A family 11 xylanase from Penicillium funiculosum is strongly inhibited by three wheat xylanase inhibitors

Carbohydrate-active enzymes in animal feed

Considering an ever-growing global population, which hit 8 billion people in the fall of 2022, it is essential to find solutions to avoid the competition between human food and animal feed for croplands. Agricultural co-products have become important components of the circular economy with their use in animal feed. Their implementation was made possible by the addition of exogenous enzymes in the diet, especially carbohydrate-active enzymes (CAZymes). In this review, we describe the diversity and versatility of microbial CAZymes targeting non-starch polysaccharides to improve the nutritional potential of diets containing cereals and protein meals. We focused our attention on cellulases, hemicellulases, pectinases which were often found to be crucial in vivo. We also highlight the performance and health benefits brought by the exogenous addition of enzymatic cocktails containing CAZymes in the diets of monogastric animals. Taking the example of the well-studied commercial cocktail Rovabio™, we discuss the evolution, constraints and future challenges faced by feed enzymes suppliers. We hope that this review will promote the use and development of enzyme solutions for industries to sustainably feed humans in the future.

Combined in situ Physical and ex-situ Biochemical Approaches to Investigate in vitro Deconstruction of Destarched Wheat Bran by Enzymes Cocktail Used in Animal Nutrition

Wheat bran is a foodstuff containing more than 40% of non-starch polysaccharides (NSPs) that are hardly digestible by monogastric animals. Therefore, cocktails enriched of hydrolytic enzymes (termed NSPases) are commonly provided as feed additives in animal nutrition. However, how these enzymes cocktails contribute to NSPs deconstruction remains largely unknown. This question was addressed by employing an original methodology that makes use of a multi-instrumented bioreactor that allows to dynamically monitor enzymes in action and to extract in-situ physical and ex-situ biochemical data from this monitoring. We report here that the deconstruction of destarched wheat bran by an industrial enzymes cocktail termed Rovabio® was entailed by two concurrent events: a particles fragmentation that caused in <2 h a 70% drop of the suspension viscosity and a solubilization that released <30 % of the wheat bran NSPs. Upon longer exposure, the fragmentation of particles continued at a very slow rate without any further solubilization. Contrary to this cocktail, xylanase C alone caused a moderate 25% drop of viscosity and a very weak fragmentation. However, the amount of xylose and arabinose from solubilized sugars after 6 h treatment with this enzyme was similar to that obtained after 2 h with Rovabio®. Altogether, this multi-scale analysis supported the synergistic action of enzymes mixture to readily solubilize complex polysaccharides, and revealed that in spite of the richness and diversity of hydrolytic enzymes in the cocktail, the deconstruction of NSPs in wheat bran was largely incomplete.

A sorghum xylanase inhibitor-like protein with highly potent antifungal, antitumor and HIV-1 reverse transcriptase inhibitory activities

•

The antifungal protein purified in this study is a pH stable and thermostable xylanase inhibitor.

•

Sorghum antifungal protein is inhibitory toward various fungal species.

•

The sorghum antifungal protein exerts suppressive action on the proliferating hepatoma (HepG2) cells and breast cancer (MCF7) cells.

•

Sorghum antifungal protein exerts a highly potent inhibitory activity against HIV-1 reverse transcriptase.

A 25-kDa protein, with an N-terminal amino acid sequence homologous to that of xylanase inhibitor and designated as xylanase inbibitor-like protein (XILP) was purified from sorghum seeds. The isolation protocol consisted of affinity chromatography, ion exchange chromatography, and gel filtration. XILP inhibited mycelial growth in various phytopathogenic fungi. The antifungal activity was thermostable and pH-stable. XILP inhibited proliferation of various cancer cell lines but did not do so in human embryonic liver (WRL 68) cells. There was no mitogenic activity toward mouse splenocytes. XILP reduced the activity of HIV-1 reverse transcriptase with an IC₅₀ of 11.1 μM, but lacked inhibitory activity toward HIV-1 integrase and SARS coronavirus proteinase. In conclusion, sorghum XILP is thermostable and pH stable and exhibits potent antifungal, antiproliferative, and HIV-1 reverse transcriptase inhibitory activities.

