In silico identification of catalytic residues and domain fold of the family GH119 sharing the catalytic machinery with the α-amylase family GH57

Dividing the α-amylase family GH57 of starch hydrolases and related enzymes into subfamilies using evolutionary, clustering and functional criteria

In the CAZy classification, four glycoside hydrolase families - GH13, GH57, GH119 and GH126 - have been designated as α-amylase families. The present study applied to a dataset of over 5000 family GH57 sequences, delivers a functionally balanced subdivision of the family GH57 into ten subfamilies. Eight of these subfamilies bear a specific enzyme activity: GH57_1, 4-α-glucanotransferases; GH57_2, amylopullulanases; GH57_3, α-amylases; GH57_4, α-galactosidases; GH57_5, α-glucan-branching enzymes; GH57_6, non-specified amylases; GH57_7, amylopullulanases-cyclomaltodextrinases; and GH57_8, maltogenic amylases. Two subfamilies, GH57_9 and GH57_10, not including any characterized members so far despite their conserved catalytic machinery, will deserve the community attention. Each subfamily is highlighted by its sequence fingerprints, through the logo of the five GH57 conserved sequence regions. The structural features are also compared with regard to domains complementing the catalytic module composed by the (β/α)7-barrel and the succeeding α-helical bundle. Characterized members in each subfamily display a strong agreement in their functional profile, indicating that the here proposed GH57 subfamily annotation results in functionally meaningful subsets. Moreover, several small groups of sequences still lacking sufficient sequence diversity and biochemical characterization did not integrate any of the created GH57 subfamilies so far; nevertheless, they could complete the overall GH57 subfamily picture in future.

Biochemical exploration of family GH119 reveals a single α-amylase specificity and confirms shared catalytic machinery with GH57 enzymes

While hundreds of starch- and glycogen-degrading enzymes have been characterized experimentally in historical families such as GH13, GH14, GH15, GH57 and GH126 of the CAZy database (www.cazy.org), the α-amylase from Bacillus circulans is the only enzyme that has been characterized in family GH119. Since glycosidase families have been shown to often group enzymes with different substrates or products, a single characterized enzyme in a family is insufficient to extrapolate enzyme function based solely on sequence similarity. Here we report the rational exploration of family GH119 through the biochemical characterization of five GH119 members. All enzymes shared single α-amylase specificity but display distinct product profile. We also report the first kinetic constants in family GH119 and the first experimental validation of previously predicted catalytic residues in family GH119, confirming that families GH119 and GH57 can be grouped in the novel clan GH-T of the CAZy database.

Functional Characterization of Recombinant Raw Starch Degrading α-Amylase from Roseateles terrae HL11 and Its Application on Cassava Pulp Saccharification

Exploring new raw starch-hydrolyzing α-amylases and understanding their biochemical characteristics are important for the utilization of starch-rich materials in bio-industry. In this work, the biochemical characteristics of a novel raw starch-degrading α-amylase (HL11 Amy) from Roseateles terrae HL11 was firstly reported. Evolutionary analysis revealed that HL11Amy was classified into glycoside hydrolase family 13 subfamily 32 (GH13_32). It contains four protein domains consisting of domain A, domain B, domain C and carbohydrate-binding module 20 (CMB20). The enzyme optimally worked at 50 °C, pH 4.0 with a specific activity of 6270 U/mg protein and 1030 raw starch-degrading (RSD) U/mg protein against soluble starch. Remarkably, HL11Amy exhibited activity toward both raw and gelatinized forms of various substrates, with the highest catalytic efficiency (kcat/Km) on starch from rice, followed by potato and cassava, respectively. HL11Amy effectively hydrolyzed cassava pulp (CP) hydrolysis, with a reducing sugar yield of 736 and 183 mg/g starch from gelatinized and raw CP, equivalent to 72% and 18% conversion based on starch content in the substrate, respectively. These demonstrated that HL11Amy represents a promising raw starch-degrading enzyme with potential applications in starch modification and cassava pulp saccharification.

