Covalent and three-dimensional structure of the cyclodextrinase from Flavobacterium sp. no. 92

Heterologous Expression and Characterization of a Novel Mesophilic Maltogenic α-Amylase AmyFlA from Flavobacterium sp. NAU1659

A novel amylase AmyFlA from Flavobacterium sp. NAU1659, AmyFlA, was cloned and expressed in Esherichia coli. Based on phylogenetic and functional analysis, it was identified as a novel member of the subfamily GH13_46, sharing high sequence identity. The protein was predicted to consist of 620 amino acids, with a putative signal peptide of 25 amino acids. The enzyme was able to hydrolyze soluble starch with a specific activity of 352.97 U/mg at 50 °C in 50 mM phosphate buffer (pH 6.0). The Km and Vmax values of AmyFlA were respectively 3.15 mg/ml and 566.36 µmol·ml-1·min-1 under optimal conditions. Its activity towards starch was enhanced by 63% in the presence of 1 mM Ca2+, indicating that AmyFlA was a Ca2+-dependent amylase. Compared to the reported maltogenic amylases, AmyFlA produced a lower variety of intermediate oligosaccharides at the start of the reaction so that the product mixture contained a higher proportion of maltose. These results indicate that AmyFlA may be potential application value in the production of high-maltose syrup.

A Novel Subfamily GH13_46 of the α-Amylase Family GH13 Represented by the Cyclomaltodextrinase from Flavobacterium sp. No. 92

In the CAZy database, the α-amylase family GH13 has already been divided into 45 subfamilies, with additional subfamilies still emerging. The presented in silico study was undertaken in an effort to propose a novel GH13 subfamily represented by the experimentally characterized cyclomaltodxtrinase from Flavobacterium sp. No. 92. Although most cyclomaltodextrinases have been classified in the subfamily GH13_20. This one has not been assigned any GH13 subfamily as yet. It possesses a non-specified immunoglobulin-like domain at its N-terminus mimicking a starch-binding domain (SBD) and the segment MPDLN in its fifth conserved sequence region (CSR) typical, however, for the subfamily GH13_36. The searches through sequence databases resulted in collecting a group of 108 homologs forming a convincing cluster in the evolutionary tree, well separated from all remaining GH13 subfamilies. The members of the newly proposed subfamily share a few exclusive sequence features, such as the "aromatic" end of the CSR-II consisting of two well-conserved tyrosines with either glycine, serine, or proline in the middle or a glutamic acid succeeding the catalytic proton donor in the CSR-III. Concerning the domain N of the representative cyclomaltodextrinase, docking trials with α-, β- and γ-cyclodextrins have indicated it may represent a new type of SBD. This new GH13 subfamily has been assigned the number GH13_46.

Starch-binding domains as CBM families-history, occurrence, structure, function and evolution

The term "starch-binding domain" (SBD) has been applied to a domain within an amylolytic enzyme that gave the enzyme the ability to bind onto raw, i.e. thermally untreated, granular starch. An SBD is a special case of a carbohydrate-binding domain, which in general, is a structurally and functionally independent protein module exhibiting no enzymatic activity but possessing potential to target the catalytic domain to the carbohydrate substrate to accommodate it and process it at the active site. As so-called families, SBDs together with other carbohydrate-binding modules (CBMs) have become an integral part of the CAZy database (http://www.cazy.org/). The first two well-described SBDs, i.e. the C-terminal Aspergillus-type and the N-terminal Rhizopus-type have been assigned the families CBM20 and CBM21, respectively. Currently, among the 85 established CBM families in CAZy, fifteen can be considered as families having SBD functional characteristics: CBM20, 21, 25, 26, 34, 41, 45, 48, 53, 58, 68, 69, 74, 82 and 83. All known SBDs, with the exception of the extra long CBM74, were recognized as a module consisting of approximately 100 residues, adopting a β-sandwich fold and possessing at least one carbohydrate-binding site. The present review aims to deliver and describe: (i) the SBD identification in different amylolytic and related enzymes (e.g., CAZy GH families) as well as in other relevant enzymes and proteins (e.g., laforin, the β-subunit of AMPK, and others); (ii) information on the position in the polypeptide chain and the number of SBD copies and their CBM family affiliation (if appropriate); (iii) structure/function studies of SBDs with a special focus on solved tertiary structures, in particular, as complexes with α-glucan ligands; and (iv) the evolutionary relationships of SBDs in a tree common to all SBD CBM families (except for the extra long CBM74). Finally, some special cases and novel potential SBDs are also introduced.

