Crystal structure of beta-amylase from Bacillus cereus var. mycoides at 2.2 A resolution

Carbohydrate binding modules: Compact yet potent accessories in the specific substrate binding and performance evolution of carbohydrate-active enzymes

Carbohydrate binding modules (CBMs) are independent non-catalytic domains widely found in carbohydrate-active enzymes (CAZymes), and they play an essential role in the substrate binding process of CAZymes by guiding the appended catalytic modules to the target substrates. Owing to their precise recognition and selective affinity for different substrates, CBMs have received increasing research attention over the past few decades. To date, CBMs from different origins have formed a large number of families that show a variety of substrate types, structural features, and ligand recognition mechanisms. Moreover, through the modification of specific sites of CBMs and the fusion of heterologous CBMs with catalytic domains, improved enzymatic properties and catalytic patterns of numerous CAZymes have been achieved. Based on cutting-edge technologies in computational biology, gene editing, and protein engineering, CBMs as auxiliary components have become portable and efficient tools for the evolution and application of CAZymes. With the aim to provide a theoretical reference for the functional research, rational design, and targeted utilization of novel CBMs in the future, we systematically reviewed the function-related characteristics and potentials of CAZyme-derived CBMs in this review, including substrate recognition and binding mechanisms, non-catalytic contributions to enzyme performances, module modifications, and innovative applications in various fields.

CBM20CP, a novel functional protein of starch metabolism in green algae

Ostreococcus tauri is a picoalga that contains a small and compact genome, which resembles that of higher plants in the multiplicity of enzymes involved in starch synthesis (ADP-glucose pyrophosphorylase, ADPGlc PPase; granule bound starch synthase, GBSS; starch synthases, SSI, SSII, SSIII; and starch branching enzyme, SBE, between others), except starch synthase IV (SSIV). Although its genome is fully sequenced, there are still many genes and proteins to which no function was assigned. Here, we identify the OT_ostta06g01880 gene that encodes CBM20CP, a plastidial protein which contains a central carbohydrate binding domain of the CBM20 family, and a coiled coil domain at the C-terminus that lacks catalytic activity. We demonstrate that CBM20CP has the ability to bind starch, amylose and amylopectin with different affinities. Furthermore, this protein interacts with OsttaSSIII-B, increasing its binding to starch granules, its catalytic efficiency and promoting granule growth. The results allow us to postulate a functional role for CBM20CP in starch metabolism in green algae. KEY MESSAGE: CBM20CP, a plastidial protein that has a modular structure but lacks catalytic activity, regulates the synthesis of starch in Ostreococcus tauri.

Expression, biochemical and structural characterization of high-specific-activity β-amylase from Bacillus aryabhattai GEL-09 for application in starch hydrolysis

BACKGROUND

β-amylase (EC 3.2.1.2) is an exo-enzyme that shows high specificity for cleaving the α-1,4-glucosidic linkage of starch from the non-reducing end, thereby liberating maltose. In this study, we heterologously expressed and characterized a novel β-amylase from Bacillus aryabhattai.

RESULTS

The amino acid-sequence alignment showed that the enzyme shared the highest sequence identity with β-amylase from Bacillus flexus (80.73%) followed by Bacillus cereus (71.38%). Structural comparison revealed the existence of an additional starch-binding domain (SBD) at the C-terminus of B. aryabhattai β-amylase, which is notably different from plant β-amylases. The recombinant enzyme purified 4.7-fold to homogeneity, with a molecular weight of ~ 57.6 kDa and maximal activity at pH 6.5 and 50 °C. Notably, the enzyme exhibited the highest specific activity (3798.9 U/mg) among reported mesothermal microbial β-amylases and the highest specificity for soluble starch, followed by corn starch. Kinetic analysis showed that the K_m and k_cat values were 9.9 mg/mL and 116961.1 s^- 1, respectively. The optimal reaction conditions to produce maltose from starch resulted in a maximal yield of 87.0%. Moreover, molecular docking suggested that B. aryabhattai β-amylase could efficiently recognize and hydrolyze maltotetraose substrate.