In Silico Analysis of Fungal and Chloride-Dependent α-Amylases within the Family GH13 with Identification of Possible Secondary Surface-Binding Sites

This study brings a detailed bioinformatics analysis of fungal and chloride-dependent α-amylases from the family GH13. Overall, 268 α-amylase sequences were retrieved from subfamilies GH13_1 (39 sequences), GH13_5 (35 sequences), GH13_15 (28 sequences), GH13_24 (23 sequences), GH13_32 (140 sequences) and GH13_42 (3 sequences). Eight conserved sequence regions (CSRs) characteristic for the family GH13 were identified in all sequences and respective sequence logos were analysed in an effort to identify unique sequence features of each subfamily. The main emphasis was given on the subfamily GH13_32 since it contains both fungal α-amylases and their bacterial chloride-activated counterparts. In addition to in silico analysis focused on eventual ability to bind the chloride anion, the property typical mainly for animal α-amylases from subfamilies GH13_15 and GH13_24, attention has been paid also to the potential presence of the so-called secondary surface-binding sites (SBSs) identified in complexed crystal structures of some particular α-amylases from the studied subfamilies. As template enzymes with already experimentally determined SBSs, the α-amylases from Aspergillus niger (GH13_1), Bacillus halmapalus, Bacillus paralicheniformis and Halothermothrix orenii (all from GH13_5) and Homo sapiens (saliva; GH13_24) were used. Evolutionary relationships between GH13 fungal and chloride-dependent α-amylases were demonstrated by two evolutionary trees—one based on the alignment of the segment of sequences spanning almost the entire catalytic TIM-barrel domain and the other one based on the alignment of eight extracted CSRs. Although both trees demonstrated similar results in terms of a closer evolutionary relatedness of subfamilies GH13_1 with GH13_42 including in a wider sense also the subfamily GH13_5 as well as for subfamilies GH13_32, GH13_15 and GH13_24, some subtle differences in clustering of particular α-amylases may nevertheless be observed.

A putative novel starch-binding domain revealed by in silico analysis of the N-terminal domain in bacterial amylomaltases from the family GH77

The family GH77 contains 4-α-glucanotransferase acting on α-1,4-glucans, known as amylomaltase in prokaryotes and disproportionating enzyme in plants. A group of bacterial GH77 members, represented by amylomaltases from Escherichia coli and Corynebacterium glutamicum, possesses an N-terminal extension that forms a distinct immunoglobulin-like fold domain, of which no function has been identified. Here, in silico analysis of 100 selected sequences of N-terminal domain homologues disclosed several well-conserved residues, among which Tyr108 (E. coli amylomaltase numbering) may be involved in α-glucan binding. These N-terminal domains, therefore, may represent a new type of starch-binding domain and define a new CBM family. This hypothesis is supported by docking of maltooligosaccharides to the N-terminal domain in amylomaltases, representing the four clusters of the phylogenetic tree.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02787-8.

Aspects and Recent Trends in Microbial α-Amylase: a Review

α-Amylases are the oldest and versatile starch hydrolysing enzymes which can replace chemical hydrolysis of starch in industries. It cleaves the α-(1,4)-D-glucosidic linkage of starch and other related polysaccharides to yield simple sugars like glucose, maltose and limit dextrin. α-Amylase covers about 30% shares of the total enzyme market. On account of their superior features, α-amylase is the most widely used among all the existing amylases for hydrolysis of polysaccharides. Endo-acting α-amylase of glycoside hydrolase family 13 is an extensively used biocatalyst and has various biotechnological applications like in starch processing, detergent, textile, paper and pharmaceutical industries. Apart from these, it has some novel applications including polymeric material for drug delivery, bioremediating agent, biodemulsifier and biofilm inhibitor. The present review will accomplish the research gap by providing the unexplored aspects of microbial α-amylase. It will allow the readers to know about the works that have already been done and the latest trends in this field. The manuscript has covered the latest immobilization techniques and the site-directed mutagenesis approaches which are readily being performed to confer the desirable property in wild-type α-amylases. Furthermore, it will state the inadequacies and the numerous obstacles coming in the way of its production during upstream and downstream steps and will also suggest some measures to obtain stable and industrial-grade α-amylase.

