Characterization of an endo-processive-type xyloglucanase having a β-1,4-glucan-binding module and an endo-type xyloglucanase from Streptomyces avermitilis

Prospection of Fungal Lignocellulolytic Enzymes Produced from Jatoba (Hymenaea courbaril) and Tamarind (Tamarindus indica) Seeds: Scaling for Bioreactor and Saccharification Profile of Sugarcane Bagasse

The lignocellulosic biomass comprises three main components: cellulose, hemicellulose, and lignin. Degradation and conversion of these three components are attractive to biotechnology. This study aimed to prospect fungal lignocellulolytic enzymes with potential industrial applications, produced through a temporal analysis using Hymenaea courbaril and Tamarindus indica seeds as carbon sources. α-L-arabinofuranosidase, acetyl xylan esterase, endo-1,5-α-L-arabinanase, β-D-galactosidase, β-D-glucosidase, β-glucanase, β-D-xylosidase, cellobiohydrolase, endoglucanase, lichenase, mannanase, polygalacturonase, endo-1,4-β-xylanase, and xyloglucanase activities were determined. The enzymes were produced for eight filamentous fungi: Aspergillus fumigatus, Trametes hirsuta, Lasiodiplodia sp., two strains of Trichoderma longibrachiatum, Neocosmospora perseae, Fusarium sp. and Thermothelomyces thermophilus. The best producers concerning enzymatic activity were T. thermophilus and T. longibrachiatum. The optimal conditions for enzyme production were the media supplemented with tamarind seeds, under agitation, for 72 h. This analysis was essential to demonstrate that cultivation conditions, static and under agitation, exert strong influences on the production of several enzymes produced by different fungi. The kind of sugarcane, pretreatment used, microorganisms, and carbon sources proved limiting sugar profile factors.

Strategic aromatic residues in the catalytic cleft of the xyloglucanase MtXgh74 modifying thermostability, mode of enzyme action, and viscosity reduction ability

The thermostable endo-processive xyloglucanase MtXgh74 from Myceliophthora thermophila was used to study the influence of aromatic amino acids in the catalytic cleft on the mode of action and the ability of enzyme to reduce xyloglucan viscosity. The enzyme derivative Mut I with mutations W64A/W67A in the "negative" subsites of the catalytic cleft resulted in a 5.5-fold increase of the K_m value. Mut I produced oligosaccharides of various lengths in addition to xyloglucan building blocks. The W320A/W321A substitutions in the "positive" subsites of the mutated enzyme Mut II catalytic cleft increased the K_m value 54-fold and resulted in an endo-dissociative mode of action. The ability of Mut II to reduce the viscosity of xyloglucan at 50 °C was much better than that of other MtXgh74 variants. Besides, Mut II efficiently reduced viscosity of a natural substrate, the pulp of xyloglucan-containing tamarind seed flour. The K_m, V_max, and k_cat values and viscosity reduction ability of the enzyme derivative Mut III (W320A/W321A/G446Y) returned to levels close to that of MtXgh74. The pattern of xyloglucan hydrolysis by Mut III was typical for endo-processive xyloglucanases. The thermostability of Mut I and Mut II at 60 °C decreased significantly compared to the wild type, whereas the thermostability of Mut III at 60 °C restored almost to the MtXgh74-wt value. All mutants lost the ability to cleave the backbone of xyloglucan building blocks which was a characteristic of MtXgh74. Instead they acquired a low branch removing activity. Molecular dynamics simulations revealed the role of mutated amino acids in the complex action mechanism of GH74 enzymes. KEY POINTS: • Endo-processive mode of action of the xyloglucanase MtXgh74 was altered by rational design. • The endo-dissociative mutant Mut II (W320A/W321A) efficiently reduced XyG viscosity. • The substitutions W320A/W321A/G446Y in Mut III recovered the endo-processive mode. • Mut II can be used to reduce the viscosity of biomass slurries containing tamarind seed flour.

Configuration of active site segments in lytic polysaccharide monooxygenases steers oxidative xyloglucan degradation

BACKGROUND

Lytic polysaccharide monooxygenases (LPMOs) are powerful enzymes that oxidatively cleave plant cell wall polysaccharides. LPMOs classified as fungal Auxiliary Activities family 9 (AA9) have been mainly studied for their activity towards cellulose; however, various members of this AA9 family have been also shown to oxidatively cleave hemicelluloses, in particularly xyloglucan (XG). So far, it has not been studied in detail how various AA9 LPMOs act in XG degradation, and in particular, how the mode-of-action relates to the structural configuration of these LPMOs.

