The functions of 4-alpha-glucanotransferases and their use for the production of cyclic glucans

Molecular Cloning, Characterization, and Application of a Novel Multifunctional Isoamylase (MIsA) from Myxococcus sp. Strain V11

A novel multifunctional isoamylase, MIsA from Myxococcus sp. strain V11, was expressed in Escherichia coli BL21(DE3). Sequence alignment revealed that MIsA is a typical isoamylase that belongs to glycoside hydrolase family 13 (GH 13). MIsA can hydrolyze the α-1,6-branches of amylopectin and pullulan, as well as the α-1,4-glucosidic bond in amylose. Additionally, MIsA demonstrates 4-α-D-glucan transferase activity, enabling the transfer of α-1,4-glucan oligosaccharides between molecules, particularly with linear maltooligosaccharides. The Km, Kcat, and Vmax values of the MIsA for amylopectin were 1.22 mM, 40.42 µmol·min-1·mg-1, and 4046.31 mM·min-1. The yields of amylopectin and amylose hydrolyzed into oligosaccharides were 10.16% and 11.70%, respectively. The hydrolysis efficiencies were 55%, 35%, and 30% for amylopectin, soluble starch, and amylose, respectively. In the composite enzyme hydrolysis of amylose, the yield of maltotetraose increased by 1.81-fold and 2.73-fold compared with that of MIsA and MTHase (MCK8499120) alone, respectively.

Trichomonas vaginalis extracellular vesicles up-regulate and directly transfer adherence factors promoting host cell colonization

Trichomonas vaginalis, a common sexually transmitted parasite that colonizes the human urogenital tract, secretes extracellular vesicles (TvEVs) that are taken up by human cells and are speculated to be taken up by parasites as well. While the crosstalk between TvEVs and human cells has led to insight into host:parasite interactions, roles for TvEVs in infection have largely been one-sided, with little known about the effect of TvEV uptake by T. vaginalis. Approximately 11% of infections are found to be coinfections of multiple T. vaginalis strains. Clinical isolates often differ in their adherence to and cytolysis of host cells, underscoring the importance of understanding the effects of TvEV uptake within the parasite population. To address this question, our lab tested the ability of a less adherent strain of T. vaginalis, G3, to take up fluorescently labeled TvEVs derived from both itself (G3-EVs) and TvEVs from a more adherent strain of the parasite (B7RC2-EVs). Here, we showed that TvEVs generated from the more adherent strain are internalized more efficiently compared to the less adherent strain. Additionally, preincubation of G3 parasites with B7RC2-EVs increases parasite aggregation and adherence to host cells. Transcriptomics revealed that TvEVs up-regulate expression of predicted parasite membrane proteins and identified an adherence factor, heteropolysaccharide binding protein (HPB2). Finally, using comparative proteomics and superresolution microscopy, we demonstrated direct transfer of an adherence factor, cadherin-like protein, from TvEVs to the recipient parasite's surface. This work identifies TvEVs as a mediator of parasite:parasite communication that may impact pathogenesis during mixed infections.

The novel chloroplast glucose transporter pGlcT2 affects adaptation to extended light periods

Intracellular sugar compartmentation is critical in plant development and acclimation to challenging environmental conditions. Sugar transport proteins are present in plasma membranes and in membranes of organelles such as vacuoles, the Golgi apparatus, and plastids. However, there may exist other transport proteins with uncharacterized roles in sugar compartmentation. Here we report one such, a novel transporter of the Monosaccharide Transporter Family (MSF), the closest phylogenetic homolog of which is the chloroplast-localized glucose transporter pGlcT and that we therefore term plastidic glucose transporter 2 (pGlcT2). We show, using gene-complemented glucose uptake deficiency of an Escherichia coli ptsG/manXYZ mutant strain and biochemical characterization, that this protein specifically facilitates glucose transport, whereas other sugars do not serve as substrates. In addition, we demonstrate pGlcT2-GFP localized to the chloroplast envelope, and that pGlcT2 is mainly produced in seedlings and in the rosette center of mature Arabidopsis plants. Therefore, in conjunction with molecular and metabolic data, we propose pGlcT2 acts as a glucose importer that can limit cytosolic glucose availability in developing pGlcT2-overexpressing seedlings. Finally, we show both overexpression and deletion of pGlcT2 resulted in impaired growth efficiency under long day and continuous light conditions, suggesting pGlcT2 contributes to a release of glucose derived from starch mobilization late in the light phase. Together, these data indicate the facilitator pGlcT2 changes the direction in which it transports glucose during plant development and suggest the activity of pGlcT2 must be controlled spatially and temporarily in order to prevent developmental defects during adaptation to periods of extended light.