Functional characterization and synergic action of fungal xylanase and arabinofuranosidase for production of xylooligosaccharides

Plant cell wall degrading enzymes are key technological components in biomass bioconversion platforms for lignocellulosic materials transformation. Cost effective production of enzymes and identification of efficient degradation routes are two economic bottlenecks that currently limit the use of renewable feedstocks through an environmental friendly pathway. The present study describes the hypersecretion of an endo-xylanase (GH11) and an arabinofuranosidase (GH54) by a fungal expression system with potential biotechnological application, along with comprehensive characterization of both enzymes, including spectrometric analysis of thermal denaturation, biochemical characterization and mode of action description. The synergistic effect of these enzymes on natural substrates such as sugarcane bagasse, demonstrated the biotechnological potential of using GH11 and GH54 for production of probiotic xylooligosaccharides from plant biomass. Our findings shed light on enzymatic mechanisms for xylooligosaccharide production, as well as provide basis for further studies for the development of novel enzymatic routes for use in biomass-to-bioethanol applications.

GH11 xylanases: Structure/function/properties relationships and applications

For technical, environmental and economical reasons, industrial demands for process-fitted enzymes have evolved drastically in the last decade. Therefore, continuous efforts are made in order to get insights into enzyme structure/function relationships to create improved biocatalysts. Xylanases are hemicellulolytic enzymes, which are responsible for the degradation of the heteroxylans constituting the lignocellulosic plant cell wall. Due to their variety, xylanases have been classified in glycoside hydrolase families GH5, GH8, GH10, GH11, GH30 and GH43 in the CAZy database. In this review, we focus on GH11 family, which is one of the best characterized GH families with bacterial and fungal members considered as true xylanases compared to the other families because of their high substrate specificity. Based on an exhaustive analysis of the sequences and 3D structures available so far, in relation with biochemical properties, we assess biochemical aspects of GH11 xylanases: structure, catalytic machinery, focus on their "thumb" loop of major importance in catalytic efficiency and substrate selectivity, inhibition, stability to pH and temperature. GH11 xylanases have for a long time been used as biotechnological tools in various industrial applications and represent in addition promising candidates for future other uses.

GH10 xylanase D from Penicillium funiculosum: biochemical studies and xylooligosaccharide production

Background

The filamentous fungus Penicillium funiculosum produces a range of glycoside hydrolases (GH). The XynD gene, encoding the sole P. funiculosum GH10 xylanase described so far, was cloned into the pPICZαA vector and expressed in methylotrophe yeast Pichia pastoris, in order to compare the results obtained with the P. funiculosum GH11 xylanases data.

Results

High level expression of recombinant XynD was obtained with a secretion of around 60 mg.L^-1. The protein was purified to homogeneity using one purification step. The apparent size on SDS-PAGE was around 64 kDa and was 46 kDa by mass spectrometry thus higher than the expected molecular mass of 41 kDa. The recombinant protein was N- and O-glycosylated, as demonstrated using glycoprotein staining and deglycosylation reactions, which explained the discrepancy in molecular mass. Enzyme-catalysed hydrolysis of low viscosity arabinoxylan (LVAX) was maximal at pH 5.0 with Km_(app) and k_cat/Km_(app) of 3.7 ± 0.2 (mg.mL^-1) and 132 (s^-1mg^-1.mL), respectively. The activity of XynD was optimal at 80°C and the recombinant enzyme has shown an interesting high thermal stability at 70°C for at least 180 min without loss of activity. The enzyme had an endo-mode of action on xylan forming mainly xylobiose and short-chain xylooligosaccharides (XOS). The initial rate data from the hydrolysis of short XOS indicated that the catalytic efficiency increased slightly with increasing their chain length with a small difference of the XynD catalytic efficiency against the different XOS.

Conclusion

Because of its attractive properties XynD might be considered for biotechnological applications. Moreover, XOS hydrolysis suggested that XynD possess four catalytic subsites with a high energy of interaction with the substrate and a fifth subsite with a small energy of interaction, according to the GH10 xylanase literature data.