RESULTS

Two Neurospora crassa (Nc) LPMOs were found to represent different mode-of-action towards XG. Interestingly, the configuration of active site segments of these LPMOs differed as well, with a shorter Segment 1 (^-Seg1) and a longer Segment 2 (⁺Seg2) present in NcLPMO9C and the opposite for NcLPMO9M (⁺Seg1^-Seg2). We confirmed that NcLPMO9C cleaved the non-reducing end of unbranched glucosyl residues within XG via the oxidation of the C4-carbon. In contrast, we found that the oxidative cleavage of the XG backbone by NcLPMO9M occurred next to both unbranched and substituted glucosyl residues. The latter are decorated with xylosyl, xylosyl-galactosyl and xylosyl-galactosyl-fucosyl units. The relationship between active site segments and the mode-of-action of these NcLPMOs was rationalized by a structure-based phylogenetic analysis of fungal AA9 LPMOs. LPMOs with a ^-Seg1⁺Seg2 configuration clustered together and appear to have a similar XG substitution-intolerant cleavage pattern. LPMOs with the ⁺Seg1^-Seg2 configuration also clustered together and are reported to display a XG substitution-tolerant cleavage pattern. A third cluster contained LPMOs with a ^-Seg1^-Seg2 configuration and no oxidative XG activity.

CONCLUSIONS

The detailed characterization of XG degradation products released by LPMOs reveal a correlation between the configuration of active site segments and mode-of-action of LPMOs. In particular, oxidative XG-active LPMOs, which are tolerant and intolerant to XG substitutions are structurally and phylogenetically distinguished from XG-inactive LPMOs. This study contributes to a better understanding of the structure-function relationship of AA9 LPMOs.

Adaptation of Syntenic Xyloglucan Utilization Loci of Human Gut Bacteroidetes to Polysaccharide Side Chain Diversity

Genome sequencing has revealed substantial variation in the predicted abilities of individual species within animal gut microbiota to metabolize the complex carbohydrates comprising dietary fiber. At the same time, a currently limited body of functional studies precludes a richer understanding of how dietary glycan structures affect the gut microbiota composition and community dynamics. Here, using biochemical and biophysical techniques, we identified and characterized differences among recombinant proteins from syntenic xyloglucan utilization loci (XyGUL) of three Bacteroides and one Dysgonomonas species from the human gut, which drive substrate specificity and access to distinct polysaccharide side chains. Enzymology of four syntenic glycoside hydrolase family 5 subfamily 4 (GH5_4) endo-xyloglucanases revealed surprising differences in xyloglucan (XyG) backbone cleavage specificity, including the ability of some homologs to hydrolyze congested branched positions. Further, differences in the complement of GH43 alpha-l-arabinofuranosidases and GH95 alpha-l-fucosidases among syntenic XyGUL confer distinct abilities to fully saccharify plant species-specific arabinogalactoxyloglucan and/or fucogalactoxyloglucan. Finally, characterization of highly sequence-divergent cell surface glycan-binding proteins (SGBPs) across syntenic XyGUL revealed a novel group of XyG oligosaccharide-specific SGBPs encoded within select BacteroidesIMPORTANCE The catabolism of complex carbohydrates that otherwise escape the endogenous digestive enzymes of humans and other animals drives the composition and function of the gut microbiota. Thus, detailed molecular characterization of dietary glycan utilization systems is essential both to understand the ecology of these complex communities and to manipulate their compositions, e.g., to benefit human health. Our research reveals new insight into how ubiquitous members of the human gut microbiota have evolved a set of microheterogeneous gene clusters to efficiently respond to the structural variations of plant xyloglucans. The data here will enable refined functional prediction of xyloglucan utilization among diverse environmental taxa in animal guts and beyond.