Designing starch derivatives with desired structures and functional properties via rearrangements of glycosidic linkages by starch-active transglycosylases

Modification of starch by transglycosylases from glycoside hydrolase families has attracted much attention recently; these enzymes can produce starch derivatives with novel properties, i.e. processability and functionality, employing highly efficient and safe methods. Starch-active transglycosylases cleave starches and transfer linear fragments to acceptors introducing α-1,4 and/or linear/branched α-1,6 glucosidic linkages, resulting in starch derivatives with excellent properties such as complexing and resistance to digestion characteristics, and also may be endowed with new properties such as thermo-reversible gel formation. This review summarizes the effects of variations in glycosidic linkage composition on structure and properties of modified starches. Starch-active transglycosylases are classified into 4 groups that form compounds: (1) in cyclic with α-1,4 glucosidic linkages, (2) with linear chains of α-1,4 glucosidic linkages, (3) with branched α-1,6 glucosidic linkages, and (4) with linear chains of α-1,6 glucosidic linkages. We discuss potential processability and functionality of starch derivatives with different linkage combinations and structures. The changes in properties caused by rearrangements of glycosidic linkages provide guidance for design of starch derivatives with desired structures and properties, which promotes the development of new starch products and starch processing for the food industry.

A 4-α-Glucanotransferase from Thermus thermophilus HB8: Secretory Expression and Characterization

4-α-glucanotransferase (4GT, EC 2.4.1.25) catalyzes the breakdown of the α-1,4 glycosidic bonds of the starch main chain and forms new α-1,4 glycosidic bonds in the side chain, which is often used to optimize the physical and chemical properties of starch and to improve the quality of starch-based food. However, the low enzyme activity of 4GT limits its production and widespread application. Herein, the 4GT gene encoding 500 amino acids from Thermus thermophilus HB8 was cloned and expressed in Escherichia coli. The purified 4GT exhibited maximum activity at pH 7.0 and 60 °C and had a good stability at pH 6.0-8.0 and 30-60 °C. It was confirmed that 4GT possessed the catalytic function of extending the branch length of potato starch. Furthermore, the 4GT gene was successfully expressed extracellularly in Bacillus subtilis. Then, the enzyme yield of 4GT increased by 4.1 times through screening of different plasmids and hosts. Additionally, the fermentation conditions were optimized to enhance 4GT extracellular enzyme yield. Finally, a recombinant Bacillus subtilis with 299.9 U/mL enzyme yield of 4GT was obtained under the optimized fermentation process. In conclusion, this study provides a valuable reference for characterization and expression of food-grade enzymes.

Heterologous expression of 4α-glucanotransferase: overproduction and properties for industrial applications

4α-Glucanotransferase (4α-GTase) is unique in its ability to form cyclic oligosaccharides, some of which are of industrial importance. Generally, low amount of enzymes is produced by or isolated from their natural sources: animals, plants, and microorganisms. Heterologous expressions of these enzymes, in an attempt to increase their production for applicable uses, have been widely studied since 1980s; however, the expressions are mostly performed in the prokaryotic bacteria, mostly Escherichia coli. Site-directed mutagenesis has added more value to these expressed enzymes to display the desired properties beneficial for their applications. The search for further suitable properties for food application leads to an extended research in expression by another group of host organism, the generally-recognized as safe host including the Bacillus and the eukaryotic yeast systems. Herein, our review focuses on two types of 4α-GTase: the cyclodextrin glycosyltransferase and amylomaltase. The updated studies on the general structure and properties of the two enzymes with emphasis on heterologous expression, mutagenesis for property improvement, and their industrial applications are provided.