Extension of the taxonomic coverage of the family GH126 outside Firmicutes and in silico characterization of its non-catalytic terminal domains

The family GH126 is a family of glycoside hydrolases established in 2011. Officially, in the CAZy database, it counts ~ 1000 sequences originating solely from bacterial phylum Firmicutes. Two members, the proteins CPF_2247 from Clostridium perfringens and PssZ from Listeria monocytogenes have been characterized as a probable α-amylase and an exopolysaccharide-specific glycosidase, respectively; their three-dimensional structures being also solved as possessing catalytic (α/α)₆-barrel fold. Previously, based on a detailed in silico analysis, the seven conserved sequence regions (CSRs) were identified for the family along with elucidating basic evolutionary relationships within the family members. The present study represents a continuation study focusing on two particular aims: (1) to find out whether the taxonomic coverage of the family GH126 might be extended outside the Firmicutes and, if positive, to deliver those out-of-Firmicutes proteins with putting them into the context of the family; and (2) to identify the family members containing the N- and/or C-terminal extensions of their polypeptide chain, additional to the catalytic (α/α)₆-barrel domain, and perform the bioinformatics characterization of the extra domains. The main results could be summarized as follows: (1) 17 bacterial proteins caught by BLAST searches outside Firmicutes (especially from phyla Proteobacteria, Actinobacteria and Bacteroidetes) have been found and convincingly suggested as new family GH126 members; and (2) a thioredoxin-like fold and various leucine-rich repeat motifs identified by Phyre2 structure homology modelling have been recognized as extra domains occurring most frequently in the N-terminal extensions of family GH126 members possessing a modular organization.

A detailed in silico analysis of the amylolytic family GH126 and its possible relatedness to family GH76

The glycoside hydrolase (GH) family 126 was established based on the X-ray structure determination of the amylolytic enzyme CPF_2247 from Clostridium perfringens genome. Its original identification as a putative carbohydrate-active enzyme was based on its low, yet significant sequence identity to members of the family GH8, which are inverting endo-β-1,4-glucanases. As the family GH8 forms the clan GH-M with GH48, the CPF_2247 protein also exhibits similarities with members of the family GH48. The original screening of the CPF_2247 on carbohydrate substrates demonstrated its activity on glycogen and amylose, thus classifying this protein as an "α-amylase". It should be pointed out, however, there are apparent inconsistencies concerning the exact enzyme specificity of the "amylase" CPF_2247, since it exhibits both the endo- and exo-fashion of action. The family GH126 currently counts ~1000 amino acid sequences solely from Bacteria; all belonging to the phylum Firmicutes. The present study delivers the first detailed bioinformatics study of 117 selected amino acid sequences from the family GH126, featuring the insightful sequence-structure comparison with the aim to define seven conserved sequence regions and elucidate the evolutionary relationships within the family. In addition, a comparative structural analysis of the GH126 members with representatives of other GH families adopting the same (α/α)₆-barrel catalytic domain fold indicates the possible sharing a catalytic residue between the families GH126 and GH76.

New groups of protein homologues in the α-amylase family GH57 closely related to α-glucan branching enzymes and 4-α-glucanotransferases

The glycoside hydrolase family GH57 is known as the second α-amylase family. Its main characteristics are as follows: (i) employing the retaining reaction mechanism; (ii) adopting the (β/α)₇-barrel (the incomplete TIM-barrel) with succeeding bundle of α-helices as the catalytic domain; (iii) sharing the five conserved sequence regions (CSRs) exhibiting the sequence fingerprints of the individual enzyme specificities; and (iv) using the catalytic machinery consisting of glutamic acid (the catalytic nucleophile) and aspartic acid (the proton donor) positioned at strands β4 (CSR-3) and β7 (CSR-4) of the (β/α)₇-barrel domain, respectively. Several years ago, a group of hypothetical proteins closely related to the specificity of α-amylase was revealed, the so-called α-amylase-like homologues, the members of which lack either one or even both catalytic residues. The novelty of the present study lies in delivering two additional groups of the "like" proteins that are homologues of α-glucan-branching enzyme (GBE) and 4-α-glucanotransferase (4AGT) specificities. Based on a recently published in silico analysis of more than 1600 family GH57 sequences, 13 GBE-like and 18 4AGT-like proteins from unique sources were collected and analyzed in a detail with respect to their taxonomical origin, sequence and structural features as well as evolutionary relationships. This in silico study could accelerate the efforts leading to experimental revealing the real function of the enzymes-like proteins in the α-amylase family GH57.