Substrate specificity, regiospecificity, and processivity in glycoside hydrolase family 74

Glycoside hydrolase family 74 (GH74) is a historically important family of endo-β-glucanases. On the basis of early reports of detectable activity on cellulose and soluble cellulose derivatives, GH74 was originally considered to be a "cellulase" family, although more recent studies have generally indicated a high specificity toward the ubiquitous plant cell wall matrix glycan xyloglucan. Previous studies have indicated that GH74 xyloglucanases differ in backbone cleavage regiospecificities and can adopt three distinct hydrolytic modes of action: exo, endo-dissociative, and endo-processive. To improve functional predictions within GH74, here we coupled in-depth biochemical characterization of 17 recombinant proteins with structural biology-based investigations in the context of a comprehensive molecular phylogeny, including all previously characterized family members. Elucidation of four new GH74 tertiary structures, as well as one distantly related dual seven-bladed β-propeller protein from a marine bacterium, highlighted key structure-function relationships along protein evolutionary trajectories. We could define five phylogenetic groups, which delineated the mode of action and the regiospecificity of GH74 members. At the extremes, a major group of enzymes diverged to hydrolyze the backbone of xyloglucan nonspecifically with a dissociative mode of action and relaxed backbone regiospecificity. In contrast, a sister group of GH74 enzymes has evolved a large hydrophobic platform comprising 10 subsites, which facilitates processivity. Overall, the findings of our study refine our understanding of catalysis in GH74, providing a framework for future experimentation as well as for bioinformatics predictions of sequences emerging from (meta)genomic studies.

Structural enzymology reveals the molecular basis of substrate regiospecificity and processivity of an exemplar bacterial glycoside hydrolase family 74 endo-xyloglucanase

Paenibacillus odorifer produces a single multimodular enzyme containing a glycoside hydrolase (GH) family 74 module (AIQ73809). Recombinant production and characterization of the GH74 module (PoGH74_cat) revealed a highly specific, processive endo-xyloglucanase that can hydrolyze the polysaccharide backbone at both branched and unbranched positions. X-ray crystal structures obtained for the free enzyme and oligosaccharide complexes evidenced an extensive hydrophobic binding platform - the first in GH74 extending from subsites -4 to +6 - and unique mobile active-site loops. Site-directed mutagenesis revealed that glycine-476 was uniquely responsible for the promiscuous backbone-cleaving activity of PoGH74_cat; replacement with tyrosine, which is conserved in many GH74 members, resulted in exclusive hydrolysis at unbranched glucose units. Likewise, systematic replacement of the hydrophobic platform residues constituting the positive subsites indicated their relative contributions to the processive mode of action. Specifically, W347 (+3 subsite) and W348 (+5 subsite) are essential for processivity, while W406 (+2 subsite) and Y372 (+6 subsite) are not strictly essential, but aid processivity.

Purification, Cloning, Functional Expression, Structure, and Characterization of a Thermostable β-Mannanase from Talaromyces trachyspermus B168 and Its Efficiency in Production of Mannooligosaccharides from Coffee Wastes

Highly thermostable β-mannanase, belonging to glycoside hydrolase family 5 subfamily 7, was purified from the culture supernatant of Talaromyces trachyspermus B168 and the cDNA of its transcript was cloned. The recombinant enzyme showed maximal activity at pH 4.5 and 85 °C. It retained more than 90 % of its activity below 60 °C. Obtaining the crystal structure of the enzyme helped us to understand the mechanism of its thermostability. An antiparallel β-sheet, salt-bridges, hydrophobic packing, proline residues in the loops, and loop shortening are considered to be related to the thermostability of the enzyme. The enzyme hydrolyzed mannans such as locust bean gum, carob galactomannan, guar gum, konjac glucomannan, and ivory nut mannan. It hydrolyzed 50.7 % of the total mannans from coffee waste, producing mannooligosaccharides. The enzyme has the highest optimum temperature among the known fungal β-mannanases and has potential for use in industrial applications.