Enhancement of large ring cyclodextrin production using pretreated starch by glycogen debranching enzyme from Corynebacterium glutamicum

Synthesis of large-ring cyclodextrins (LR-CDs) in any significant amount has been challenging. This study enhanced the LR-CDs production by Thermus filiformis amylomaltase (TfAM) enzyme by starch pretreatment using glycogen debranching enzyme from Corynebacterium glutamicum (CgGDE). CgGDE pretreated tapioca starch gave LR-CD conversion of 31.2 ± 2.2%, compared with LR-CDs produced from non-treated tapioca starch (16.0 ± 2.4%). CgGDE pretreatment enhanced amylose content by approximately 30%. Notably, a shorter incubation time of 1 h is sufficient for CgGDE starch pretreatment to produce high LR-CD yield, compared with 6 h required for the commercial isoamylase. High-Performance Anion Exchange Chromatography coupled with Pulsed Amperometric Detection (HPAEC-PAD) and Gel Permeable Chromatography (GPC) revealed that CgGDE is more efficient than the commercial isoamylase in debranching tapioca starch and gave lower molecular weight products. In addition, lower amount of by-products (linear oligosaccharides) were detected in cyclization reaction when using CgGDE-pretreated starch. In conclusion, CgGDE is a highly effective enzyme to promote LR-CD synthesis from starch with a shorter incubation time than the commercial isoamylase.

Amylomaltases in Extremophilic Microorganisms

Amylomaltases (4-α-glucanotransferases, E.C. 2.4.1.25) are enzymes which can perform a double-step catalytic process, resulting in a transglycosylation reaction. They hydrolyse glucosidic bonds of α-1,4'-d-glucans and transfer the glucan portion with the newly available anomeric carbon to the 4'-position of an α-1,4'-d-glucan acceptor. The intramolecular reaction produces a cyclic α-1,4'-glucan. Amylomaltases can be found only in prokaryotes, where they are involved in glycogen degradation and maltose metabolism. These enzymes are being studied for possible biotechnological applications, such as the production of (i) sugar substitutes; (ii) cycloamyloses (molecules larger than cyclodextrins), which could potentially be useful as carriers and encapsulating agents for hydrophobic molecules and also as effective protein chaperons; and (iii) thermoreversible starch gels, which could be used as non-animal gelatin substitutes. Extremophilic prokaryotes have been investigated for the identification of amylomaltases to be used in the starch modifying processes, which require high temperatures or extreme conditions. The aim of this article is to present an updated overview of studies on amylomaltases from extremophilic Bacteria and Archaea, including data about their distribution, activity, potential industrial application and structure.

Retarding Oxidative and Enzymatic Degradation of Phenolic Compounds Using Large-Ring Cycloamylose

The phenolic compounds (PCs) abundant in fruits and vegetables are easily browned by oxygen and browning enzymes, with subsequent destruction of nutrients during food processing and storage. Therefore, natural anti-browning additives are required to control these reactions. The aim of the present study was to investigate the feasibility of cycloamylose (CA) complexation as a way to improve stability of PCs against oxidation and browning enzymes. The complex was prepared by reacting enzymatically produced CA with a degree of polymerization of 23-45 with PCs in aqueous solution. No significant differences were observed between the PCs and their CA complexes in 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical scavenging experiments. However, the reduction rate of their antioxidant activity was clearly reduced in the presence of CA for as long as 4 weeks. At the studied concentrations, the activity of polyphenol oxidase on all of the tested PC species was inhibited in the presence of CA, although this effect was less evident as the substrate concentration increased. The higher the CA concentration added to apple juice, the lower the variation in the total color difference (ΔE*) during storage, confirming that CA could be used as an effective natural anti-browning agent. Our study is the first to study the potential of CA as a natural material for browning control. The results obtained will provide useful information for active food applications requiring oxidative stability in fruit products.