CONCLUSIONS

These results suggested that B. aryabhattai β-amylase could be a potential candidate for use in the industrial production of maltose from starch.

Carbohydrate Binding Modules: Diversity of Domain Architecture in Amylases and Cellulases From Filamentous Microorganisms

Enzymatic degradation of abundant renewable polysaccharides such as cellulose and starch is a field that has the attention of both the industrial and scientific community. Most of the polysaccharide degrading enzymes are classified into several glycoside hydrolase families. They are often organized in a modular manner which includes a catalytic domain connected to one or more carbohydrate-binding modules. The carbohydrate-binding modules (CBM) have been shown to increase the proximity of the enzyme to its substrate, especially for insoluble substrates. Therefore, these modules are considered to enhance enzymatic hydrolysis. These properties have played an important role in many biotechnological applications with the aim to improve the efficiency of polysaccharide degradation. The domain organization of glycoside hydrolases (GHs) equipped with one or more CBM does vary within organisms. This review comprehensively highlights the presence of CBM as ancillary modules and explores the diversity of GHs carrying one or more of these modules that actively act either on cellulose or starch. Special emphasis is given to the cellulase and amylase distribution within the filamentous microorganisms from the genera of Streptomyces and Aspergillus that are well known to have a great capacity for secreting a wide range of these polysaccharide degrading enzyme. The potential of the CBM and other ancillary domains for the design of improved polysaccharide decomposing enzymes is discussed.

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.

Identification of active site residues of Fenugreek β-amylase: chemical modification and in silico approach

The amino acid sequence of Fenugreek β-amylase is not available in protein data bank. Therefore, an attempt has been made to identify the catalytic amino acid residues of enzyme by employing studies of pH dependence of enzyme catalysis, chemical modification and bioinformatics. Treatment of purified Fenugreek β-amylase with EDAC in presence of glycine methyl ester and sulfhydryl group specific reagents (IAA, NEM and p-CMB), followed a pseudo first-order kinetics and resulted in effective inactivation of enzyme. The reaction with EDAC in presence of NTEE (3-nitro-l-tyrosine ethylester) resulted into modification of two carboxyl groups per molecule of enzyme and presence of one accessible sulfhydryl group at the active site, per molecule of enzyme was ascertained by titration with DTNB. The above results were supported by the prevention of inactivation of enzyme in presence of substrate. Based on MALDI-TOF analysis of purified Fenugreek β-amylase and MASCOT search, β-amylase of Medicago sativa was found to be the best match. To further confirm the amino acid involved in catalysis, homology modelling of β-amylase of M. sativa was performed. The sequence alignment, superimposition of template and target models, along with study of interactions involved in docking of sucrose and maltose at the active site, led to identification of Glu187, Glu381 and Cys344 as active site residues.

Chemical genetics and cereal starch metabolism: structural basis of the non-covalent and covalent inhibition of barley β-amylase

There are major issues regarding the proposed pathway for starch degradation in germinating cereal grain. Given the commercial importance but genetic intractability of the problem, we have embarked on a program of chemical genetics studies to identify and dissect the pathway and regulation of starch degradation in germinating barley grains. As a precursor to in vivo studies, here we report systematic analysis of the reversible and irreversible inhibition of the major β-amylase of the grain endosperm (BMY1). The molecular basis of inhibitor action was defined through high resolution X-ray crystallography studies of unliganded barley β-amylase, as well as its complexes with glycone site binder disaccharide iminosugar G1M, irreversible inhibitors α-epoxypropyl and α-epoxybutyl glucosides, which target the enzyme's catalytic residues, and the aglycone site binders acarbose and α-cyclodextrin.