GH30 Glucuronoxylan-Specific Xylanase from Streptomyces turgidiscabies C56

Endoxylanases are important enzymes in bioenergy research because they specifically hydrolyze xylan, the predominant polysaccharide in the hemicellulose fraction of lignocellulosic biomass. For effective biomass utilization, it is important to understand the mechanism of substrate recognition by these enzymes. Recent studies have shown that the substrate specificities of bacterial and fungal endoxylanases classified into glycoside hydrolase family 30 (GH30) were quite different. While the functional differences have been described, the mechanism of substrate recognition is still unknown. Therefore, a gene encoding a putative GH30 endoxylanase was cloned from Streptomyces turgidiscabies C56, and the recombinant enzyme was purified and characterized. GH30 glucuronoxylan-specific xylanase A of Streptomyces turgidiscabies (StXyn30A) showed hydrolytic activity with xylans containing both glucuronic acid and the more common 4-O-methyl-glucuronic acid side-chain substitutions but not on linear xylooligosaccharides, suggesting that this enzyme requires the recognition of glucuronic acid side chains for hydrolysis. The StXyn30A limit product structure was analyzed following a secondary β-xylosidase treatment by thin-layer chromatography and mass spectrometry analysis. The hydrolysis products from both glucuronoxylan and 4-O-methylglucuronoxylan by StXyn30A have these main-chain substitutions on the second xylopyranosyl residue from the reducing end. Because previous structural studies of bacterial GH30 enzymes and molecular modeling of StXyn30A suggested that a conserved arginine residue (Arg296) interacts with the glucuronic acid side-chain carboxyl group, we focused on this residue, which is conserved at subsite -2 of bacterial but not fungal GH30 endoxylanases. To help gain an understanding of the mechanism of how StXyn30A recognizes glucuronic acid substitutions, Arg296 mutant enzymes were studied. The glucuronoxylan hydrolytic activities of Arg296 mutants were significantly reduced in comparison to those of the wild-type enzyme. Furthermore, limit products other than aldotriouronic acid were observed for these Arg296 mutants upon secondary β-xylosidase treatment. These results indicate that a disruption of the highly conserved Arg296 interaction leads to a decrease of functional specificity in StXyn30A, as indicated by the detection of alternative hydrolysis products. Our studies allow a better understanding of the mechanism of glucuronoxylan recognition and enzyme specificity by bacterial GH30 endoxylanases and provide further definition of these unique enzymes for their potential application in industry.IMPORTANCE Hemicellulases are important enzymes that hydrolyze hemicellulosic polysaccharides to smaller sugars for eventual microbial assimilation and metabolism. These hemicellulases include endoxylanases that cleave the β-1,4-xylose main chain of xylan, the predominant form of hemicellulose in lignocellulosic biomass. Endoxylanases play an important role in the utilization of plant biomass because in addition to their general utility in xylan degradation, they can also be used to create defined compositions of xylooligosaccharides. For this, it is important to understand the mechanism of substrate recognition. Recent studies have shown that the substrate specificities of bacterial and fungal endoxylanases that are classified into glycoside hydrolase family 30 (GH30) were distinct, but the difference in the mechanisms of substrate recognition is still unknown. We performed characterization and mutagenesis analyses of a new bacterial GH30 endoxylanase for comparison with previously reported fungal GH30 endoxylanases. Our study results in a better understanding of the mechanism of substrate specificity and recognition for bacterial GH30 endoxylanases. The experimental approach and resulting data support the conclusions and provide further definition of the structure and function of GH30 endoxylanases for their application in bioenergy research.

In vitro and in vivo characterization of three Cellvibrio japonicus glycoside hydrolase family 5 members reveals potent xyloglucan backbone-cleaving functions

BACKGROUND

Xyloglucan (XyG) is a ubiquitous and fundamental polysaccharide of plant cell walls. Due to its structural complexity, XyG requires a combination of backbone-cleaving and sidechain-debranching enzymes for complete deconstruction into its component monosaccharides. The soil saprophyte Cellvibrio japonicus has emerged as a genetically tractable model system to study biomass saccharification, in part due to its innate capacity to utilize a wide range of plant polysaccharides for growth. Whereas the downstream debranching enzymes of the xyloglucan utilization system of C. japonicus have been functionally characterized, the requisite backbone-cleaving endo-xyloglucanases were unresolved.