Amyrel, a novel glucose-forming α-amylase from Drosophila with 4-α-glucanotransferase activity by disproportionation and hydrolysis of maltooligosaccharides

The α-amylase paralogue Amyrel present in true flies (Diptera Muscomorpha) has been classified as a glycoside hydrolase in CAZy family GH13 on the basis of its primary structure. Here we report that, in fact, Amyrel is currently unique amongst Animals as it possesses both the hydrolytic α-amylase activity (EC 3.2.1.1) and a 4-α-glucanotransferase (EC 2.4.1.25) transglycosylation activity. Amyrel reacts specifically on α-(1-4) glycosidic bonds of starch and related polymers but produces a complex mixture of maltooligosaccharides, in sharp contrast with canonical animal α-amylases. With model maltooligosaccharides G2 (maltose) to G7, the Amyrel reaction starts by a disproportionation leading to Gn-1 and Gn + 1 products, which become themselves substrates for new disproportionation cycles. As a result, all detectable odd- and even-numbered maltooligosaccharides at least up to G12 were observed. However, hydrolysis of these products proceeds simultaneously, as shown by p-nitrophenyl-tagged oligosaccharides and microcalorimetry, and upon prolonged reaction, glucose is the major end product followed by maltose. The main structural determinant of these atypical activities was found to be a Gly-His-Gly-Ala deletion in the so-called flexible loop bordering the active site. Indeed, engineering this deletion in pig pancreatic and D. melanogaster α-amylases results in reaction patterns similar to those of Amyrel. It is proposed that this deletion provides more freedom to the substrate for subsites occupancy and allows a less constrained action pattern resulting in versatile activities at the active site.

Streptococcus agalactiae amylomaltase offers insight into the transglycosylation mechanism and the molecular basis of thermostability among amylomaltases

Amylomaltase (AM) catalyzes transglycosylation of starch to form linear or cyclic oligosaccharides with potential applications in biotechnology and industry. In the present work, a novel AM from the mesophilic bacterium Streptococcus agalactiae (SaAM), with 18–49% sequence identity to previously reported AMs, was characterized. Cyclization and disproportionation activities were observed with the optimum temperature of 30 °C and 40 °C, respectively. Structural determination of SaAM, the first crystal structure of small AMs from the mesophiles, revealed a glycosyl-enzyme intermediate derived from acarbose and a second acarbose molecule attacking the intermediate. This pre-transglycosylation conformation has never been before observed in AMs. Structural analysis suggests that thermostability in AMs might be mainly caused by an increase in salt bridges since SaAM has a lower number of salt bridges compared with AMs from the thermophiles. Increase in thermostability by mutation was performed. C446 was substituted with A/S/P. C446A showed higher activities and higher k_cat/K_m values for starch in comparison to the WT enzyme. C446S exhibited a 5 °C increase in optimum temperature and the threefold increase in half-life time at 45 °C, most likely resulting from H-bonding interactions. For all enzymes, the main large-ring cyclodextrin (LR-CD) products were CD24-CD26 with CD22 as the smallest. C446S produced more CD35-CD42, especially at a longer incubation time.

Comparison of Catalyzing Properties of Bacterial 4-α-Glucanotransferases Focusing on Their Cyclizing Activity

A newly cloned 4-α-glucanotransferase (αGT) from Deinococcus geothermalis and two typical bacterial αGTs from Thermus scotoductus and Escherichia coli (MalQ) were investigated. Among 4 types of catalysis, the cyclization activity of αGTs that produces cycloamylose (CA), a valuable carbohydrate making inclusion complexes, was intensively studied. The new αGT, DgαGT, showed close protein sequence to the αGT from T. scotoductus (TsαGT). MalQ was clearly separated from the other two αGTs in the phylogenetic and the conserved regions analyses. The reaction velocities of disproportionation, cyclization, coupling, and hydrolysis of three αGTs were determined. Intriguingly, MalQ exhibited more than 100-fold lower cyclization activity than the others. To lesser extent, the disproportionation activity of MalQ was relatively low. DgαGT and TsαGT showed similar kinetics results, but TsαGT had nearly 10-fold lower hydrolysis activity than DgαGT. Due to the very low cyclizing activity of MalQ, DgαGT and TsαGT were selected for further analyses. When amylose was treated with DgαGT or TsαGT, CA with a broad DP range was generated immediately. The DP distribution of CA had a bimodal shape (DP 7 and 27 as peaks) for the both enzymes, but larger DPs of CA quickly decreased in the DgαGT. Cyclomaltopentaose, a rare cyclic sugar, was produced at early reaction stage and accumulated as the reactions went on in the both enzymes, but the increase was more profound in the TsαGT. Taken together, we clearly demonstrated the catalytic differences between αGT groups from thermophilic and pathogenic bacteria that and showed that αGTs play different roles depending on their lifestyle.