The carbohydrate-binding module family 20--diversity, structure, and function

Starch-active enzymes often possess starch-binding domains (SBDs) mediating attachment to starch granules and other high molecular weight substrates. SBDs are divided into nine carbohydrate-binding module (CBM) families, and CBM20 is the earliest-assigned and best characterized family. High diversity characterizes CBM20s, which occur in starch-active glycoside hydrolase families 13, 14, 15, and 77, and enzymes involved in starch or glycogen metabolism, exemplified by the starch-phosphorylating enzyme glucan, water dikinase 3 from Arabidopsis thaliana and the mammalian glycogen phosphatases, laforins. The clear evolutionary relatedness of CBM20s to CBM21s, CBM48s and CBM53s suggests a common clan hosting most of the known SBDs. This review surveys the diversity within the CBM20 family, and makes an evolutionary comparison with CBM21s, CBM48s and CBM53s, discussing intrafamily and interfamily relationships. Data on binding to and enzymatic activity towards soluble ligands and starch granules are summarized for wild-type, mutant and chimeric fusion proteins involving CBM20s. Noticeably, whereas CBM20s in amylolytic enzymes confer moderate binding affinities, with dissociation constants in the low micromolar range for the starch mimic beta-cyclodextrin, recent findings indicate that CBM20s in regulatory enzymes have weaker, low millimolar affinities, presumably facilitating dynamic regulation. Structures of CBM20s, including the first example of a full-length glucoamylase featuring both the catalytic domain and the SBD, are summarized, and distinct architectural and functional features of the two SBDs and roles of pivotal amino acids in binding are described. Finally, some applications of SBDs as affinity or immobilization tags and, recently, in biofuel and in planta bioengineering are presented.

Crystal structures of the starch-binding domain from Rhizopus oryzae glucoamylase reveal a polysaccharide-binding path

GA (glucoamylase) hydrolyses starch and polysaccharides to β-D-glucose. RoGA (Rhizopus oryzae GA) consists of two functional domains, an N-terminal SBD (starch-binding domain) and a C-terminal catalytic domain, which are connected by an O-glycosylated linker. In the present study, the crystal structures of the SBD from RoGA (RoGACBM21) and the complexes with β-cyclodextrin (SBD–βCD) and maltoheptaose (SBD–G7) were determined. Two carbohydrate binding sites, I (Trp47) and II (Tyr32), were resolved and their binding was co-operative. Besides the hydrophobic interaction, two unique polyN loops comprising consecutive asparagine residues also participate in the sugar binding. A conformational change in Tyr32 was observed between unliganded and liganded SBDs. To elucidate the mechanism of polysaccharide binding, a number of mutants were constructed and characterized by a quantitative binding isotherm and Scatchard analysis. A possible binding path for long-chain polysaccharides in RoGACBM21 was proposed.

Kinetic and structural analysis of enzyme sliding on a substrate: multiple attack in beta-amylase

Beta-amylase (EC 3.2.1.2) is starch-hydrolyzing exo-type enzyme that can catalyze the successive liberation of beta-maltose from the nonreducing ends of alpha-1,4-linked glucopyranosyl polymers. There is a well-known phenomenon called multiple or repetitive attack where the enzyme releases several maltose molecules in a single enzyme-substrate complex. In order to understand it further, we examined the beta-amylase-catalyzed reaction using maltooligosaccharides. The Monte Carlo method was applied for simulation of the beta-amylase-catalyzed reaction including the multiple attack mechanism. Through site-directed mutagenesis, we have successfully prepared a mutant enzyme which may be simulated as a multiple attack action reduced one with retaining significant hydrolytic activity. From the results of X-ray structure analysis of the mutant enzyme, it was clarified that one carboxyl residue plays a very important role in the multiple attack. The multiple attack action needs the force of enzyme sliding on the substrate. In addition, it is important for the multiple attack that the enzyme and substrate have the characteristics of a stable productive substrate-enzyme complex through a hydrogen bond between the nonreducing end of the substrate and the carboxyl residue of the enzyme.

