Enzymatic circularization of a malto-octaose linear chain studied by stochastic reaction path calculations on cyclodextrin glycosyltransferase

Modification of Bacillus clarkii γ-Cyclodextrin Glycosyltransferase and Addition of Complexing Agents to Increase γ-Cyclodextrin Production

γ-Cyclodextrin (γ-CD), a cyclic oligosaccharide containing eight glucose units linked by α-1,4-glycosidic bonds, can be produced from starch using cyclodextrin glycosyltransferase (CGTase). Unfortunately, this enzymatic process produces mixtures of α-, β-, and γ-CD. In this study, amino acid residues in the subsite -3 (T47 and F91) and the central subsite (Y186) of Bacillus clarkii γ-CGTase were modified to improve the γ-CD production. The cyclization activities and product specificities of mutants T47H and F91W were similar to those of the wild-type. The cyclization activities of mutants F91N and F91L were significantly greater than those of the wild-type but their γ-CD product specificities were lower. Finally, the central subsite mutant Y186W displayed a γ-CD specificity (94.6%) significantly greater than that of the wild-type (77.1%). To maximize the γ-CD yield, the effects of added complexing agents were investigated. Among the cyclic complexing agents tested, low-boiling cyclododecanone was the smallest that precipitated with γ-CD. When cyclododecanone was used with Y186W, the total CD yield reached 72.6%, and 96.6% of the product was γ-CD. These results, which represent the highest γ-CD yield ever reported, may provide a way to improve large-scale γ-CD preparation and expand the uses of γ-CD in the future.

Structural features of a bacterial cyclic α-maltosyl-(1→6)-maltose (CMM) hydrolase critical for CMM recognition and hydrolysis

Cyclic α-maltosyl-(1→6)-maltose (CMM, cyclo-{→6)-α-d-Glcp-(1→4)-α-d-Glcp-(1→6)-α-d-Glcp-(1→4)-α-d-Glcp-(1→})is a cyclic glucotetrasaccharide with alternating α-1,4 and α-1,6 linkages. CMM is composed of two maltose units and is one of the smallest cyclic glucooligosaccharides. Although CMM is resistant to usual amylases, it is efficiently hydrolyzed by CMM hydrolase (CMMase), belonging to subfamily 20 of glycoside hydrolase family 13 (GH13_20). Here, we determined the ligand-free crystal structure of CMMase from the soil-associated bacterium Arthrobacter globiformis and its structures in complex with maltose, panose, and CMM to elucidate the structural basis of substrate recognition by CMMase. The structures disclosed that although the monomer structure consists of three domains commonly adopted by GH13 and other α-amylase-related enzymes, CMMase forms a unique wing-like dimer structure. The complex structure with CMM revealed four specific subsites, namely -3', -2, -1, and +1'. We also observed that the bound CMM molecule adopts a low-energy conformer compared with the X-ray structure of a single CMM crystal, also determined here. Comparison of the CMMase active site with those in other enzymes of the GH13_20 family revealed that three regions forming the wall of the cleft, denoted PYF (Pro-203/Tyr-204/Phe-205), CS (Cys-163/Ser-164), and Y (Tyr-168), are present only in CMMase and are involved in CMM recognition. Combinations of multiple substitutions in these regions markedly decreased the activity toward CMM, indicating that the specificity for this cyclic tetrasaccharide is supported by the entire shape of the pocket. In summary, our work uncovers the mechanistic basis for the highly specific interactions of CMMase with its substrate CMM.

Synthesis and investigation of a self-assembled hydrogel based on hydroxyethyl cellulose and its in vitro ibuprofen drug release characteristics

IBU is solubilized and encapsulated by β-CDP. Then C₁₂ side-chain grafting onto HEC forms inclusion complexes with the cavities of β-CDP in β-CDP/IBU through host–guest interactions to form a new self-assembled hydrogel gel-(β)CDP-HEC/IBU.