Recombinant expression, purification, and characterization of a cyclodextrinase from Massilia timonae

Some microorganisms can produce cyclodextrin glycosyltransferases, which degrades starch by catalyzing cyclization and giving rise to cyclodextrin. Thus, to fully degrade starch, microorganisms can also synthesize cyclodextrinases, which hydrolyze cyclodextrins. In this work, a truncated gene, without the signal peptide coding sequence, encoding a cyclodextrinase from Massilia timonae was PCR amplified, cloned, and expressed in E. coli. The histidine-tagged recombinant enzyme was purified by immobilized metal ion affinity chromatography. The purified protein was found to be a tetramer of about 260 kDa, with monomers of about 65 kDa, as estimated by gel filtration and SDS-PAGE, respectively. The enzyme presented an optimum temperature of 40 °C, optimum pH of 7.0, and remained stable after 30 min of incubation at 45 °C, with a T₅₀ of 48.45 °C. The enzyme showed a higher activity toward β-cyclodextrin compared to that for maltodextrin and starch. K_M for β-cyclodextrin was 2.1 mM, V_max was 0.084 μmol/min, k_cat was 8326 min^-1, and k_cat/K_M was 4.1 × 10⁶ M^-1min^-1. Calcium acted as an activator and SDS, CTAB, several cations, and EDTA acted as strong inhibitors. The purified cyclodextrinase produced glucose and maltose as final products by hydrolysis of β-cyclodextrin, maltotetraose, and maltoheptaose. This novel cyclodextrinase could be a promising alternative for the enzymatic hydrolysis of starch.

Introduction of novel thermostable α-amylases from genus Anoxybacillus and proposing to group the Bacillaceae related α-amylases under five individual GH13 subfamilies

Among the thermophilic Bacillaceae family members, α-amylase production of 15 bacilli from genus Anoxybacillus was investigated, some of which are biotechnologically important. These Anoxybacillus α-amylase genes displayed ≥ 91.0% sequence similarities to Anoxybacillus enzymes (ASKA, ADTA and GSX-BL), but relatively lower similarities to Geobacillus (≤ 69.4% to GTA, Gt-amyII), and Bacillus aquimaris (≤ 61.3% to BaqA) amylases, all formerly proposed only in a Glycoside Hydrolase 13 (GH13) subfamily. The phylogenetic analyses of 63 bacilli-originated protein sequences among 93 α-amylases revealed the overall relationships within Bacillaceae amylolytic enzymes. All bacilli α-amylases formed 5 clades different from 15 predefined GH13 subfamilies. Their phylogenetic findings, taxonomic relationships, temperature requirements, and comparisonal structural analyses (including their CSR-I-VII regions, 12 sugar- and 4 calcium-binding sites, presence or absence of the complete catalytic machinery, and their currently unassigned status in a valid GH13 subfamiliy) revealed that these five GH13 α-amylase clades related to familly share some common characteristics, but also display differentiative features from each other and the preclassified ones. Based on these findings, we proposed to divide Bacillaceae related GH13 subfamilies into 5 individual groups: the novel a2 subfamily clustered around α-amylase B2M1-A (Anoxybacillus sp.), the a1, a3 and a4 subfamilies (including the representatives E184aa-A (Anoxybacillus sp.), ATA (Anoxybacillus tepidamans), and BaqA,) all of which were composed from the division of the previously grouped single subfamily around α-amylase BaqA, and the undefinite subfamily formerly defined as xy including Bacillus megaterium NL3.

A new group of glycoside hydrolase family 13 α-amylases with an aberrant catalytic triad

α-Amylases are glycoside hydrolase enzymes that act on the α(1→4) glycosidic linkages in glycogen, starch, and related α-glucans, and are ubiquitously present in Nature. Most α-amylases have been classified in glycoside hydrolase family 13 with a typical (β/α)₈-barrel containing two aspartic acid and one glutamic acid residue that play an essential role in catalysis. An atypical α-amylase (BmaN1) with only two of the three invariant catalytic residues present was isolated from Bacillus megaterium strain NL3, a bacterial isolate from a sea anemone of Kakaban landlocked marine lake, Derawan Island, Indonesia. In BmaN1 the third residue, the aspartic acid that acts as the transition state stabilizer, was replaced by a histidine. Three-dimensional structure modeling of the BmaN1 amino acid sequence confirmed the aberrant catalytic triad. Glucose and maltose were found as products of the action of the novel α-amylase on soluble starch, demonstrating that it is active in spite of the peculiar catalytic triad. This novel BmaN1 α-amylase is part of a group of α-amylases that all have this atypical catalytic triad, consisting of aspartic acid, glutamic acid and histidine. Phylogenetic analysis showed that this group of α-amylases comprises a new subfamily of the glycoside hydrolase family 13.