Amylomaltase from Thermus filiformis: expression in Saccharomyces cerevisiae and its use in starch modification

AIM

To express amylomaltase from Thermus filiformis (TfAM) in a generally recognized as safe (GRAS) organism and to use the enzyme in starch modification.

METHODS AND RESULTS

TfAM was expressed in Saccharomyces cerevisiae, using 2% (w/v) galactose inducer under GAL1 promoter. The enzyme was thermostable with high disproportionation and cyclization activities. The main large-ring cyclodextrin (CD) products were CD24-CD29, with CD26 as maximum at all incubation times. TfAM was used to modify cassava and pea starches, the amylose content decreased 18% and 30%, respectively, when 5% (w/v) starch was treated with 0·5 U TfAM g-1 starch. The increase in short branched chain (DP, degree of polymerization, 1-5) and the broader chain length distribution pattern which extended to the longer chain (DP40) after TfAM treatment were observed. The thermal property was changed, with an increase in retrogradation of starch as suggested by a lower enthalpy.

CONCLUSIONS

TfAM was successfully expressed in S. cerevisiae and was used to make starches with new functionality.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report on the expression of AM in the GRAS yeast and the production of a modified starch gel from pea starch to improve the versatility of starch for food use.

Structural analysis and reaction mechanism of the disproportionating enzyme (D-enzyme) from potato

Starch produced by plants is a stored form of energy and is an important dietary source of calories for humans and domestic animals. Disproportionating enzyme (D-enzyme) catalyzes intramolecular and intermolecular transglycosylation reactions of α-1, 4-glucan. D-enzyme is essential in starch metabolism in the potato. We present the crystal structures of potato D-enzyme, including two different types of complex structures: a primary Michaelis complex (substrate binding mode) for 26-meric cycloamylose (CA26) and a covalent intermediate for acarbose. Our study revealed that the acarbose and CA26 reactions catalyzed by potato D-enzyme involve the formation of a covalent intermediate with the donor substrate. HPAEC of reaction substrates and products revealed the activity of the potato D-enzyme on acarbose and CA26 as donor substrates. The structural and chromatography analyses provide insight into the mechanism of the coupling reaction of CA and glucose catalyzed by the potato D-enzyme. The enzymatic reaction mechanism does not involve residual hydrolysis. This could be particularly useful in preventing unnecessary starch degradation leading to reduced crop productivity. Optimization of this mechanism would be important for improvements of starch storage and productivity in crops.

An ultra-stable glucanotransferase-cum-exoamylase from the hyperthermophile archaeon Thermococcus onnurineus

The genome of the hyperthermophile archaeon Thermococcus onnurineus (strain NA1) encodes a 652 residues-long putative 4-α-glucanotransferase of the GH 57 family which we have expressed in Escherichia coli. The enzyme (TonAmyGT) appears to remove glucose from the reducing end of a donor glucan and transfers it to the non-reducing end of an acceptor glucan, creating a pool of oligosaccharides through disproportionation of any substrate maltooligosaccharide, with maltose acting substantively as the smallest donor glucan as well as the smallest acceptor glucan. Additionally, glucose is also cleaved from maltooligosaccharides and released into solution without being transferred to an acceptor, causing the enzyme to function as an exo-amylase (which can digest starch) in addition to its activity as a glucanotransferase. TonAmyGT functions over a broad range of temperature (20-100 °C) and pH (4.0-9.0), and shows extreme resistance to chemical and thermal denaturation, displaying a melting temperature of 104 °C, at a pressure of 35 psi, in a differential scanning calorimeter. An interesting characteristic is that the glucanotransferase activity shows feedback inhibition through glucose (which the enzyme itself generates), indicating that the exo-amylase and glucanotransferase activities regulate each other.