Molecular dynamic analysis of mutant Y195I α-cyclodextrin glycosyltransferase with switched product specificity from α-cyclodextrin to γ-cyclodextrin

Alpha-cyclodextrin (α-CD) glycosyltransferase (α-CGTase) can convert starch into α-CD blended with various proportions of β-cyclodextrin (β-CD) and/or γ-cyclodextrin (γ-CD). In this study, we verified the catalytic characteristics of purified Y195I α-CGTase and elucidated the mechanism of action with molecular dynamic (MD) simulations. We found that purified Y195I α-CGTase produced less α-CD, slightly more β-CD, and significantly more γ-CD than wild-type α-CGTase. Correspondingly, α-CD-based K m values increased, and β-CD- and γ-CD-based K m values decreased. MD simulation studies revealed that the dynamic trajectories of the substrate oligosaccharide chain in the mutant CGTase binding site were significantly different from those in the wild-type enzyme, with reduced hydrophobic interaction, finally resulting in different product specificity and more γ-CD formation.

Recent advances in discovery, heterologous expression, and molecular engineering of cyclodextrin glycosyltransferase for versatile applications

Cyclodextrin glycosyltransferase (CGTase) is an important enzyme with multiple functions, in particular the production of cyclodextrins. It is also widely applied in baking and carbohydrate glycosylation because it participates in various types of catalytic reactions. New applications are being found with novel CGTases being isolated from various organisms. Heterologous expression is performed for the overproduction of CGTases to meet the requirements of these applications. In addition, various directed evolution techniques have been applied to modify the molecular structure of CGTase for improved performance in industrial applications. In recent years, substantial progress has been made in the heterologous expression and molecular engineering of CGTases. In this review, we systematically summarize the heterologous expression strategies used for enhancing the production of CGTases. We also outline and discuss the molecular engineering approaches used to improve the production, secretion, and properties (e.g., product and substrate specificity, catalytic efficiency, and thermal stability) of CGTase.

Iterative saturation mutagenesis of -6 subsite residues in cyclodextrin glycosyltransferase from Paenibacillus macerans to improve maltodextrin specificity for 2-O-D-glucopyranosyl-L-ascorbic acid synthesis

2-O-d-Glucopyranosyl-l-ascorbic acid (AA-2G), a stable l-ascorbic acid derivative, is usually synthesized by cyclodextrin glycosyltransferase (CGTase), which contains nine substrate-binding subsites (from +2 to -7). In this study, iterative saturation mutagenesis (ISM) was performed on the -6 subsite residues (Y167, G179, G180, and N193) in the CGTase from Paenibacillus macerans to improve its specificity for maltodextrin, which is a cheap and easily soluble glycosyl donor for AA-2G synthesis. Site saturation mutagenesis of four sites-Y167, G179, G180, and N193-was first performed and revealed that four mutants-Y167S, G179R, N193R, and G180R-produced AA-2G yields higher than those of other mutant and wild-type CGTases. ISM was then conducted with the best positive mutant as a template. Under optimal conditions, mutant Y167S/G179K/N193R/G180R produced the highest AA-2G titer of 2.12 g/liter, which was 84% higher than that (1.15 g/liter) produced by the wild-type CGTase. Kinetics analysis of AA-2G synthesis using mutant CGTases confirmed the enhanced maltodextrin specificity and showed that compared to the wild-type CGTase, the mutants had no cyclization activity but high hydrolysis and disproportionation activities. A possible mechanism for the enhanced substrate specificity was also analyzed through structure modeling of the mutant and wild-type CGTases. These results indicated that the -6 subsite played crucial roles in the substrate binding and catalytic reactions of CGTase and that the obtained CGTase mutants, especially Y167S/G179K/N193R/G180R, are promising starting points for further development through protein engineering.

Structural and functional analysis of substrate recognition by the 250s loop in amylomaltase from Thermus brockianus

Amylomaltase, or 4-α-glucanotransferase (EC 2.4.1.25), is involved in glycogen and maltooligosaccharide metabolism in microorganisms, catalyzing both the hydrolysis and transfer of an α-1,4-oligosacchraride to other sugar molecules. In this study, we determined the crystal structure of amylomaltase from Thermus brockianus at a resolution of 2.3 Å and conducted a biochemical study to understand the detailed mechanism for its activity. Careful comparison with previous amylomaltase structures showed a pattern of conformational flexibility in the 250s loop with higher B-factor. Amylomaltase from T. brockianus exhibited a high transglycosylation factor for glucose and a lower value for maltose. Mutation of Gln256 resulted in increased K(m) for maltotriose and a sharp decrease of the transglycosylation factor for maltose, suggesting the involvement of Gln 256 in substrate binding between subsites +1 and +2. Mutation of Phe251 resulted in significantly lower glucose production but increased maltose production from maltopentose substrates, showing an altered substrate-binding affinity. The mutational data suggest the conformational flexibility of the loop may be involved in substrate binding in the GH77 family. Here, we present an action model of the 250s loop providing the molecular basis for the involvement of residues Phe251, Gln256, and Trp258 in the hydrolysis and transglycosylation activities in amylomaltase.