A new GH13 subfamily represented by the α-amylase from the halophilic archaeon Haloarcula hispanica

α-Amylase catalyzes the endohydrolysis of α-1,4-glucosidic linkages in starch and related α-glucans. In the CAZy database, most α-amylases have been classified into the family GH13 counting at present more than 80,000 sequences and ~ 30 different enzyme specificities. The family has already been divided into 42 subfamilies, but additional subfamilies are still emerging. The present bioinformatics study was undertaken in an effort to propose a novel GH13 subfamily around the experimentally characterized α-amylase from the halophilic archaeon Haloarcula hispanica, which until now has not been assigned to any GH13 subfamily. The in silico analysis resulted in collecting a convincing group of putative haloarchaeal α-amylase homologues sharing sequence similarities mainly in their conserved sequence regions (CSRs) and forming a cluster in the evolutionary tree, which is well separated from representatives of established GH13 subfamilies. One of the most exclusive sequence features of the novel GH13 subfamily is the tyrosine (Tyr79 in H. hispanica α-amylase numbering) succeeding the glycine at the beginning of the CSR-VI at the β2 strand of the catalytic TIM-barrel. Evolutionarily, the novel GH13 α-amylase subfamily was most closely related to two clusters of GH13 subfamilies with the specificity of α-amylase, i.e. subfamilies GH13_5, 6 and 7 as well as GH13_15, 24, 27 and 28.

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.

In silico analysis of the α-amylase family GH57: eventual subfamilies reflecting enzyme specificities

Glycoside hydrolases (GHs) have been classified in the CAZy database into 153 GH families. Currently, there might be four α-amylase families: the main family GH13, the family GH57 with related GH119 and, eventually, also GH126. The family GH57 was established in 1996 as the second and smaller α-amylase family. In addition to α-amylase, it contains 4-α-glucanotransferase, α-glucan branching enzyme, amylopullulanase, dual-specificity amylopullulanase–cyclomaltodextrinase, non-specified amylase, maltogenic amylase and α-galactosidase. The family GH57 enzymes employ the retaining reaction mechanism, share five typical conserved sequence regions and possess catalytic (β/α)₇-barrel succeeded by a four-helix bundle with the catalytic machinery consisting of catalytic nucleophile and proton donor (glutamic acid and aspartic acid at strands β4 and β7, respectively). The present bioinformatics study delivers a detailed sequence comparison of 1602 family GH57 sequences with the aim to highlight the uniqueness of each enzyme’s specificity and all eventual protein groups. This was achieved by creating the evolutionary tree focused on both the enzyme specificities and taxonomical origin. The substantial increase of numbers of sequences from recent comparisons done more than 5 years ago has allowed to refine the details of the sequence logos for the individual enzyme specificities. The study identifies a new evolutionary distinct group of α-galactosidase-related enzymes with until-now-undefined enzyme specificity but positioned on the evolutionary tree on a branch adjacent to α-galactosidases. The specificity of α-galactosidase is, moreover, the only one of the entire family GH57 for which there is no structural support for the proposal of the proton donor based on sequence analysis. The analysis also suggests a few so-called “like” protein groups related to some family GH57 enzyme specificities but lacking one or both catalytic residues.

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.

Complete Genome of the Starch-Degrading Myxobacteria Sandaracinus amylolyticus DSM 53668T

Myxobacteria are members of δ-proteobacteria and are typified by large genomes, well-coordinated social behavior, gliding motility, and starvation-induced fruiting body formation. Here, we report the 10.33 Mb whole genome of a starch-degrading myxobacterium Sandaracinus amylolyticus DSM 53668^T that encodes 8,962 proteins, 56 tRNA, and two rRNA operons. Phylogenetic analysis, in silico DNA-DNA hybridization and average nucleotide identity reveal its divergence from other myxobacterial species and support its taxonomic characterization into a separate family Sandaracinaceae, within the suborder Sorangiineae. Sequence similarity searches using the Carbohydrate-active enzymes (CAZyme) database help identify the enzyme repertoire of S. amylolyticus involved in starch, agar, chitin, and cellulose degradation. We identified 16 α-amylases and two γ-amylases in the S. amylolyticus genome that likely play a role in starch degradation. While many of the amylases are seen conserved in other δ-proteobacteria, we notice several novel amylases acquired via horizontal transfer from members belonging to phylum Deinococcus-Thermus, Acidobacteria, and Cyanobacteria. No agar degrading enzyme(s) were identified in the S. amylolyticus genome. Interestingly, several putative β-glucosidases and endoglucanases proteins involved in cellulose degradation were identified. However, the absence of cellobiohydrolases/exoglucanases corroborates with the lack of cellulose degradation by this bacteria.