Starch granule initiation and morphogenesis-progress in Arabidopsis and cereals

Starch, the major storage carbohydrate in plants, is synthesized in plastids as semi-crystalline, insoluble granules. Many organs and cell types accumulate starch at some point during their development and maturation. The biosynthesis of the starch polymers, amylopectin and amylose, is relatively well understood and mostly conserved between organs and species. However, we are only beginning to understand the mechanism by which starch granules are initiated, and the factors that control the number of granules per plastid and the size/shape of granules. Here, we review recent progress in understanding starch granule initiation and morphogenesis. In Arabidopsis, granule initiation requires several newly discovered proteins with specific locations within the chloroplast, and also on the availability of maltooligosaccharides which act as primers for initiation. We also describe progress in understanding granule biogenesis in the endosperm of cereal grains-within which there is large interspecies variation in granule initiation patterns and morphology. Investigating whether this diversity results from differences between species in the functions of known proteins, and/or from the presence of novel, unidentified proteins, is a promising area of future research. Expanding our knowledge in these areas will lead to new strategies for improving the quality of cereal crops by modifying starch granule size and shape in vivo.

Significance of H461 at subsite +1 in substrate binding and transglucosylation activity of amylomaltase from Corynebacterium glutamicum

Amylomaltase (AM) catalyzes inter- and intra-molecular transglycosylation reactions of glucan to yield linear and cyclic oligosaccharide products. The functional roles of the conserved histidine at position 461 in the active site of AM from Corynebacterium glutamicum (CgAM) was investigated. H461 A/S/D/R/W were constructed, their catalytic properties were compared to the wild-type (WT). A significant decrease in transglucosylation activities was observed, especially in H461A mutant, while hydrolysis activity was barely affected. The transglucosylation factor of the H461A-CgAM was decreased by 8.6 folds. WT preferred maltotriose (G3) as substrate for disproportionation reaction, but all H461 mutants showed higher preference for maltose (G2). Using G3 substrate, k_cat/K_m values of H461 mutated CgAMs were 40-64 folds lower, while the K_m values were twice higher than those of WT. All mutants could not produce large-ring cyclodextrin (LR-CD) product. The heat capacity profile indicated that WT had higher thermal stability than H461A. The X-ray structure of WT showed two H-bonds between H461 and heptasaccharide analog at subsite +1, while no such bonding was observed from the model structure of H461A. The importance of H461 on substrate binding with CgAM was evidenced. We are the first to mutate an active site histidine in AM to explore its function.

Y418 in 410s loop is required for high transglucosylation activity and large-ring cyclodextrin production of amylomaltase from Corynebacterium glutamicum

Amylomaltase catalyzes α-1,4 glucosyl transfer reaction to yield linear or cyclic oligosaccharide products. The aim of this work is to investigate functional roles of 410s loop unique to amylomaltase from Corynebacterium glutamicum (CgAM). Site-directed mutagenesis of Y418, the residue at the loop tip, was performed. Y418A/S/D/R/W/F - CgAMs were characterized and compared to the wild-type (WT). A significant decrease in starch transglucosylation, disproportionation and cyclization activities was observed. Specificity for G3 substrate in disproportionation reaction was not changed; however, Y418F showed an increase in preference for longer oligosaccharides G5 to G7. The catalytic efficiency of Y418 mutated CgAMs, except for Y418F, was significantly lower (up to 8- and 12- fold for the W and R mutants, respectively) than that of WT. The change was in the k_cat, not the K_m values which were around 16-20 mM. The profile of large-ring cyclodextrin (LR-CD) product was different; the principal product of Y418A/D/S was shifted to the larger size (CD36-CD40) while that of the WT and Y418F peaked at CD29-CD33. The product yield was reduced especially in W and R mutants. Hence Y418 in 410s loop of CgAM not only contributes to transglucosylation activities but also controls the amount and size of LR-CD products through the proposed hydrophobic stacking interaction and the suitable distance of loop channel for substrate entering. This is the first report to show the effect of the loop tip residue on LR-CD product formation.