The Sus operon: a model system for starch uptake by the human gut Bacteroidetes

Resident bacteria in the densely populated human intestinal tract must efficiently compete for carbohydrate nutrition. The Bacteroidetes, a dominant bacterial phylum in the mammalian gut, encode a plethora of discrete polysaccharide utilization loci (PULs) that are selectively activated to facilitate glycan capture at the cell surface. The most well-studied PUL-encoded glycan-uptake system is the starch utilization system (Sus) of Bacteroides thetaiotaomicron. The Sus includes the requisite proteins for binding and degrading starch at the surface of the cell preceding oligosaccharide transport across the outer membrane for further depolymerization to glucose in the periplasm. All mammalian gut Bacteroidetes possess analogous Sus-like systems that target numerous diverse glycans. In this review, we discuss what is known about the eight Sus proteins of B. thetaiotaomicron that define the Sus-like paradigm of nutrient acquisition that is exclusive to the Gram-negative Bacteroidetes. We emphasize the well-characterized outer membrane proteins SusDEF and the α-amylase SusG, each of which have unique structural features that allow them to interact with starch on the cell surface. Despite the apparent redundancy in starch-binding sites among these proteins, each has a distinct role during starch catabolism. Additionally, we consider what is known about how these proteins dynamically interact and cooperate in the membrane and propose a model for the formation of the Sus outer membrane complex.

Recombinant cyclodextrinase from Thermococcus kodakarensis KOD1: expression, purification, and enzymatic characterization

A gene encoding a cyclodextrinase from Thermococcus kodakarensis KOD1 (CDase-Tk) was identified and characterized. The gene encodes a protein of 656 amino acid residues with a molecular mass of 76.4 kDa harboring four conserved regions found in all members of the α-amylase family. A recombinant form of the enzyme was purified by ion-exchange chromatography, and its catalytic properties were examined. The enzyme was active in a broad range of pH conditions (pHs 4.0–10.0), with an optimal pH of 7.5 and a temperature optimum of 65°C. The purified enzyme preferred to hydrolyze β-cyclodextrin (CD) but not α- or γ-CD, soluble starch, or pullulan. The final product from β-CD was glucose. The V_max and K_m values were 3.13 ± 0.47 U mg⁻¹ and 2.94 ± 0.16 mg mL⁻¹ for β-CD. The unique characteristics of CDase-Tk with a low catalytic temperature and substrate specificity are discussed, and the starch utilization pathway in a broad range of temperatures is also proposed.

Crystal structure of a compact α-amylase from Geobacillus thermoleovorans

A truncated form of an α-amylase, GTA, from thermophilic Geobacillus thermoleovorans CCB_US3_UF5 was biochemically and structurally characterized. The recombinant GTA, which lacked both the N- and C-terminal transmembrane regions, functioned optimally at 70°C and pH 6.0. While enzyme activity was not enhanced by the addition of CaCl2, GTA's thermostability was significantly improved in the presence of CaCl2. The structure, in complex with an acarbose-derived pseudo-hexasaccharide, consists of the typical three domains and binds one Ca(2+) ion. This Ca(2+) ion was strongly bound and not chelated by EDTA. A predicted second Ca(2+)-binding site, however, was disordered. With limited subsites, two novel substrate-binding residues, Y147 and Y182, may help increase substrate affinity. No distinct starch-binding domain is present, although two regions rich in aromatic residues have been observed. GTA, with a smaller domain B and several shorter loops compared to other α-amylases, has one of the most compact α-amylase folds that may contribute greatly to its tight Ca(2+) binding and thermostability.