RESULTS

Combined bioinformatic and transcriptomic analyses implicated three glycoside hydrolase family 5 subfamily 4 (GH5_4) members, with distinct modular organization, as potential keystone endo-xyloglucanases in C. japonicus. Detailed biochemical and enzymatic characterization of the GH5_4 modules of all three recombinant proteins confirmed particularly high specificities for the XyG polysaccharide versus a panel of other cell wall glycans, including mixed-linkage beta-glucan and cellulose. Moreover, product analysis demonstrated that all three enzymes generated XyG oligosaccharides required for subsequent saccharification by known exo-glycosidases. Crystallographic analysis of GH5D, which was the only GH5_4 member specifically and highly upregulated during growth on XyG, in free, product-complex, and active-site affinity-labelled forms revealed the molecular basis for the exquisite XyG specificity among these GH5_4 enzymes. Strikingly, exhaustive reverse-genetic analysis of all three GH5_4 members and a previously biochemically characterized GH74 member failed to reveal a growth defect, thereby indicating functional compensation in vivo, both among members of this cohort and by other, yet unidentified, xyloglucanases in C. japonicus. Our systems-based analysis indicates distinct substrate-sensing (GH74, GH5E, GH5F) and attack-mounting (GH5D) functions for the endo-xyloglucanases characterized here.

CONCLUSIONS

Through a multi-faceted, molecular systems-based approach, this study provides a new insight into the saccharification pathway of xyloglucan utilization system of C. japonicus. The detailed structural-functional characterization of three distinct GH5_4 endo-xyloglucanases will inform future bioinformatic predictions across species, and provides new CAZymes with defined specificity that may be harnessed in industrial and other biotechnological applications.

Thermostable multifunctional GH74 xyloglucanase from Myceliophthora thermophila: high-level expression in Pichia pastoris and characterization of the recombinant protein

A xyloglucanase of the GH74 family was identified in the thermophilic fungus strain Myceliophthora thermophila VKPM F-244, and its gene sequence was optimized for cloning and expression in Pichia pastoris. The recombinant xyloglucanase MtXgh74 exhibited the highest activity toward tamarind seed xyloglucan with a K _M value of 0.51 ± 0.06 mg/mL. The activities on barley β-glucan and carboxymethylcellulose were about 4 and 2%, respectively, compared to xyloglucan. Maximum xyloglucanase activity was observed at 70-75 °C and pH 6.5. After pre-incubation at 50 °C, pH 6.0 for 3 h, the enzyme retained 100% of its activity. The half-life of MtXgh74 at 60 °C, pH 6.0 was 40 min. In P. pastoris, MtXgh74 was produced in glycosylated form. The enzyme production in a 1 L bioreactor resulted in a yield of 118 U/mL or 5.3 g/L after 51 h fermentation. Kinetic studies of the hydrolysis product formation suggest that MtXgh74 has an endo-processive mode of action. The final products were the standard xyloglucan building blocks XXXG, XXLG, XLXG, and XLLG. Additionally, MtXgh74 hydrolyzed various linkages within the xyloglucan building blocks XXXG, XXLG, and XLXG (except XLLG) producing diverse low molecular weight oligosaccharides which may be identified by MALDI-TOF as XG, XX, XXG/GXX/XGX, XXX, LG, LX/XL, XLX/XXL, LLG, GXXXG, GXLLG, XLLGX. The unique combination of different activities within one enzyme along with its high thermostability and specificity toward xyloglucan makes MtXgh74 a promising candidate enzyme for industrial applications.

Xyloglucan breakdown by endo-xyloglucanase family 74 from Aspergillus fumigatus

Xyloglucan is the most abundant hemicellulose in primary walls of spermatophytes except for grasses. Xyloglucan-degrading enzymes are important in lignocellulosic biomass hydrolysis because they remove xyloglucan, which is abundant in monocot-derived biomass. Fungal genomes encode numerous xyloglucanase genes, belonging to at least six glycoside hydrolase (GH) families. GH74 endo-xyloglucanases cleave xyloglucan backbones with unsubstituted glucose at the -1 subsite or prefer xylosyl-substituted residues in the -1 subsite. In this work, 137 GH74-related genes were detected by examining 293 Eurotiomycete genomes and Ascomycete fungi contained one or no GH74 xyloglucanase gene per genome. Another interesting feature is that the triad of tryptophan residues along the catalytic cleft was found to be widely conserved among Ascomycetes. The GH74 from Aspergillus fumigatus (AfXEG74) was chosen as an example to conduct comprehensive biochemical studies to determine the catalytic mechanism. AfXEG74 has no CBM and cleaves the xyloglucan backbone between the unsubstituted glucose and xylose-substituted glucose at specific positions, along the XX motif when linked to regions deprived of galactosyl branches. It resembles an endo-processive activity, which after initial random hydrolysis releases xyloglucan-oligosaccharides as major reaction products. This work provides insights on phylogenetic diversity and catalytic mechanism of GH74 xyloglucanases from Ascomycete fungi.