Thermus thermophilus glycoside hydrolase family 57 branching enzyme: crystal structure, mechanism of action, and products formed

Branching enzyme (EC 2.4.1.18; glycogen branching enzyme; GBE) catalyzes the formation of α1,6-branching points in glycogen. Until recently it was believed that all GBEs belong to glycoside hydrolase family 13 (GH13). Here we describe the cloning and expression of the Thermus thermophilus family GH57-type GBE and report its biochemical properties and crystal structure at 1.35-Å resolution. The enzyme has a central (β/α)(7)-fold catalytic domain A with an inserted domain B between β2 and α5 and an α-helix-rich C-terminal domain, which is shown to be essential for substrate binding and catalysis. A maltotriose was modeled in the active site of the enzyme which suggests that there is insufficient space for simultaneously binding of donor and acceptor substrates, and that the donor substrate must be cleaved before acceptor substrate can bind. The biochemical assessment showed that the GH57 GBE possesses about 4% hydrolytic activity with amylose and in vitro forms a glucan product with a novel fine structure, demonstrating that the GH57 GBE is clearly different from the GH13 GBEs characterized to date.

The evolution of cyclodextrin glucanotransferase product specificity

Cyclodextrin glucanotransferases (CGTases) have attracted major interest from industry due to their unique capacity of forming large quantities of cyclic alpha-(1,4)-linked oligosaccharides (cyclodextrins) from starch. CGTases produce a mixture of cyclodextrins from starch consisting of 6 (alpha), 7 (beta) and 8 (gamma) glucose units. In an effort to identify the structural factors contributing to the evolutionary diversification of product specificity amongst this group of enzymes, we selected nine CGTases from both mesophilic, thermophilic and hyperthermophilic organisms for comparative product analysis. These enzymes displayed considerable variation regarding thermostability, initial rates, percentage of substrate conversion and ratio of alpha-, beta- and gamma-cyclodextrins formed from starch. Sequence comparison of these CGTases revealed that specific incorporation and/or substitution of amino acids at the substrate binding sites, during the evolutionary progression of these enzymes, resulted in diversification of cyclodextrin product specificity.

Elimination of competing hydrolysis and coupling side reactions of a cyclodextrin glucanotransferase by directed evolution

Thermoanaerobacterium thermosulfurigenes cyclodextrin glucanotransferase primarily catalyses the formation of cyclic alpha-(1,4)-linked oligosaccharides (cyclodextrins) from starch. This enzyme also possesses unusually high hydrolytic activity as a side reaction, thought to be due to partial retention of ancestral enzyme function. This side reaction is undesirable, since it produces short saccharides that are responsible for the breakdown of the cyclodextrins formed, thus limiting the yield of cyclodextrins produced. To reduce the competing hydrolysis reaction, while maintaining the cyclization activity, we applied directed evolution, introducing random mutations throughout the cgt gene by error-prone PCR. Mutations in two residues, Ser-77 and Trp-239, on the outer region of the active site, lowered the hydrolytic activity up to 15-fold with retention of cyclization activity. In contrast, mutations within the active site could not lower hydrolytic rates, indicating an evolutionary optimized role for cyclodextrin formation by residues within this region. The crystal structure of the most effective mutant, S77P, showed no alterations to the peptide backbone. However, subtle conformational changes to the side chains of active-site residues had occurred, which may explain the increased cyclization/hydrolysis ratio. This indicates that secondary effects of mutations located on the outer regions of the catalytic site are required to lower the rates of competing side reactions, while maintaining the primary catalytic function. Subsequent functional analysis of various glucanotransferases from the superfamily of glycoside hydrolases also suggests a gradual evolutionary progression of these enzymes from a common 'intermediate-like' ancestor towards specific transglycosylation activity.

