear that the synthesis of granulose in Cl. pasteurianum is not subject to the positive, fine control that is a feature of glycogen biosynthesis in most bacteria.

[On the activity of synthetase and phosphorylase in maize leaves at different starch levels]

The activities of phosphorylase and synthetase in the bundle sheath cells of maize leaves were investigated in plants that as a result of different light-dark treatments contained various amounts of starch. The material coming from the dark (67 h) had almost no starch and scarcely any synthetase activity if starch granules were not added to the assay mixture as a primer. In the presence of this primer a poor synthetase activity could be detected. After 2 h in the light the leaves produced a small amount of starch and the synthetase activity increased. When starch granules were included in the test the synthetase activity was increased 3.5-fold. This value was 1.5-fold higher than the corresponding one in the dark. Plants that were in the light for 28 h contained fair amounts of starch and the synthetase activity was independent of the addition of primer. The values were 3 fold higher than those found in plants in the dark. A further increase in the synthetase activity and decrease in starch content were brought about by a dark period of 2 h following the illumination of 28 h.The total activity of phosphorylase remained high and almost constant in all these materials and did not require the addition of primer to attain a maximal value. In the material coming from the dark, the product produced by the activity of phosphorylase in absence of added primer could serve as a good primer for the synthetase. From the comparison of the amounts of starch produced in vivo under different conditions and the enzymic activities found we conclude that the first steps of starch synthesis are carried out by phosphorylase and that further on both enzymes participate in the process.