Recent structural insights into the enzymology of the ubiquitous plant cell wall glycan xyloglucan

The xyloglucans (XyGs) constitute a family of highly decorated β(1→4)-glucans whose members are widespread and abundant across the plant kingdom. As such, XyGs constitute a significant reserve of metabolically accessible monosaccharides for diverse phytopathogenic, saprophytic, and gut symbiotic micro-organisms. To overcome the intrinsic stability of the diverse glycosidic bonds in XyGs, bacteria and fungi have evolved extensive repertoires of xyloglucan-active enzymes from manifold families, whose exquisitely adapted tertiary structures are recently coming to light.

Screening, identification, and characterization of α-xylosidase from a soil metagenome

A novel α-xylosidase, MeXyl31, was isolated and characterized from a soil metagenomic library. The amino acid sequence of MeXyl31 showed a slight homology with other characterized α-xylosidases. The optimal pH and temperature of recombinant MeXyl31 were pH 5.5 and 45°C, respectively. Recombinant MeXyl31 had a higher α-xylosidase activity toward pNP α-d-xylopyranoside than pNP α-d-glucopyranoside, isoprimeverose, and other xyloglucan oligosaccharides. The kcat/Km value toward pNP α-d-xylopyranoside was about 750-fold higher than that of isoprimeverose. MeXyl31 activity was strongly inactivated in the presence of zinc and copper ions. MeXyl31 is the first α-xylosidase isolated from the metagenome and, relative to other xyloglucan oligosaccharides, shows higher activity toward pNP α-d-xylopyranoside.

Functional and structural characterization of a potent GH74 endo-xyloglucanase from the soil saprophyte Cellvibrio japonicus unravels the first step of xyloglucan degradation

The heteropolysaccharide xyloglucan (XyG) comprises up to one-quarter of the total carbohydrate content of terrestrial plant cell walls and, as such, represents a significant reservoir in the global carbon cycle. The complex composition of XyG requires a consortium of backbone-cleaving endo-xyloglucanases and side-chain cleaving exo-glycosidases for complete saccharification. The biochemical basis for XyG utilization by the model Gram-negative soil saprophytic bacterium Cellvibrio japonicus is incompletely understood, despite the recent characterization of associated side-chain cleaving exo-glycosidases. We present a detailed functional and structural characterization of a multimodular enzyme encoded by gene locus CJA_2477. The CJA_2477 gene product comprises an N-terminal glycoside hydrolase family 74 (GH74) endo-xyloglucanase module in train with two carbohydrate-binding modules (CBMs) from families 10 and 2 (CBM10 and CBM2). The GH74 catalytic domain generates Glc4 -based xylogluco-oligosaccharide (XyGO) substrates for downstream enzymes through an endo-dissociative mode of action. X-ray crystallography of the GH74 module, alone and in complex with XyGO products spanning the entire active site, revealed a broad substrate-binding cleft specifically adapted to XyG recognition, which is composed of two seven-bladed propeller domains characteristic of the GH74 family. The appended CBM10 and CBM2 members notably did not bind XyG, nor other soluble polysaccharides, and instead were specific cellulose-binding modules. Taken together, these data shed light on the first step of xyloglucan utilization by C. japonicus and expand the repertoire of GHs and CBMs for selective biomass analysis and utilization.

DATABASE

Structural data have been deposited in the RCSB protein database under the Protein Data Bank codes: 5FKR, 5FKS, 5FKT and 5FKQ.

Structural basis for xyloglucan specificity and α-d-Xylp(1 → 6)-D-Glcp recognition at the -1 subsite within the GH5 family