Purification and characterization of cyclic maltosyl-(1-->6)-maltose hydrolase and alpha-glucosidase from an Arthrobacter globiformis strain

Cyclic maltosyl-maltose [CMM, cyclo-[-->6)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->6)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->]], a novel cyclic tetrasaccharide, has a unique structure. Its four glucose residues are joined by alternate alpha-1,4 and alpha-1,6 linkages. CMM is synthesized from starch by the action of 6-alpha-maltosyltransferase from Arthrobacter globiformis M6. Recently, we determined the mechanism of extracellular synthesis of CMM, but the degrading pathway of the saccharide remains unknown. Hence we tried to identify the enzymes involved in the degradation of CMM to glucose from the cell-free extract of the strain, and identified CMM hydrolase (CMMase) and alpha-glucosidase as the responsible enzymes. The molecular mass of CMMase was determined to be 48.6 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and 136 kDa by gel filtration column chromatography. The optimal pH and temperature for CMMase activity were 6.5 and 30 degrees C. The enzyme remained stable from pH 5.5 to 8.0 and up to 25 degrees C. CMMase hydrolyzed CMM to maltose via maltosyl-maltose as intermediates, but it did not hydrolyze CMM to glucose, suggesting that it is a novel hydrolase that hydrolyzes the alpha-1,6-linkage of CMM. The molecular mass of alpha-glucosidase was determined to be 60.1 kDa by SDS-PAGE and 69.5 kDa by gel filtration column chromatography. The optimal pH and temperature for alpha-glucosidase activity were 7.0 and 35 degrees C. The enzyme remained stable from pH 7.0 to 9.5 and up to 35 degrees C. alpha-Glucosidase degraded maltosyl-maltose to glucose via panose and maltose as intermediates, but it did not degrade CMM. Furthermore, when CMMase and alpha-glucosidase existed simultaneously in a reaction mixture containing CMM, glucose was detected as the final product. It was found that CMM was degraded to glucose by the synergistic action of CMMase and alpha-glucosidase.

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.

Role of Phe283 in enzymatic reaction of cyclodextrin glycosyltransferase from alkalophilic Bacillus sp.1011: Substrate binding and arrangement of the catalytic site

Cyclodextrin glycosyltransferase (CGTase) belonging to the alpha-amylase family mainly catalyzes transglycosylation and produces cyclodextrins from starch and related alpha-1,4-glucans. The catalytic site of CGTase specifically conserves four aromatic residues, Phe183, Tyr195, Phe259, and Phe283, which are not found in alpha-amylase. To elucidate the structural role of Phe283, we determined the crystal structures of native and acarbose-complexed mutant CGTases in which Phe283 was replaced with leucine (F283L) or tyrosine (F283Y). The temperature factors of the region 259-269 in native F283L increased >10 A(2) compared with the wild type. The complex formation with acarbose not only increased the temperature factors (>10 A(2)) but also changed the structure of the region 257-267. This region is stabilized by interactions of Phe283 with Phe259 and Leu260 and plays an important role in the cyclodextrin binding. The conformation of the side-chains of Glu257, Phe259, His327, and Asp328 in the catalytic site was altered by the mutation of Phe283 with leucine, and this indicates that Phe283 partly arranges the structure of the catalytic site through contacts with Glu257 and Phe259. The replacement of Phe283 with tyrosine decreased the enzymatic activity in the basic pH range. The hydroxyl group of Tyr283 forms hydrogen bonds with the carboxyl group of Glu257, and the pK(a) of Glu257 in F283Y may be lower than that in the wild type.

Purification and characterization of an intracellular cycloalternan-degrading enzyme from Bacillus sp. NRRL B-21195

A novel intracellular cycloalternan-degrading enzyme (CADE) was purified to homogeneity from the cell pellet of Bacillus sp. NRRL B-21195. The enzyme has a molecular mass of 125 kDa on SDS-PAGE. The pH optimum was 7.0, and the enzyme was stable from pH 6.0 to 9.2. The temperature optimum was 35 degrees C and the enzyme exhibited stability up to 50 degrees C. The enzyme hydrolyzed cycloalternan [CA; cyclo(-->6)-alpha-d-Glcp-(1-->3)-alpha-d-Glcp-(1-->6)-alpha-d-Glcp-(-->3)-alpha-d-Glcp-(1-->)] as the best substrate, to produce only isomaltose via an intermediate, alpha-isomaltosyl-(1-->3)-isomaltose. This enzyme also hydrolyzed isomaltosyl substrates, such as panose, alpha-isomaltosyl-(1-->4)-maltooligosaccharides, alpha-isomaltosyl-(1-->3)-glucose, and alpha-isomaltosyl-(1-->3)-isomaltose to liberate isomaltose. Neither maltooligosaccharides nor isomaltooligosaccharides were hydrolyzed by the enzyme, indicating that CADE requires alpha-isomaltosyl residues connected with (1-->4)- or (1-->3)-linkages. The K(m) value of cycloalternan (1.68 mM) was 20% of that of panose (8.23 mM). The k(cat) value on panose (14.4s(-1)) was not significantly different from that of cycloalternan (10.8 s(-1)). Judging from its specificity, the systematic name of the enzyme should be cycloalternan isomaltosylhydrolase. This intracellular enzyme is apparently involved in the metabolism of starch via cycloalternan in Bacillus sp. NRRL B-21195, its role being to hydrolyze cycloalternan inside the cells.