Molecular determinants of substrate and inhibitor specificities of the Penicillium griseofulvum family 11 xylanases

Penicillium griseofulvum possesses two endo-(1,4)-beta-xylanase genes, PgXynA and PgXynB, belonging to family 11 glycoside hydrolases. The enzymes share 69% identity, a similar hydrolysis profile i.e. the predominant production of xylobiose and xylotriose as end products from wheat arabinoxylan and a specificity region of six potential xylose subsites, but differ in terms of catalytic efficiency which can be explained by subtle structural differences in the positioning of xylohexaose in the PgXynB model. Site-directed mutagenesis of the "thumb" region revealed structural basis of PgXynB substrate and inhibitor specificities. We produced variants displaying increased catalytic efficiency towards wheat arabinoxylan and xylo-oligosaccharides and identified specific determinants in PgXynB "thumb" region responsible for resistance to the wheat xylanase inhibitor XIP-I. Based on kinetic analysis and homology modeling, we suggested that Pro130(PgXynB), Lys131(PgXynB) and Lys132(PgXynB) hamper flexibility of the loop forming the "thumb" and interfere by steric hindrance with the inhibitor.

Purification and characterization of a XIP-type endoxylanase inhibitor from Rice (Oryza sativa)

A rice XIP-type inhibitor was purified by affinity chromatography with an immobilized Aspergillus aculeatus family 10 endoxylanase. Rice XIP is a monomeric protein, with a molecular mass of ca. 32 kDa and a pI of ca. 5.6. Its N-terminal amino acid sequence was identical to that of a rice chitinase homologue, demonstrating the difficulty when using sequence information to differentiate between endoxylanase inhibitors and (putative) chitinases in rice. Rice XIP inhibited different endoxylanases to a varying degree. In particular, it most strongly inhibited family 10 endoxylanases from A. niger and A. oryzae, while several family 11 enzymes from Bacillus subtilis, A. niger and Trichoderma sp. were not sensitive to inhibition. The above mentioned A. aculeatus endoxylanase was not inhibited either, although gel permeation chromatography revealed that it complexed rice XIP in a 1:1 molar stoichiometric ratio.

Substrate and product hydrolysis specificity in family 11 glycoside hydrolases: an analysis of Penicillium funiculosum and Penicillium griseofulvum xylanases

Two genes encoding family 11 endo-(1,4)-beta-xylanases from Penicillium griseofulvum (PgXynA) and Penicillium funiculosum (PfXynC) were heterologously expressed in Escherichia coli as glutathione S-transferase fusion proteins, and the recombinant enzymes were purified after affinity chromatography and proteolysis. PgXynA and PfXynC were identical to their native counterparts in terms of molecular mass, pI, N-terminal sequence, optimum pH, and enzymatic activity towards arabinoxylan. Further investigation of the rate and pattern of hydrolysis of PgXynA and PfXynC on wheat soluble arabinoxylan showed the predominant production of xylotriose and xylobiose as end products. The initial rate data from the hydrolysis of short xylo-oligosaccharides indicated that the catalytic efficiency increased with increasing chain length (n) of oligomer up to n = 6, suggesting that the specificity region of both Penicillium xylanases spans about six xylose units. In contrast to PfXynC, PgXynA was found insensitive to the wheat xylanase inhibitor protein XIP-I.

ZDOCK and RDOCK performance in CAPRI rounds 3, 4, and 5

We present an evaluation of the results of our ZDOCK and RDOCK algorithms in Rounds 3, 4, and 5 of the protein docking challenge CAPRI. ZDOCK is a Fast Fourier Transform (FFT)-based, initial-stage rigid-body docking algorithm, and RDOCK is an energy minimization algorithm for refining and reranking ZDOCK results. Of the 9 targets for which we submitted predictions, we attained at least acceptable accuracy for 7, at least medium accuracy for 6, and high accuracy for 3. These results are evidence that ZDOCK in combination with RDOCK is capable of making accurate predictions on a diverse set of protein complexes.