The evolution of putative starch-binding domains

The present bioinformatics analysis was focused on the starch-binding domains (SBDs) and SBD-like motifs sequentially related to carbohydrate-binding module (CBM) families CBM20 and CBM21. Originally, these SBDs were known from microbial amylases only. At present homologous starch- and glycogen-binding domains (or putative SBD sequences) have been recognised in various plant and animal proteins. The sequence comparison clearly showed that the SBD-like sequences in genethonin-1, starch synthase III and glucan branching enzyme should possess the real SBD function since the two tryptophans (or at least two aromatics) of the typical starch-binding site 1 are conserved in their sequences. The same should apply also for the sequences corresponding with the so-called KIS-domain of plant AKINbetagamma protein that is a homologue of the animal AMP-activated protein kinase (AMPK). The evolutionary tree classified the compared SBDs into three distinct groups: (i) the family CBM20 (the motifs from genethonins, laforins, starch excess 4 protein, beta-subunits of the animal AMPK and all plant and yeast homologues, and eventually from amylopullulanases); (ii) the family CBM21 (the motifs from regulatory subunits of protein phosphatase 1 together with those from starch synthase III); and (iii) the (CBM20+CBM21)-related group (the motifs from the pullulanase subfamily consisting of pullulanase, branching enzyme, isoamylase and maltooligosyl trehalohydrolase).

The family 21 carbohydrate-binding module of glucoamylase from Rhizopus oryzae consists of two sites playing distinct roles in ligand binding

The starch-hydrolysing enzyme GA (glucoamylase) from Rhizopus oryzae is a commonly used glycoside hydrolase in industry. It consists of a C-terminal catalytic domain and an N-terminal starch-binding domain, which belong to the CBM21 (carbohydrate-binding module, family 21). In the present study, a molecular model of CBM21 from R. oryzae GA (RoGACBM21) was constructed according to PSSC (progressive secondary structure correlation), modified structure-based sequence alignment, and site-directed mutagenesis was used to identify and characterize potential ligand-binding sites. Our model suggests that RoGACBM21 contains two ligand-binding sites, with Tyr32 and Tyr67 grouped into site I, and Trp47, Tyr83 and Tyr93 grouped into site II. The involvement of these aromatic residues has been validated using chemical modification, UV difference spectroscopy studies, and both qualitative and quantitative binding assays on a series of RoGACBM21 mutants. Our results further reveal that binding sites I and II play distinct roles in ligand binding, the former not only is involved in binding insoluble starch, but also facilitates the binding of RoGACBM21 to long-chain soluble polysaccharides, whereas the latter serves as the major binding site mediating the binding of both soluble polysaccharide and insoluble ligands. In the present study we have for the first time demonstrated that the key ligand-binding residues of RoGACBM21 can be identified and characterized by a combination of novel bioinformatics methodologies in the absence of resolved three-dimensional structural information.

A new clan of CBM families based on bioinformatics of starch-binding domains from families CBM20 and CBM21

Approximately 10% of amylolytic enzymes are able to bind and degrade raw starch. Usually a distinct domain, the starch-binding domain (SBD), is responsible for this property. These domains have been classified into families of carbohydrate-binding modules (CBM). At present, there are six SBD families: CBM20, CBM21, CBM25, CBM26, CBM34, and CBM41. This work is concentrated on CBM20 and CBM21. The CBM20 module was believed to be located almost exclusively at the C-terminal end of various amylases. The CBM21 module was known as the N-terminally positioned SBD of Rhizopus glucoamylase. Nowadays many nonamylolytic proteins have been recognized as possessing sequence segments that exhibit similarities with the experimentally observed CBM20 and CBM21. These facts have stimulated interest in carrying out a rigorous bioinformatics analysis of the two CBM families. The present analysis showed that the original idea of the CBM20 module being at the C-terminus and the CBM21 module at the N-terminus of a protein should be modified. Although the CBM20 functionally important tryptophans were found to be substituted in several cases, these aromatics and the regions around them belong to the best conserved parts of the CBM20 module. They were therefore used as templates for revealing the corresponding regions in the CBM21 family. Secondary structure prediction together with fold recognition indicated that the CBM21 module structure should be similar to that of CBM20. The evolutionary tree based on a common alignment of sequences of both modules showed that the CBM21 SBDs from alpha-amylases and glucoamylases are the closest relatives to the CBM20 counterparts, with the CBM20 modules from the glycoside hydrolase family GH13 amylopullulanases being possible candidates for the intermediate between the two CBM families.