Remarkable evolutionary relatedness among the enzymes and proteins from the α-amylase family

The α-amylase is a ubiquitous starch hydrolase catalyzing the cleavage of the α-1,4-glucosidic bonds in an endo-fashion. Various α-amylases originating from different taxonomic sources may differ from each other significantly in their exact substrate preference and product profile. Moreover, it also seems to be clear that at least two different amino acid sequences utilizing two different catalytic machineries have evolved to execute the same α-amylolytic specificity. The two have been classified in the Cabohydrate-Active enZyme database, the CAZy, in the glycoside hydrolase (GH) families GH13 and GH57. While the former and the larger α-amylase family GH13 evidently forms the clan GH-H with the families GH70 and GH77, the latter and the smaller α-amylase family GH57 has only been predicted to maybe define a future clan with the family GH119. Sequences and several tens of enzyme specificities found throughout all three kingdoms in many taxa provide an interesting material for evolutionarily oriented studies that have demonstrated remarkable observations. This review emphasizes just the three of them: (1) a close relatedness between the plant and archaeal α-amylases from the family GH13; (2) a common ancestry in the family GH13 of animal heavy chains of heteromeric amino acid transporter rBAT and 4F2 with the microbial α-glucosidases; and (3) the unique sequence features in the primary structures of amylomaltases from the genus Borrelia from the family GH77. Although the three examples cannot represent an exhaustive list of exceptional topics worth to be interested in, they may demonstrate the importance these enzymes possess in the overall scientific context.

Aspergillus Oryzae S2 α-Amylase Domain C Involvement in Activity and Specificity: In Vivo Proteolysis, Molecular and Docking Studies

We previously reported that Aspergillus oryzae strain S2 had produced two α-amylase isoforms named AmyA and AmyB. The apparent molecular masses revealed by SDS-PAGE were 50 and 42 kDa, respectively. Yet AmyB has a higher catalytic efficiency. Based on a monitoring study of the α-amylase production in both the presence and absence of different protease inhibitors, a chymotrypsin proteolysis process was detected in vivo generating AmyB. A. oryzae S2 α-amylase gene was amplified, cloned and sequenced. The sequence analysis revealed nine exons, eight introns and an encoding open reading frame of 1500 bp corresponding to AmyA isoform. The amino-acid sequence analysis revealed aY371 potential chymotrypsin cleaving site, likely to be the AmyB C-Terminal end and two other potential sites at Y359, and F379. A zymogram with a high acrylamide concentration was used. It highlighted two other closed apparent molecular mass α-amylases termed AmyB1 and AmyB2 reaching40 kDa and 43 kDa. These isoforms could be possibly generated fromY359, and F379secondary cut, respectively. The molecular modeling study showed that AmyB preserved the (β/α)8 barrel domain and the domain B but lacked the C-terminal domain C. The contact map analysis and the docking studies strongly suggested a higher activity and substrate binding affinity for AmyB than AmyA which was previously experimentally exhibited. This could be explained by the easy catalytic cleft accessibility.

Structural features underlying the selective cleavage of a novel exo-type maltose-forming amylase fromPyrococcussp. ST04

A novel maltose-forming α-amylase (PSMA) was recently found in the hyperthermophilic archaeonPyrococcussp. ST04. This enzyme shows <13% amino-acid sequence identity to other known α-amylases and displays a unique enzymatic property in that it hydrolyzes both α-1,4-glucosidic and α-1,6-glucosidic linkages of substrates, recognizing only maltose units, in an exo-type manner. Here, the crystal structure of PSMA at a resolution of 1.8 Å is reported, showing a tight ring-shaped tetramer with monomers composed of two domains: an N-domain (amino acids 1–341) with a typical GH57 family (β/α)7-barrel fold and a C-domain (amino acids 342–597) composed of α-helical bundles. A small closed cavity observed in proximity to the catalytic residues Glu153 and Asp253 at the domain interface has the appropriate volume and geometry to bind a maltose unit, accounting for the selective exo-type maltose hydrolysis of the enzyme. A narrow gate at the putative subsite +1 formed by residue Phe218 and Phe452 is essential for specific cleavage of glucosidic bonds. The closed cavity at the active site is connected to a short substrate-binding channel that extends to the central hole of the tetramer, exhibiting a geometry that is significantly different from classical maltogenic amylases or β-amylases. The structural features of this novel exo-type maltose-forming α-amylase provide a molecular basis for its unique enzymatic characteristics and for its potential use in industrial applications and protein engineering.