Roles of N287 in catalysis and product formation of amylomaltase from Corynebacterium glutamicum

Amylomaltase catalyzes intermolecular and intramolecular transglucosylation reactions to form linear and cyclic oligosaccharides, respectively. The aim of this work is to investigate the structure-function relationship of amylomaltase from a mesophilic Corynebacterium glutamicum (CgAM). Site-directed mutagenesis was performed to substitute Tyr for Asn287 (N287Y) to determine its role in controlling amylomaltase activity and product formation. Expression of the wild-type (WT) and N287Y was achieved by cultivating recombinant cells in the medium containing lactose at 16 °C for 14 h. The purified mutated enzyme showed a significant decrease in all transglucosylation activities while hydrolysis activity was not changed. Optimum temperature and pH for disproportionation reaction were slightly changed upon mutation while those for cyclization reaction were not changed. Interestingly, N287Y showed a change in large-ring cyclodextrin (LR-CD) product profile in which the larger size was observed together with an increase in thermostability and substrate preference for G5 in addition to G3. The secondary structure of the mutated enzyme was slightly changed in related to the WT as evidenced from circular dichroism analysis. This work thus demonstrates that N287 is required for transglucosylation activities of CgAM. Having an aromatic residue in this position increased thermostability, changed product profile and substrate preference but demolished most enzyme activities.

Crystal Structure of Amylomaltase from Corynebacterium glutamicum

Amylomaltase is an essential enzyme in maltose utilization and maltodextrin metabolism, and it has been industrially used for the production of cyclodextrin and modification of starch. We determined the crystal structure of amylomaltase from Corynebacterium glutamicum (CgAM) at a resolution of 1.7 Å. Although CgAM forms a dimer without NaCl, it exists as a monomer in physiological concentration of NaCl. CgAM is composed of N- and C-terminal domains, which can be further divided into two and four subdomains, respectively. It exhibits a unique structural feature at the functionally unknown N-domain and also shows two striking differences at the C-domain compared to other amylomaltases. These differences at extended edge of the substrate-binding site might affect substrate specificity for large cyclodextrin formation. The bis-tris methane and sulfate molecules bound at the substrate-binding site of our current structure mimic the binding of the hydroxyl groups of glucose bound at subsites -1 and -2, respectively.

Pcal_0768, a hyperactive 4-α-glucanotransferase from Pyrobacculum calidifontis

Genome sequence of hyperthermophilic archaeon Pyrobaculum calidifontis revealed the presence of an open reading frame, Pcal_0768, corresponding to a putative 4-α-glucanotranferase belonging to glycoside hydrolases (GH) family 77. We have produced, in Escherichia coli, and purified recombinant Pcal_0768 which exhibited high disproportionation (690 U mg(-1)) activity. To the best of our knowledge, this is the highest ever reported activity for any member of family GH77. Maltooligosaccharides, when used as sole substrates, were disproportionated into linear maltooligohomologues. The analysis of the reaction end products revealed no evidence for the production of cycloamyloses. Catalytic activity of the enzyme remained unchanged in the presence or the absence of ionic and nonionic detergents. γ-cyclodextrin, an inhibitor of 4-α-glucanotransferases, did not show any inhibitory effect on Pcal_0768 activity. These properties make Pcal_0768 a potential candidate for starch processing industry.

Mutagenesis for improvement of activity and thermostability of amylomaltase from Corynebacterium glutamicum

This work aims to improve thermostability of amylomaltase from a mesophilic Corynebacterium glutamicum (CgAM) by random and site-directed mutagenesis. From error prone PCR, a mutated CgAM with higher thermostability at 50 °C compared to the wild-type was selected and sequenced. The result showed that the mutant contains a single mutation of A406V. Site-directed mutagenesis was then performed to construct A406V and A406L. Both mutated CgAMs showed higher intermolecular transglucosylation activity with an upward shift in the optimum temperature and a slight increase in the optimum pH for disproportionation and cyclization reactions. Thermostability of both mutated CgAMs at 35-40 °C was significantly increased with a higher peak temperature from DSC spectra when compared to the wild-type. A406V had a greater effect on activity and thermostability than A406L. The catalytic efficiency values kcat/Km of A406V- and A406L-CgAMs were 2.9 and 1.4 times higher than that of the wild-type, respectively, mainly due to a significant increase in kcat. LR-CD product analysis demonstrated that A406V gave higher product yield, especially at longer incubation time and higher temperature, in comparison to the wild-type enzyme.