GH5 is one of the largest glycoside hydrolase families, comprising at least 20 distinct activities within a common structural scaffold. However, the molecular basis for the functional differentiation among GH5 members is still not fully understood, principally for xyloglucan specificity. In this work, we elucidated the crystal structures of two novel GH5 xyloglucanases (XEGs) retrieved from a rumen microflora metagenomic library, in the native state and in complex with xyloglucan-derived oligosaccharides. These results provided insights into the structural determinants that differentiate GH5 XEGs from parental cellulases and a new mode of action within the GH5 family related to structural adaptations in the -1 subsite. The oligosaccharide found in the XEG5A complex, permitted the mapping, for the first time, of the positive subsites of a GH5 XEG, revealing the importance of the pocket-like topology of the +1 subsite in conferring the ability of some GH5 enzymes to attack xyloglucan. Complementarily, the XEG5B complex covered the negative subsites, completing the subsite mapping of GH5 XEGs at high resolution. Interestingly, XEG5B is, to date, the only GH5 member able to cleave XXXG into XX and XG, and in the light of these results, we propose that a modification in the -1 subsite enables the accommodation of a xylosyl side chain at this position. The stereochemical compatibility of the -1 subsite with a xylosyl moiety was also reported for other structurally nonrelated XEGs belonging to the GH74 family, indicating it to be an essential attribute for this mode of action.

Characterisation of a novel endo-xyloglucanase (XcXGHA) from Xanthomonas that accommodates a xylosyl-substituted glucose at subsite -1

A xyloglucan-specific endo-1,4β-glucanase (XcXGHA) from Xanthomonas citri pv. mangiferaeindicae has been cloned, expressed in Escherichia coli, purified and characterised. The XcXGHA enzyme belongs to CAZy family GH74 and has catalytic site residues conserved with other xyloglucanases in this family. At its optimal reaction conditions, pH 7.0 and 40 °C, the enzyme has a k cat/K M value of 2.2 × 10(7) min(-1) M(-1) on a tamarind seed xyloglucan substrate. XcXGHA is relatively stable within a broad pH range (pH 4-9) and up to 50 °C (t 1/2, 50 °C of 74 min). XcXGHA is proven to be xyloglucan-specific, and a glycan microarray study verifies that XcXGHA catalyses cleavage of xyloglucan extracted from both monocot and dicot plant species. The enzyme catalyses hydrolysis of tamarind xyloglucan in a unique way by cleaving XXXG into XX and XG (X is xylosyl-substituted glucose; G is unsubstituted glucose), is able to degrade more complex xyloglucans and notably is able to cleave near more substituted xyloglucan motifs such as L [i.e. α-L-Fucp-(1 → 2)-β-D-Galp-(1 → 2)-α-D-Xylp-(1 → 6)-β-D-Glcp]. LC-MS/MS analysis of product profiles of tamarind xyloglucan which had been catalytically degraded by XcXGHA revealed that XcXGHA has specificity for X in subsite -1. The 3D model suggests that XcXGHA consists of two seven-bladed β-propeller domains with the catalytic center formed by the interface of these two domains, which is conserved in xyloglucanases in the GH74 family. However, the XcXGHA has two amino acids (D264 and R472) that differ from the conserved residues of other GH74 xyloglucanases. These two amino acids were predicted to be located on the opposite side of the active site pocket, facing each other and forming a closing surface above the active site pocket. These two amino acids may contribute to the unique substrate specificity of the XcXGHA enzyme.

Key amino acid residues for the endo-processive activity of GH74 xyloglucanase

Unlike endo-dissociative-xyloglucanases, Paenibacillus XEG74 is an endo-processive xyloglucanase that contains four unique tryptophan residues in the negative subsites (W61 and W64) and the positive subsites (W318 and W319), as indicated by three-dimensional homology modelling. Selective replacement of the positive subsite residues with alanine mutations reduced the degree of processive activity and resulted in the more endo-dissociative-activity. The results showed that W318 and W319, which are found in the positive subsites, are essential for processive degradation and are responsible for maintaining binding interactions with xyloglucan polysaccharide through a stacking effect.

Crystallization and preliminary X-ray diffraction analysis of a new xyloglucanase from Xanthomonas campestris pv. campestris

Xyloglucanases (Xghs) are important enzymes involved in xyloglucan modification and degradation. Xanthomonas campestris pv. campestris (Xcc) is a phytopathogenic bacterium which produces a large number of glycosyl hydrolases (GH), but has only one family 74 GH (Xcc-Xgh). This enzyme was overexpressed in Escherichia coli, purified and crystallized. Diffraction data sets were collected for the native enzyme and its complex with glucose to maximum resolutions of 2.0 and 2.1 Å, respectively. The data were indexed in a hexagonal crystal system with unit-cell parameters a = b = 153.4, c = 84.9 Å. As indicated by molecular-replacement solution, the crystals belonged to space group P6(1).