Engineering of the pH optimum of Bacillus cereus beta-amylase: conversion of the pH optimum from a bacterial type to a higher-plant type

The optimum pH of Bacillus cereus beta-amylase (BCB, pH 6.7) differs from that of soybean beta-amylase (SBA, pH 5.4) due to the substitution of a few amino acid residues near the catalytic base residue (Glu 380 in SBA and Glu 367 in BCB). To explore the mechanism for controlling the optimum pH of beta-amylase, five mutants of BCB (Y164E, Y164F, Y164H, Y164Q, and Y164Q/T47M/Y164E/T328N) were constructed and characterized with respect to enzymatic properties and X-ray structural crystal analysis. The optimum pH of the four single mutants shifted to 4.2-4.8, approximately 2 pH units and approximately 1 pH unit lower than those of BCB and SBA, respectively, and their k(cat) values decreased to 41-3% of that of the wild-type enzyme. The X-ray crystal analysis of the enzyme-maltose complexes showed that Glu 367 of the wild type is surrounded by two water molecules (W1 and W2) that are not found in SBA. W1 is hydrogen-bonded to both side chains of Glu 367 and Tyr 164. The mutation of Tyr 164 to Glu and Phe resulted in the disruption of the hydrogen bond between Tyr 164 Oeta and W1 and the introduction of two additional water molecules near position 164. In contrast, the triple mutant of BCB with a slightly decreased pH optimum at pH 6.0 has no water molecules (W1 and W2) around Glu 367. These results suggested that a water-mediated hydrogen bond network (Glu 367...W1...Tyr 164...Thr 328) is the primary requisite for the increased pH optimum of wild-type BCB. This strategy is completely different from that of SBA, in which a hydrogen bond network (Glu 380...Thr 340...Glu 178) reduces the optimum pH in a hydrophobic environment.

Structural and enzymatic analysis of soybean beta-amylase mutants with increased pH optimum

Comparison of the architecture around the active site of soybean beta-amylase and Bacillus cereus beta-amylase showed that the hydrogen bond networks (Glu380-(Lys295-Met51) and Glu380-Asn340-Glu178) in soybean beta-amylase around the base catalytic residue, Glu380, seem to contribute to the lower pH optimum of soybean beta-amylase. To convert the pH optimum of soybean beta-amylase (pH 5.4) to that of the bacterial type enzyme (pH 6.7), three mutants of soybean beta-amylase, M51T, E178Y, and N340T, were constructed such that the hydrogen bond networks were removed by site-directed mutagenesis. The kinetic analysis showed that the pH optimum of all mutants shifted dramatically to a neutral pH (range, from 5.4 to 6.0-6.6). The Km values of the mutants were almost the same as that of soybean beta-amylase except in the case of M51T, while the Vmax values of all mutants were low compared with that of soybean beta-amylase. The crystal structure analysis of the wild type-maltose and mutant-maltose complexes showed that the direct hydrogen bond between Glu380 and Asn340 was completely disrupted in the mutants M51T, E178Y, and N340T. In the case of M51T, the hydrogen bond between Glu380 and Lys295 was also disrupted. These results indicated that the reduced pKa value of Glu380 is stabilized by the hydrogen bond network and is responsible for the lower pH optimum of soybean beta-amylase compared with that of the bacterial beta-amylase.