Tracing the evolution of the α-amylase subfamily GH13_36 covering the amylolytic enzymes intermediate between oligo-1,6-glucosidases and neopullulanases

Among the glycoside hydrolases (GHs) classified within the Carbohydrate-Active enZymes (CAZy) server, the α-amylase family GH13 belongs to the largest GH families. It has been divided into the official 36 subfamilies by the CAZy curators. Originally the subfamilies of oligo-1,6-glucosidase and neopullulanase were defined using the sequence of the fifth conserved sequence region (CSR) as a selection marker. It is localized outside the catalytic α-amylase (β/α)(8)-barrel in the domain B, that is, in a longer loop connecting the strand β3 with the helix α3 of the barrel. It is sequentially positioned 26-28 residues in front of the invariant aspartic acid residue in the β4-strand acting as the GH13 catalytic nucleophile. The CSR V is characteristic as QpDln and MpKln for the former and latter subfamilies, respectively. A group of intermediate sequences possessing the CSR V as a mix of the two above-mentioned subfamilies, that is, MpDln, was also proposed previously. The present bioinformatics analysis was done in an effort to reveal as many as possible GH13 members of this intermediary group, currently classified as the subfamily GH13_36, and to discuss their evolutionary relationships to known GH13 specificities as well as with regard to their taxonomic origin. Using the BLAST tool with the sequence of the α-amylase from Halothermothrix orenii AmyA exhibiting the intermediary features, 152 GH13 enzymes, and hypothetical proteins were retrieved covering defined specificities (GH13 subfamilies 4, 16, 17, 18, 20, 21, 23, 29, 30, 31, 34, and 35) and intermediary enzymes and proteins (GH13_36). In both evolutionary trees-based on the alignment of CSRs and complete sequences-most of the 'intermediary' proteins (i.e., those with MPDLN signature) were positioned in several closely related clusters forming, however, a single GH13_36 large part of the trees. A few novel GH13 subfamilies were proposed as well as the specificity implications were discussed based on the presented in silico analysis. The results may also be helpful in assigning any GH13-like amino acid sequence the subfamily GH13_36 affiliation without additional biochemical characterization.

Structural investigation of the thermostability and product specificity of amylosucrase from the bacterium Deinococcus geothermalis

Amylosucrases are sucrose-utilizing α-transglucosidases that naturally catalyze the synthesis of α-glucans, linked exclusively through α1,4-linkages. Side products and in particular sucrose isomers such as turanose and trehalulose are also produced by these enzymes. Here, we report the first structural and biophysical characterization of the most thermostable amylosucrase identified so far, the amylosucrase from Deinoccocus geothermalis (DgAS). The three-dimensional structure revealed a homodimeric quaternary organization, never reported before for other amylosucrases. A sequence signature of dimerization was identified from the analysis of the dimer interface and sequence alignments. By rigidifying the DgAS structure, the quaternary organization is likely to participate in the enhanced thermal stability of the protein. Amylosucrase specificity with respect to sucrose isomer formation (turanose or trehalulose) was also investigated. We report the first structures of the amylosucrases from Deinococcus geothermalis and Neisseria polysaccharea in complex with turanose. In the amylosucrase from N. polysaccharea (NpAS), key residues were found to force the fructosyl moiety to bind in an open state with the O3' ideally positioned to explain the preferential formation of turanose by NpAS. Such residues are either not present or not similarly placed in DgAS. As a consequence, DgAS binds the furanoid tautomers of fructose through a weak network of interactions to enable turanose formation. Such topology at subsite +1 is likely favoring other possible fructose binding modes in agreement with the higher amount of trehalulose formed by DgAS. Our findings help to understand the inter-relationships between amylosucrase structure, flexibility, function, and stability and provide new insight for amylosucrase design.

Calcium ion-dependent increase in thermostability of dextran glucosidase from Streptococcus mutans

Dextran glucosidase from Streptococcus mutans (SmDG), which belongs to glycoside hydrolase family 13 (GH13), hydrolyzes the non-reducing terminal glucosidic linkage of isomaltooligosaccharides and dextran. Thermal deactivation of SmDG did not follow the single exponential decay but rather the two-step irreversible deactivation model, which involves an active intermediate having 39% specific activity. The presence of a low concentration of CaCl2 increased the thermostability of SmDG, mainly due to a marked reduction in the rate constant of deactivation of the intermediate. The addition of MgCl2 also enhanced thermostability, while KCl and NaCl were not effective. Therefore, divalent cations, particularly Ca2+, were considered to stabilize SmDG. On the other hand, CaCl2 had no significant effect on catalytic reaction. The enhanced stability by Ca2+ was probably related to calcium binding in the β→α loop 1 of the (β/α)(8) barrel of SmDG. Because similar structures and sequences are widespread in GH13, these GH13 enzymes might have been stabilized by calcium ions.