Tyrosine 105 and Threonine 212 at Outermost Substrate Binding Subsites –6 and +4 Control Substrate Specificity, Oligosaccharide Cleavage Patterns, and Multiple Binding Modes of Barley α-Amylase 1

The role in activity of outer regions in the substrate binding cleft in alpha-amylases is illustrated by mutational analysis of Tyr(105) and Thr(212) localized at subsites -6 and +4 (substrate cleavage occurs between subsites -1 and +1) in barley alpha-amylase 1 (AMY1). Tyr(105) is conserved in plant alpha-amylases whereas Thr(212) varies in these and related enzymes. Compared with wild-type AMY1, the subsite -6 mutant Y105A has 140, 15, and <1% activity (k(cat)/K(m)) on starch, amylose DP17, and 2-chloro-4-nitrophenyl beta-d-maltoheptaoside, whereas T212Y at subsite +4 has 32, 370, and 90% activity, respectively. Thus engineering of aromatic stacking interactions at the ends of the 10-subsite long binding cleft affects activity very differently, dependent on the substrate. Y105A dominates in dual subsite -6/+4 [Y105A/T212(Y/W)]AMY1 mutants having almost retained and low activity on starch and oligosaccharides, respectively. Bond cleavage analysis of oligosaccharide degradation by wild-type and mutant AMY1 supports that Tyr(105) is critical for binding at subsite -6. Substrate binding is improved by T212(Y/W) introduced at subsite +4 and the [Y105A/T212(Y/W)]AMY1 double mutants synergistically enhanced productive binding of the substrate aglycone. The enzymatic properties of the series of AMY1 mutants suggest that longer substrates adopt several binding modes. This is in excellent agreement with computed distinct multiple docking solutions observed for maltododecaose at outer binding areas of AMY1 beyond subsites -3 and +3.

Crystal structures of 4-alpha-glucanotransferase from Thermococcus litoralis and its complex with an inhibitor

Thermococcus litoralis 4-alpha-glucanotransferase (TLGT) belongs to glucoside hydrolase family 57 and catalyzes the disproportionation of amylose and the formation of large cyclic alpha-1,4-glucan (cycloamylose) from linear amylose. We determined the crystal structure of TLGT with and without an inhibitor, acarbose. TLGT is composed of two domains: an N-terminal domain (domain I), which contains a (beta/alpha)7 barrel fold, and a C-terminal domain (domain II), which has a twisted beta-sandwich fold. In the structure of TLGT complexed with acarbose, the inhibitor was bound at the cleft within domain I, indicating that domain I is a catalytic domain of TLGT. The acarbose-bound structure also clarified that Glu123 and Asp214 were the catalytic nucleophile and acid/base catalyst, respectively, and revealed the residues involved in substrate binding. It seemed that TLGT produces large cyclic glucans by preventing the production of small cyclic glucans by steric hindrance, which is achieved by three lids protruding into the active site cleft, as well as an extended active site cleft. Interestingly, domain I of TLGT shares some structural features with the catalytic domain of Golgi alpha-mannosidase from Drosophila melanogaster, which belongs to glucoside hydrolase family 38. Furthermore, the catalytic residue of the two enzymes is located in the same position. These observations suggest that families 57 and 38 evolved from a common ancestor.