The xylanolytic enzyme system from the genus Penicillium

In nature, there are numerous microorganisms that efficiently degrade xylan, a major component of lignocellulose. In particular, filamentous fungi have demonstrated a great capability for secreting a wide range of xylanases, being the genus Aspergillus and Trichoderma the most extensively studied and reviewed among the xylan-producing fungi. However, an important amount of information about the production and genetics of xylanases from fungi of the genus Penicillium has accumulated in recent years. A great number of Penicillia are active producers of xylanolytic enzymes, and the use of xylanases from these species has acquired growing importance in biotechnological applications. This review summarizes our current knowledge about the properties, genetics, expression and biotechnological potential of xylanases from the genus Penicillium.

Mapping of residues involved in the interaction between the Bacillus subtilis xylanase A and proteinaceous wheat xylanase inhibitors

The Bacillus subtilis xylanase A was subjected to site-directed mutagenesis, aimed at changing the interaction with Triticum aestivum xylanase inhibitor, the only wheat endogenous proteinaceous xylanase inhibitor interacting with this xylanase. The published structure of Bacillus circulans XynA was used to target amino acids surrounding the active site cleft of B.subtilis XynA for mutation. Twenty-two residues were mutated, resulting in 62 different variants. The catalytic activity of active mutants ranged from 563 to 5635 XU/mg and the interaction with T.aestivum xylanase inhibitor showed a similar variation. The results indicate that T.aestivum xylanase inhibitor interacts with several amino acid residues surrounding the active site of the enzyme. Three different amino acid substitutions in one particular residue (D11) completely abolished the interaction between T.aestivum xylanase inhibitor and B.subtilis xylanase A.

Purification and properties of an extracellular endo-1,4-β-xylanase from Penicillium citrinum and characterization of the encoding gene

An extracellular endo-1,4-beta-xylanase was purified from the culture filtrate of a filamentous fungus Penicillium citrinum FERM P-15944 grown on birch-wood xylan. The purified enzyme showed a single band on SDS-PAGE with an apparent M(r) of 20,000 and had an isoelectric point below 3.5. Xylanase activity was optimal at pH 5.0 and 55 degrees C. The genomic DNA and cDNAs encoding this protein were cloned and sequenced. Southern blot analysis indicated that the xylanase gene (xynA) was present as a single copy in the genome. An open reading frame of 657 bp was interrupted by two introns of 65 and 55 bp, and encoded a presumed prepropeptide of 27 amino acids and a mature protein of 190 amino acids. Three distinct transcription start points were observed at positions -20 (A), -31 (A), and -36 (A) from the start codon. The 5'-noncoding region had a putative TATA box at nt -66 (TATAAA). The xynA cDNA was functionally expressed under the control of the alcohol oxidase I gene promoter in the methylotrophic yeast Pichia pastoris. A neighbor-joining tree showed that the P. citrinum enzyme is closely related to several other fungal xylanases belonging to the glycoside hydrolase family 11: Trichoderma reesei XYN2, Aspergillus niger xynNB, Penicillium funiculosum xynC, Penicillium sp. strain 40 xynA, Chaetomium gracile cgxB, and Aspergillus nidulans xlnA and xlnB.

His374 of wheat endoxylanase inhibitor TAXI-I stabilizes complex formation with glycoside hydrolase family 11 endoxylanases