Crystal structure of a catalytic site mutant of beta-amylase from Bacillus cereus var. mycoides cocrystallized with maltopentaose

The X-ray crystal structure of a catalytic site mutant of beta-amylase, E172A (Glu172 --> Ala), from Bacillus cereus var. mycoides complexed with a substrate, maltopentaose (G5), and the wild-type enzyme complexed with maltose were determined at 2.1 and 2.0 A resolution, respectively. Clear and continuous density corresponding to G5 was observed in the active site of E172A, and thus, the substrate, G5, was not hydrolyzed. All glucose residues adopted a relaxed (4)C(1) conformation, and the conformation of the maltose unit for Glc2 and Glc3 was much different from those of other maltose units, where each glucose residue of G5 is named Glc1-Glc5 (Glc1 is at the nonreducing end). A water molecule was observed 3.3 A from the C1 atom of Glc2, and 3.0 A apart from the OE1 atom of Glu367 which acts as a general base. In the wild-type enzyme-maltose complex, two maltose molecules bind at subsites -2 and -1 and at subsites +1 and +2 in tandem. The conformation of the maltose molecules was similar to that of the condensation product of soybean beta-amylase, but differed from that of G5 in E172A. When the substrate flips between Glc2 and Glc3, the conformational energy of the maltose unit was calculated to be 20 kcal/mol higher than that of the cis conformation by MM3. We suggest that beta-amylase destabilizes the bond that is to be broken in the ES complex, decreasing the activation energy, DeltaG(++), which is the difference in free energy between this state and the transition state.

Oligosaccharide and sucrose complexes of amylosucrase. Structural implications for the polymerase activity

The glucosyltransferase amylosucrase is structurally quite similar to the hydrolase alpha-amylase. How this switch in functionality is achieved is an important and fundamental question. The inactive E328Q amylosucrase variant has been co-crystallized with maltoheptaose, and the structure was determined by x-ray crystallography to 2.2 A resolution, revealing a maltoheptaose binding site in the B'-domain somewhat distant from the active site. Additional soaking of these crystals with maltoheptaose resulted in replacement of Tris in the active site with maltoheptaose, allowing the mapping of the -1 to +5 binding subsites. Crystals of amylosucrase were soaked with sucrose at different concentrations. The structures at approximately 2.1 A resolution revealed three new binding sites of different affinity. The highest affinity binding site is close to the active site but is not in the previously identified substrate access channel. Allosteric regulation seems necessary to facilitate access from this binding site. The structures show the pivotal role of the B'-domain in the transferase reaction. Based on these observations, an extension of the hydrolase reaction mechanism valid for this enzyme can be proposed. In this mechanism, the glycogen-like polymer is bound in the widest access channel to the active site. The polymer binding introduces structural changes that allow sucrose to migrate from its binding site into the active site and displace the polymer.

Crystal structures of amylosucrase from Neisseria polysaccharea in complex with D-glucose and the active site mutant Glu328Gln in complex with the natural substrate sucrose

The structure of amylosucrase from Neisseria polysaccharea in complex with beta-D-glucose has been determined by X-ray crystallography at a resolution of 1.66 A. Additionally, the structure of the inactive active site mutant Glu328Gln in complex with sucrose has been determined to a resolution of 2.0 A. The D-glucose complex shows two well-defined D-glucose molecules, one that binds very strongly in the bottom of a pocket that contains the proposed catalytic residues (at the subsite -1), in a nonstrained (4)C(1) conformation, and one that binds in the packing interface to a symmetry-related molecule. A third weaker D-glucose-binding site is located at the surface near the active site pocket entrance. The orientation of the D-glucose in the active site emphasizes the Glu328 role as the general acid/base. The binary sucrose complex shows one molecule bound in the active site, where the glucosyl moiety is located at the alpha-amylase -1 position and the fructosyl ring occupies subsite +1. Sucrose effectively blocks the only visible access channel to the active site. From analysis of the complex it appears that sucrose binding is primarily obtained through enzyme interactions with the glucosyl ring and that an important part of the enzyme function is a precise alignment of a lone pair of the linking O1 oxygen for hydrogen bond interaction with Glu328. The sucrose specificity appears to be determined primarily by residues Asp144, Asp394, Arg446, and Arg509. Both Asp394 and Arg446 are located in an insert connecting beta-strand 7 and alpha-helix 7 that is much longer in amylosucrase compared to other enzymes from the alpha-amylase family (family 13 of the glycoside hydrolases).