Kinetics of cytochrome C folding: atomically detailed simulations

The vast range of time scales (from nanoseconds to seconds) during protein folding is a challenge for experiments and computations. To make concrete predictions on folding mechanisms, atomically detailed simulations of protein folding, using potentials derived from chemical physics principles, are desired. However, due to their computational complexity, straightforward molecular dynamics simulations of protein folding are impossible today. An alternative algorithm is used that makes it possible to compute approximate atomically detailed long time trajectories (the Stochastic Difference Equation in Length). This algorithm is used to compute 26 atomically detailed folding trajectories of cytochrome c (a millisecond process). The early collapse of the protein chain (with marginal formation of secondary structure), and the earlier formation of the N and C helices (compare to the 60's helix) are consistent with the experiment. The existence of an energy barrier upon entry to the molten globule is examined as well. In addition to (favorable) comparison to experiments, we show that non-native contacts drive the formation of the molten globule. In contrast to popular folding models, the non-native contacts do not form off-pathway kinetic traps in cytochrome c.

Mutations converting cyclodextrin glycosyltransferase from a transglycosylase into a starch hydrolase

Cyclodextrin glycosyltransferase (CGTase) efficiently catalyzes transglycosylation of oligo-maltodextrins, although the enzyme also has a low hydrolytic activity. Its +2 substrate binding subsite, which contains the conserved Phe184 and Phe260 residues, has been shown to be important for this transglycosylation activity [Nakamura et al. (1994) Biochemistry 33, 9929-9936]. Here we show that the amino acid side chain at position 260 also controls the hydrolytic activity of CGTase. Three Phe260 mutants of Thermoanaerobacterium thermosulfurigenes CGTase were obtained with a higher hydrolytic activity than ever observed before for a CGTase. These Phe260 mutations even changed CGTase from a transglycosylase into a starch hydrolase.

The remote substrate binding subsite -6 in cyclodextrin-glycosyltransferase controls the transferase activity of the enzyme via an induced-fit mechanism

Cyclodextrin-glycosyltransferase (CGTase) catalyzes the formation of alpha-, beta-, and gamma-cyclodextrins (cyclic alpha-(1,4)-linked oligosaccharides of 6, 7, or 8 glucose residues, respectively) from starch. Nine substrate binding subsites were observed in an x-ray structure of the CGTase from Bacillus circulans strain 251 complexed with a maltononaose substrate. Subsite -6 is conserved in CGTases, suggesting its importance for the reactions catalyzed by the enzyme. To investigate this in detail, we made six mutant CGTases (Y167F, G179L, G180L, N193G, N193L, and G179L/G180L). All subsite -6 mutants had decreased k(cat) values for beta-cyclodextrin formation, as well as for the disproportionation and coupling reactions, but not for hydrolysis. Especially G179L, G180L, and G179L/G180L affected the transglycosylation activities, most prominently for the coupling reactions. The results demonstrate that (i) subsite -6 is important for all three CGTase-catalyzed transglycosylation reactions, (ii) Gly-180 is conserved because of its importance for the circularization of the linear substrates, (iii) it is possible to independently change cyclization and coupling activities, and (iv) substrate interactions at subsite -6 activate the enzyme in catalysis via an induced-fit mechanism. This article provides for the first time definite biochemical evidence for such an induced-fit mechanism in the alpha-amylase family.

Hydrophobic amino acid residues in the acceptor binding site are main determinants for reaction mechanism and specificity of cyclodextrin-glycosyltransferase

Cyclodextrin-glycosyltransferases (CGTases) (EC ) preferably catalyze transglycosylation reactions with glucosyl residues as acceptor, whereas the homologous alpha-amylases catalyze hydrolysis reactions using water as acceptor. This difference in reaction specificity is most likely caused by the acceptor binding site. To investigate this in detail we altered the acceptor site residues Lys-232, Phe-183, Phe-259, and Glu-264 of Bacillus circulans strain 251 CGTase using site-directed mutagenesis. Lys-232 is of general importance for catalysis, which appears to result mainly from stabilization of the conformation of the loop containing the catalytic nucleophile Asp-229 and His-233, a residue that has been implied in transition state stabilization. Glu-264 contributes to the disproportionation reaction only, where it is involved in initial binding of the (maltose) acceptor. Phe-183 and Phe-259 play important and distinct roles in the transglycosylation reactions catalyzed by CGTase. Mutation of Phe-183 affects especially the cyclization and coupling reactions, whereas Phe-259 is most important for the cyclization and disproportionation reactions. Moreover, the hydrophobisity of Phe-183 and Phe-259 limits the hydrolyzing activity of the enzyme. Hydrolysis can be enhanced by making these residues more polar, which concomitantly results in a lower transglycosylation activity. A double mutant was constructed that yielded an enzyme preferring hydrolysis over cyclization (15:1), whereas the wild type favors cyclization over hydrolysis (90:1).