Wheat endoxylanase inhibitor TAXI-I inhibits microbial glycoside hydrolase family 11 endoxylanases. Crystallographic data of an Aspergillus niger endoxylanase-TAXI-I complex showed His374 of TAXI-I to be a key residue in endoxylanase inhibition. Its role in enzyme-inhibitor interaction was further investigated by site-directed mutagenesis of His374 into alanine, glutamine or lysine. Binding kinetics and affinities of the molecular interactions between A. niger, Bacillus subtilis, Trichoderma longibrachiatumendoxylanases and wild-type TAXI-I and TAXI-I His374 mutants were determined by surface plasmon resonance analysis. Enzyme-inhibitor binding was in accordance with a simple 1 : 1 binding model. Association and dissociation rate constants of wild-type TAXI-I towards the endoxylanases were in the range between 1.96 and 36.1 x 10(4)m(-1) x s(-1) and 0.72-3.60 x 10(-4) x s(-1), respectively, resulting in equilibrium dissociation constants in the low nanomolar range. Mutation of TAXI-I His374 to a variable degree reduced the inhibition capacity of the inhibitor mainly due to higher complex dissociation rate constants (three- to 80-fold increase). The association rate constants were affected to a smaller extent (up to eightfold decrease). Substitution of TAXI-I His374 therefore strongly affects the affinity of the inhibitor for the enzymes. In addition, the results show that His374 plays a critical role in the stabilization of the endoxylanase-TAXI-I complex rather than in the docking of inhibitor onto enzyme.

Emergence of a subfamily of xylanase inhibitors within glycoside hydrolase family 18

The xylanase inhibitor protein I (XIP-I), recently identified in wheat, inhibits xylanases belonging to glycoside hydrolase families 10 (GH10) and 11 (GH11). Sequence and structural similarities indicate that XIP-I is related to chitinases of family GH18, despite its lack of enzymatic activity. Here we report the identification and biochemical characterization of a XIP-type inhibitor from rice. Despite its initial classification as a chitinase, the rice inhibitor does not exhibit chitinolytic activity but shows specificities towards fungal GH11 xylanases similar to that of its wheat counterpart. This, together, with an analysis of approximately 150 plant members of glycosidase family GH18 provides compelling evidence that xylanase inhibitors are largely represented in this family, and that this novel function has recently emerged based on a common scaffold. The plurifunctionality of GH18 members has major implications for genomic annotations and predicted gene function. This study provides new information which will lead to a better understanding of the biological significance of a number of GH18 'inactivated' chitinases.

The inhibition specificity of recombinant Penicillium funiculosum xylanase B towards wheat proteinaceous inhibitors

The filamentous fungus Penicillium funiculosum produces a mixture of modular and non-modular xylanases belonging to different glycoside hydrolase (GH) families. In the present study, we heterologously expressed the cDNA encoding GH11 xylanase B (XYNB) and studied the enzymatic properties of the recombinant enzyme. Expression in Escherichia coli led to the partial purification of a glutathione fusion protein from the soluble fraction whereas the recombinant protein produced in Pichia pastoris was successfully purified using a one-step chromatography. Despite O-glycosylation heterogeneity, the purified enzyme efficiently degraded low viscosity xylan [K(m)=40+/-3 g l(-1), V(max)=16.1+/-0.8 micromol xylose min(-1) and k(cat)=5405+/-150 s(-1) at pH 4.2 and 45 degrees C] and medium viscosity xylan [K(m)=34.5+/-3.2 g l(-1), V(max)=14.9+/-1.0 micromol xylose min(-1)k(cat)=4966+/-333 s(-1) at pH 4.2 and 45 degrees C]. XYNB was further tested for its ability to interact with wheat xylanase inhibitors. The xylanase activity of XYNB produced in P. pastoris was strongly inhibited by both XIP-I and TAXI-I in a competitive manner, with a K(i) of 89.7+/-8.5 and 2.9+/-0.3 nM, respectively, whereas no inhibition was detected with TAXI-II. Physical interaction of both TAXI-I and XIP-I with XYNB was observed using titration curves across a pH range 3-9.

High-level expression, purification, and characterization of recombinant wheat xylanase inhibitor TAXI-I secreted by the yeast Pichia pastoris