Multi-wavelength colorimetric determination of large-ring cyclodextrin content for the cyclization activity of 4-α-glucanotransferase

Large-ring cyclodextrins (LR-CDs) have a number of intriguing properties for potential use in pharmaceutical and food industry. To date, no colorimetric method has been reported for LR-CD content quantification. In this study, triple wavelength colorimetry (TWC) and orthogonal-function spectrophotometry (OFS) have been successfully applied to determine ingredient concentrations in a mixture of amylose and LR-CDs. Both TWC and OFS yielded precise amylose content data in good agreement with expected values. For quantification of LR-CD content, OFS provided a higher accuracy than TWC, which resulted in a slight over-determination. As a comparison, single-wavelength colorimetry performed at the corresponding absorption maximum led to a significant over-determination of both amylose and LR-CD contents. The validity of TWC and OFS allowed their application for discriminative detection of the cyclization and total activity of a 4-α-glucanotransferase (4 αGTase) from Thermus aquaticus regarding the synthesis of LR-CDs and the conversion of amylose to small molecules, respectively. High pressure size exclusion chromatography analysis of the post-reaction mixtures following 4 αGTase-catalyzed conversion of amylose revealed the presence of linear malto-oligosaccharides in the LR-CD fraction. By introduction of a correction factor, the interference caused by linear malto-oligosaccharides was eliminated for a more accurate determination of LR-CD cyclization activity.

Identification of essential tryptophan in amylomaltase from Corynebacterium glutamicum

This work aims to identify essential tryptophan residue(s) of amylomaltase from Corynebacterium glutamicum (CgAM) through chemical modification and site-directed mutagenesis techniques. The recombinant enzyme expressed by Escherichia coli was purified and treated with N-bromosuccinimide (NBS), a modifying agent for tryptophan. A significant decrease in enzyme activity was observed indicating that tryptophan is important for catalysis. Inactivation kinetics with NBS resulted in pseudo first-order rate constant (kinact) of 2.31 min(-1). Substrate protection experiment confirmed the active site localization of the NBS-modified tryptophan residue(s) in CgAM. Site-directed mutagenesis was performed on W330, W425 and W673 to localize essential tryptophan residues. Substitution by alanine resulted in the loss of intra- and intermolecular transglucosylation activities for all mutated CgAMs. Analysis of circular dichroism spectra showed no change in the secondary structure of W425A but a significant change for W330A and W673A from that of the WT. From these results in combination with X-ray structural data and interpretation from the binding interactions in the active site region, W425 was confirmed to be essential for catalytic activity of CgAM. The hydrophobicity of this tryptophan was thought to be critical for substrate binding and supporting catalytic action of the three carboxylate residues at the active site.

Crystallization and preliminary X-ray crystallographic analysis of the amylomaltase from Corynebacterium glutamicum

Amylomaltase (AM; EC 2.4.1.25) belongs to the 4-α-glucanotransferase group of the α-amylase family. The enzyme can produce cycloamylose or large-ring cyclodextrin through intramolecular transglycosylation or cyclization reactions of α-1,4-glucan. Amylomaltase from the mesophilic bacterium Corynebacterium glutamicum (CgAM) contains extra residues at the N-terminus for which the three-dimensional structure is not yet known. In this study, CgAM was overexpressed and purified to homogeneity using DEAE FF and Phenyl FF columns. The purified CgAM was crystallized by the vapour-diffusion method. Preliminary X-ray data showed that the CgAM crystal diffracted to 1.7 Å resolution and belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 73.28, b = 82.61, c = 118.64 Å. To obtain the initial phases, crystals of selenomethionyl-substituted amylomaltase were produced, and multiple-wavelength anomalous dispersion phasing and structure refinement are now in progress.