SusG: a unique cell-membrane-associated alpha-amylase from a prominent human gut symbiont targets complex starch molecules

SusG is an alpha-amylase and part of a large protein complex on the outer surface of the bacterial cell and plays a major role in carbohydrate acquisition by the animal gut microbiota. Presented here, the atomic structure of SusG has an unusual extended, bilobed structure composed of amylase at one end and an unprecedented internal carbohydrate-binding motif at the other. Structural studies further demonstrate that the carbohydrate-binding motif binds maltooligosaccharide distal to, and on the opposite side of, the amylase catalytic site. SusG has an additional starch-binding site on the amylase domain immediately adjacent to the active cleft. Mutagenesis analysis demonstrates that these two additional starch-binding sites appear to play a role in catabolism of insoluble starch. However, elimination of these sites has only a limited effect, suggesting that they may have a more important role in product exchange with other Sus components.

Biochemical and structural features of a novel cyclodextrinase from cow rumen metagenome

A novel enzyme, RA.04, belonging to the alpha-amylase family was obtained after expression of metagenomic DNA from rumen fluid (Ferrer et al.: Environ. Microbiol. 2005, 7, 1996-2010). The purified RA.04 has a tetrameric structure (280 kDa) and exhibited maximum activity (5000 U/mg protein) at 70 degrees C and was active within an unusually broad pH range from 5.5 to 9.0. It maintained 80% activity at pH 5.0 and 9.5 and 75 degrees C. The enzyme hydrolyzed alpha-D-(1,4) bonds 13-fold faster than alpha-D-(1,6) bonds to yield maltose and glucose as the main products, and it exhibited transglycosylation activity. Its preferred substrates, in the descending order, were maltooligosaccharides (C3-C7), cyclomaltoheptaose (beta-CD), cyclomaltohexaose (alpha-CD), cyclomaltooctaose (gamma-CD), soluble starch, amylose, pullulan and amylopectin. The biochemical properties and amino acid sequence alignments suggested that this enzyme is a cyclomaltodextrinase. However, despite the similarity in the catalytic module (with Glu359 and Asp331 being the catalytic nucleophile and substrate-binding residues, respectively), the enzyme bears a shorter N-terminal domain that may keep the active site more accessible for both starch and pullulan, compared to the other known CDases. Moreover, RA.04 lacks the well-conserved N-terminal Trp responsible for the substrate preference typical of CDases/MAases/PNases, suggesting a new residue is implicated in the preference for cyclic maltooligosaccharides. This study has demonstrated the usefulness of a metagenomic approach to gain novel debranching enzymes, important for the bread/food industries, from microbial environments with a high rate of plant polymer turnover, exemplified by the cow rumen.

Structure of a complex of Thermoactinomyces vulgaris R-47 α-amylase 2 with maltohexaose demonstrates the important role of aromatic residues at the reducing end of the substrate binding cleft

Thermoactinomyces vulgaris R-47 alpha-amylase 2 (TVAII) can efficiently hydrolyze both starch and cyclomaltooligosaccharides (cyclodextrins). The crystal structure of an inactive mutant TVAII in a complex with maltohexaose was determined at a resolution of 2.1A. TVAII adopts a dimeric structure to form two catalytic sites, where substrates are found to bind. At the catalytic site, there are many hydrogen bonds between the enzyme and substrate at the non-reducing end from the hydrolyzing site, but few hydrogen bonds at the reducing end, where two aromatic residues, Trp356 and Tyr45, make effective interactions with a substrate. Trp356 drastically changes its side-chain conformation to achieve a strong stacking interaction with the substrate, and Tyr45 from another molecule forms a water-mediated hydrogen bond with the substrate. Kinetic analysis of the wild-type and mutant enzymes in which Trp356 and/or Tyr45 were replaced with Ala suggested that Trp356 and Tyr45 are essential to the catalytic reaction of the enzyme, and that the formation of a dimeric structure is indispensable for TVAII to hydrolyze both starch and cyclodextrins.