α-Amylase: an enzyme specificity found in various families of glycoside hydrolases

α-Amylase (EC 3.2.1.1) represents the best known amylolytic enzyme. It catalyzes the hydrolysis of α-1,4-glucosidic bonds in starch and related α-glucans. In general, the α-amylase is an enzyme with a broad substrate preference and product specificity. In the sequence-based classification system of all carbohydrate-active enzymes, it is one of the most frequently occurring glycoside hydrolases (GH). α-Amylase is the main representative of family GH13, but it is probably also present in the families GH57 and GH119, and possibly even in GH126. Family GH13, known generally as the main α-amylase family, forms clan GH-H together with families GH70 and GH77 that, however, contain no α-amylase. Within the family GH13, the α-amylase specificity is currently present in several subfamilies, such as GH13_1, 5, 6, 7, 15, 24, 27, 28, 36, 37, and, possibly in a few more that are not yet defined. The α-amylases classified in family GH13 employ a reaction mechanism giving retention of configuration, share 4-7 conserved sequence regions (CSRs) and catalytic machinery, and adopt the (β/α)8-barrel catalytic domain. Although the family GH57 α-amylases also employ the retaining reaction mechanism, they possess their own five CSRs and catalytic machinery, and adopt a (β/α)7-barrel fold. These family GH57 attributes are likely to be characteristic of α-amylases from the family GH119, too. With regard to family GH126, confirmation of the unambiguous presence of the α-amylase specificity may need more biochemical investigation because of an obvious, but unexpected, homology with inverting β-glucan-active hydrolases.

Two potentially novel amylolytic enzyme specificities in the prokaryotic glycoside hydrolase α-amylase family GH57

Glycoside hydrolase (GH) family 57 consists of more than 900 proteins from Archaea (roughly one-quarter) and Bacteria (roughly three-quarters), mostly from thermophiles. Fewer than 20 GH57 members have already been biochemically characterized as real, (almost exclusively) amylolytic enzymes. In addition to a recently described dual-specificity amylopullulanase-cyclomaltodextrinase, five enzyme specificities have been well established in the family--α-amylase, amylopullulanase, branching enzyme, 4-α-glucanotransferase and α-galactosidase--plus a group of the so-called α-amylase-like homologues probably without the enzyme activity. A (β/α)7-barrel succeeded by a bundle of a few α-helices forming the catalytic domain, and five conserved sequence regions (CSRs), are the main characteristics of family GH57. The main goal of the present bioinformatics study was to describe two novel groups within family GH57 that represent potential non-specified amylases (127 sequences mostly from Bacteria) and maltogenic amylases (12 sequences from Archaea). These were collected from sequence databases based on an indication of their biochemical characterization. Although both the non-specified amylases and the maltogenic amylases share the in silico identified catalytic machinery and predicted fold with the experimentally determined GH57 members, the two novel groups may define new GH57 subfamilies. They are distinguishable from the other, previously recognized, subfamilies by specific sequence features present especially in their CSRs (the so-called sequence fingerprints), also reflecting their own evolutionary histories.

Phylogenomic relationships between amylolytic enzymes from 85 strains of fungi

Fungal amylolytic enzymes, including α-amylase, gluocoamylase and α-glucosidase, have been extensively exploited in diverse industrial applications such as high fructose syrup production, paper making, food processing and ethanol production. In this paper, amylolytic genes of 85 strains of fungi from the phyla Ascomycota, Basidiomycota, Chytridiomycota and Zygomycota were annotated on the genomic scale according to the classification of glycoside hydrolase (GH) from the Carbohydrate-Active enZymes (CAZy) Database. Comparisons of gene abundance in the fungi suggested that the repertoire of amylolytic genes adapted to their respective lifestyles. Amylolytic enzymes in family GH13 were divided into four distinct clades identified as heterologous α- amylases, eukaryotic α-amylases, bacterial and fungal α-amylases and GH13 α-glucosidases. Family GH15 had two branches, one for gluocoamylases, and the other with currently unknown function. GH31 α-glucosidases showed diverse branches consisting of neutral α-glucosidases, lysosomal acid α-glucosidases and a new clade phylogenetically related to the bacterial counterparts. Distribution of starch-binding domains in above fungal amylolytic enzymes was related to the enzyme source and phylogeny. Finally, likely scenarios for the evolution of amylolytic enzymes in fungi based on phylogenetic analyses were proposed. Our results provide new insights into evolutionary relationships among subgroups of fungal amylolytic enzymes and fungal evolutionary adaptation to ecological conditions.