Triticum aestivum xylanase inhibitor I (TAXI-I) is a wheat protein that inhibits microbial xylanases belonging to glycoside hydrolase family 11. In the present study, recombinant TAXI-I (rTAXI-I) was successfully produced by the methylotrophic yeast Pichia pastoris at high expression levels (approximately 75 mg/L). The rTAXI-I protein was purified from the P. pastoris culture medium using cation exchange and gel filtration chromatographic steps. rTAXI-I has an iso-electric point of at least 9.3 and a mass spectrometry molecular mass of 42,013 Da indicative of one N-linked glycosylation. The recombinant protein fold was confirmed by circular dichroism spectroscopy. Xylanase inhibition by rTAXI-I was optimal at 20-30 degrees C and at pH 5.0. rTAXI-I still showed xylanase inhibition activity at 30 degrees C after a 40 min pre-incubation step at temperatures between 4 and 70 degrees C and after 2 h pre-incubation at room temperature at a pH ranging from 3.0 to 12.0, respectively. All tested glycoside hydrolase family 11 xylanases were inhibited by rTAXI-I whereas those belonging to family 10 were not. Specific inhibition activities against family 11 Aspergillus niger and Bacillus subtilis xylanases were 3570 and 2940IU/mg protein, respectively. The obtained biochemical characteristics of rTAXI-I produced by P. pastoris (no proteolytical cleft) were similar to those of natural TAXI-I (mixture of proteolytically processed and non-processed forms) and non-glycosylated rTAXI-I expressed in Escherichia coli. The present results show that xylanase inhibition activity of TAXI-I is only affected to a limited degree by its glycosylation or proteolytic processing.

The dual nature of the wheat xylanase protein inhibitor XIP-I: structural basis for the inhibition of family 10 and family 11 xylanases

The xylanase inhibitor protein I (XIP-I) from wheat Triticum aestivum is the prototype of a novel class of cereal protein inhibitors that inhibit fungal xylanases belonging to glycoside hydrolase families 10 (GH10) and 11 (GH11). The crystal structures of XIP-I in complex with Aspergillus nidulans (GH10) and Penicillium funiculosum (GH11) xylanases have been solved at 1.7 and 2.5 A resolution, respectively. The inhibition strategy is novel because XIP-I possesses two independent enzyme-binding sites, allowing binding to two glycoside hydrolases that display a different fold. Inhibition of the GH11 xylanase is mediated by the insertion of an XIP-I Pi-shaped loop (Lalpha(4)beta(5)) into the enzyme active site, whereas residues in the helix alpha7 of XIP-I, pointing into the four central active site subsites, are mainly responsible for the reversible inactivation of GH10 xylanases. The XIP-I strategy for inhibition of xylanases involves substrate-mimetic contacts and interactions occluding the active site. The structural determinants of XIP-I specificity demonstrate that the inhibitor is able to interact with GH10 and GH11 xylanases of both fungal and bacterial origin. The biological role of the xylanase inhibitors is discussed in light of the present structural data.

Properties of TAXI-type endoxylanase inhibitors

Two types of proteinaceous endoxylanase inhibitors occur in different cereals, i.e. the TAXI [Triticum aestivum endoxylanase inhibitor]-type and XIP [endoxylanase inhibiting protein]-type inhibitors. The present paper focuses on the TAXI-type proteins and deals with their structural characteristics and the identification, characterisation and heterologous expression of a TAXI gene from wheat. In addition, to shed light on the mechanism by which TAXI-type endoxylanase inhibitors work, the enzyme specificity, the optimal conditions for maximal inhibition activity, the molar complexation ratio and the inhibition kinetics of the inhibitors are explained and the effect of mutations of an endoxylanase on the inhibition by TAXIs is discussed.

Potential physiological role of plant glycosidase inhibitors

Carbohydrate-active enzymes including glycosidases, transglycosidases, glycosyltransferases, polysaccharide lyases and carbohydrate esterases are responsible for the enzymatic processing of carbohydrates in plants. A number of carbohydrate-active enzymes are produced by microbial pathogens and insects responsible of severe crop losses. Plants have evolved proteinaceous inhibitors to modulate the activity of several of these enzymes. The continuing discovery of new inhibitors indicates that this research area is still unexplored and may lead to new exciting developments. To date, the role of the inhibitors is not completely understood. Here we review recent results obtained on the best characterised inhibitors, pointing to their possible biological role in vivo. Results recently obtained with plant transformation technology indicate that this class of inhibitors has potential biotechnological applications.