Complexes of Thermoactinomyces vulgaris R-47 alpha-amylase 1 and pullulan model oligossacharides provide new insight into the mechanism for recognizing substrates with alpha-(1,6) glycosidic linkages

Thermoactinomyces vulgaris R-47 alpha-amylase 1 (TVAI) has unique hydrolyzing activities for pullulan with sequence repeats of alpha-(1,4), alpha-(1,4), and alpha-(1,6) glycosidic linkages, as well as for starch. TVAI mainly hydrolyzes alpha-(1,4) glycosidic linkages to produce a panose, but it also hydrolyzes alpha-(1,6) glycosidic linkages with a lesser efficiency. X-ray structures of three complexes comprising an inactive mutant TVAI (D356N or D356N/E396Q) and a pullulan model oligosaccharide (P2; [Glc-alpha-(1,6)-Glc-alpha-(1,4)-Glc-alpha-(1,4)]2 or P5; [Glc-alpha-(1,6)-Glc-alpha-(1,4)-Glc-alpha-(1,4)]5) were determined. The complex D356N/P2 is a mimic of the enzyme/product complex in the main catalytic reaction of TVAI, and a structural comparison with Aspergillus oryzaealpha-amylase showed that the (-) subsites of TVAI are responsible for recognizing both starch and pullulan. D356N/E396Q/P2 and D356N/E396Q/P5 provided models of the enzyme/substrate complex recognizing the alpha-(1,6) glycosidic linkage at the hydrolyzing site. They showed that only subsites -1 and -2 at the nonreducing end of TVAI are effective in the hydrolysis of alpha-(1,6) glycosidic linkages, leading to weak interactions between substrates and the enzyme. Domain N of TVAI is a starch-binding domain acting as an anchor in the catalytic reaction of the enzyme. In this study, additional substrates were also found to bind to domain N, suggesting that domain N also functions as a pullulan-binding domain.

Complex Structures of Thermoactinomyces vulgaris R-47 α-Amylase 2 with Acarbose and Cyclodextrins Demonstrate the Multiple Substrate Recognition Mechanism

Thermoactinomyces vulgaris R-47 alpha-amylase 2 (TVAII) has the unique ability to hydrolyze cyclodextrins (CDs), with various sized cavities, as well as starch. To understand the relationship between structure and substrate specificity, x-ray structures of a TVAII-acarbose complex and inactive mutant TVAII (D325N/D421N)/alpha-, beta- and gamma-CDs complexes were determined at resolutions of 2.9, 2.9, 2.8, and 3.1 A, respectively. In all complexes, the interactions between ligands and enzymes at subsites -1, -2, and -3 were almost the same, but striking differences in the catalytic site structure were found at subsites +1 and +2, where Trp(356) and Tyr(374) changed the conformation of the side chain depending on the structure and size of the ligands. Trp(356) and Tyr(374) are thought to be responsible for the multiple substrate-recognition mechanism of TVAII, providing the unique substrate specificity. In the beta-CD complex, the beta-CD maintains a regular conical structure, making it difficult for Glu(354) to protonate the O-4 atom at the hydrolyzing site as a previously proposed hydrolyzing mechanism of alpha-amylase. From the x-ray structures, it is suggested that the protonation of the O-4 atom is possibly carried out via a hydrogen atom of the inter-glucose hydrogen bond at the hydrolyzing site.

Complex structures of Thermoactinomyces vulgaris R-47 alpha-amylase 1 with malto-oligosaccharides demonstrate the role of domain N acting as a starch-binding domain

The X-ray structures of complexes of Thermoactinomyces vulgaris R-47 alpha-amylase 1 (TVAI) with an inhibitor acarbose and an inactive mutant TVAI with malto-hexaose and malto-tridecaose have been determined at 2.6, 2.0 and 1.8A resolution, and the structures have been refined to R-factors of 0.185 (R(free)=0.225), 0.184 (0.217) and 0.164 (0.200), respectively, with good chemical geometries. Acarbose binds to the catalytic site of TVAI, and interactions between acarbose and the enzyme are very similar to those found in other structure-solved alpha-amylase/acarbose complexes, supporting the proposed catalytic mechanism. Based on the structure of the TVAI/acarbose complex, the binding mode of pullulan containing alpha-(1,6) glucoside linkages could be deduced. Due to the structural difference caused by the replaced amino acid residue (Gln396 for Glu) in the catalytic site, malto-hexaose and malto-tridecaose partially bind to the catalytic site, giving a mimic of the enzyme/product complex. Besides the catalytic site, four sugar-binding sites on the molecular surface are found in these X-ray structures. Two sugar-binding sites in domain N hold the oligosaccharides with a regular helical structure of amylose, which suggests that the domain N is a starch-binding domain acting as an anchor to starch in the catalytic reaction of the enzyme. An assay of hydrolyzing activity for the raw starches confirmed that TVAI can efficiently hydrolyze raw starch.