XIP-I, a xylanase inhibitor protein from wheat: a novel protein function

Endo-(1,4)-beta-xylanases of plant and fungal origin play an important role in the degradation of arabinoxylans. Two distinct classes of proteinaceous endoxylanase inhibitors, the Triticum aestivum xylanase inhibitor (TAXI) and the xylanase inhibitor protein (XIP), have been identified in cereals. Engineering of proteins in conjunction with enzyme kinetics, thermodynamic, real-time interaction, and X-ray crystallographic studies has provided knowledge on the mechanism of inhibition of XIP-I towards endoxylanases. XIP-I is a 30 kDa protein which belongs to glycoside hydrolase family 18, and folds as a typical (beta/alpha)8 barrel. Although the inhibitor shows highest homology with plant chitinases, XIP-I does not hydrolyse chitin; probably due to structural differences in the XIP-I binding cleft. The inhibitor is specific for fungal xylanases from glycoside hydrolases families 10 and 11, but does not inhibit bacterial enzymes. The inhibition is competitive and, depending on the xylanase, the Ki value can be as low as 3.4 nM. Site-directed mutagenesis of a xylanase from Aspergillus niger suggested that the XIP-I binding site was the conserved hairpin loop "thumb" region of family 11 xylanases. Furthermore, XIP-I shows the ability to inhibit barley alpha-amylases of glycoside hydrolase family 13, providing the first example of a protein able to inhibit members of different glycoside hydrolase families (10, 11, and 13), and additionally a novel function for a protein of glycoside hydrolase family 18.

Structural analysis of xylanase inhibitor protein I (XIP-I), a proteinaceous xylanase inhibitor from wheat (Triticum aestivum, var. Soisson)

A novel class of proteinaceous inhibitors exhibiting specificity towards microbial xylanases has recently been discovered in cereals. The three-dimensional structure of xylanase inhibitor protein I (XIP-I) from wheat (Triticum aestivum, var. Soisson) was determined by X-ray crystallography at 1.8 A (1 A=0.1 nm) resolution. The inhibitor possesses a (beta/alpha)(8) barrel fold and has structural features typical of glycoside hydrolase family 18, namely two consensus regions, approximately corresponding to the third and fourth barrel strands, and two non-proline cis -peptide bonds, Ser(36)-Phe and Trp(256)-Asp (in XIP-I numbering). However, detailed structural analysis of XIP-I revealed several differences in the region homologous with the active site of chitinases. The catalytic glutamic acid residue of family 18 chitinases [Glu(127) in hevamine, a chitinase/lysozyme from the rubber tree (Hevea brasiliensis)] is conserved in the structure of the inhibitor (Glu(128)), but its side chain is fully engaged in salt bridges with two neighbouring arginine residues. Gly(81), located in subsite -1 of hevamine, where the reaction intermediate is formed, is replaced by Tyr(80) in XIP-I. The tyrosine side chain fills the subsite area and makes a strong hydrogen bond with the side chain of Glu(190) located at the opposite side of the cleft, preventing access of the substrate to the catalytic glutamic acid. The structural differences in the inhibitor cleft structure probably account for the lack of activity of XIP-I towards chitin.

Comparison of modular and non-modular xylanases as carrier proteins for the efficient secretion of heterologous proteins from Penicillium funiculosum

Genes encoding three enzymes with xylanase activity from the filamentous fungus Penicillium funiculosum are described. Two of the encoded xylanases are predicted to be modular in structure with catalytic and substrate-binding domains separated by a serine and threonine-rich linker region; the other had none of these properties and was non-modular. In order to develop P. funiculosum as a host for the secreted production of heterologous proteins, each of the xylanases was assessed for use as a carrier protein in a fusion strategy. We show that one of the modular xylanases (encoded by xynA) was an effective carrier protein but the other (encoded by xynB) and the non-modular xylanase (encoded by xynC) were not effective as secretion carriers. We show that the beta-glucuronidase (GUS) protein from Escherichia coli is secreted by P. funiculosum when expressed as an XYNA fusion but that the secreted GUS protein, cleaved in vivo from XYNA, is glycosylated and enzymatically inactive.