An Aspergillus oryzae acetyl xylan esterase: Molecular cloning and characteristics of recombinant enzyme expressed in Pichia pastoris

Recombinant acetylxylan esterase of Halalkalibacterium halodurans NAH-Egypt: molecular and biochemical study

Acetylxylan esterase plays a crucial role in xylan hydrolysis as the acetyl side-groups restrict endoxylanase action by stearic hindrance. In this study, an acetylxylan esterase (AXE-HAS10: 960 bp & 319 a.a) putative ORF from Halalkalibacterium halodurans NAH-Egypt was extensively studied through heterologous overexpression in Escherichia coli, biochemical characterization, and structural modeling. The AXE-HAS10 tertiary structure was predicted by the Local Meta Threading Server. AXE-HAS10 belongs to the carbohydrate esterase Family 7. Purified to homogeneity AXE-HAS10 showed specific activity (36.99 U/mg), fold purification (11.42), and molecular mass (41.39 kDa). AXE-HAS10 showed optimal pH (8.5) and temperature (40 oC). After 15 h of incubation at pH 7.0–9.0, AXE-HAS10 maintained 100% activity. After 120 min at 35 and 40 oC, the retained activity was 80 and 50%, respectively. At 10 mM Mn2+, Fe3+, K+, and Ca2+ after 30 min, retained activity was 329 ± 15, 212 ± 5.2, 123 ± 1.4, and 120 ± 3.0%, respectively. After 30 min of preincubation with triton x-100, SDS, and CTAB at 0.1% (v/v), the retained activity was 150 ± 19, 88 ± 4, and 82 ± 7%, respectively. At 6.0 M NaCl after 30 min, retained activity was 58%. A 1.44-fold enhancement of beechwood xylan hydrolysis was achieved by AXE-HAS10 and Penicillium chrysogenum DSM105774 β-xylanase concurrently. Present data underpins AXE-HAS10 as a promising AXE for industrial exploitation.

Gelation of konjac glucomannan by acetylmannan esterases from Aspergillus oryzae

Konjac glucomannan (KGM) is a principal component of the gelatinous food Konjac. Konjac production through alkali treatment releases an undesirable amine-odor. Two acetylesterases (AME1 and AME2) active against konjac glucomannan (polymer or oligomer) were purified from the supernatant of Aspergillus oryzae RIB40 culture. We cloned the genes encoding AME1 and AME2 based on the genomic information of A. oryzae, constructed their expression systems in A. oryzae, and obtained the recombinant enzymes (rAME1 and rAME2). rAME1 did not act on the KGM polymer but only on the KGM oligomer, releasing approximately 60% of the acetic acid in the substrate. However, rAME2 was active against both KGM substrates, releasing approximately 80% and 100% of acetic acid from the polymer and oligomer, respectively. Both enzymes were active against xylan and exhibited a trace activity on ethyl ferulate. The acetyl group position specificities of both enzymes were analyzed via heteronuclear single quantum correlation NMR using oligosaccharides of glucomannan prepared from Aloe vera (AGM), which has a higher acetyl group content than KGM. rAME1 acted specifically on single-substituted acetyl groups and not on double-substituted ones. In contrast, rAME2 appeared to act on all the acetyl groups in AGM. Treatment of 3% KGM with rAME2 followed by heating to 90 °C resulted in gel formation under weakly acidic conditions. This is the first study to induce gelation of KGM under these conditions. A comparison of the breaking and brittleness properties of gels formed by alkaline and enzymatic treatments revealed similar texture of the two gels. Furthermore, scanning electron microscopy of the surface structure of both gels revealed that both formed a fine mesh structure. Our findings on enzymatic gelation of KGM should lead to the development of new applications in food manufacturing industry.

Characterisation of a Novel Acetyl Xylan Esterase (BaAXE) Screened from the Gut Microbiota of the Common Black Slug (Arion ater)

Acetyl xylan esterases (AXEs) are enzymes capable of hydrolysing the acetyl bonds in acetylated xylan, allowing for enhanced activity of backbone-depolymerizing enzymes. Bioprospecting novel AXE is essential in designing enzyme cocktails with desired characteristics targeting the complete breakdown of lignocellulose. In this article, we report the characterisation of a novel AXE identified as Gene_id_40363 in the metagenomic library analysed from the gut microbiota of the common black slug. The conserved domain description was identified with an NCBI BLASTp search using the translated nucleotide sequence as a query. The activity of the recombinant enzyme was tested on various synthetic substrates and acetylated substrates. The protein sequence matched the conserved domain described as putative hydrolase and aligned closely to an uncharacterized esterase from Buttiauxella agrestis, hence the designation as BaAXE. BaAXE showed low sequence similarity among characterized CE family proteins with an available 3D structure. BaAXE was active on 4-nitrophenyl acetate, reporting a specific activity of 78.12 U/mg and a Km value of 0.43 mM. The enzyme showed optimal activity at 40 °C and pH 8 and showed high thermal stability, retaining over 40% activity after 2 h of incubation from 40 °C to 100 °C. BaAXE hydrolysed acetyl bonds, releasing acetic acid from acetylated xylan and β-D-glucose pentaacetate. BaAXE has great potential for biotechnological applications harnessing its unique characteristics. In addition, this proves the possibility of bioprospecting novel enzymes from understudied environments.

Identification and characterization of an acetyl xylan esterase from Aspergillus oryzae

In this study, we report the identification and characterization of an acetyl xylan esterase, designated as AoAXEC, which was previously annotated as a hypothetical protein encoded by AO090023000158 in the Aspergillus oryzae genomic database. Based on its amino acid sequence, a low sequence identity to known acetyl xylan esterases was observed in the sequence of characterized acetyl xylan esterase. The gene fused with α-factor signal sequence of Saccharomyces cerevisiae instead of the native signal sequence was cloned into a vector, pPICZαC, and expressed successfully in Pichia pastoris as an active extracellular protein. The purified recombinant protein had pH and temperature optima of 7.0 and 50 °C, respectively, and was stable up to 50 °C. The optimal substrate for hydrolysis by the purified recombinant AoAXEC, among a panel of α-naphthyl esters (C2-C16), was α-naphthyl propionate (C3), with an activity of 0.35 ± 0.006 units/mg protein. No significant difference of the K_m value was observed between C3 (2.3 ± 0.7 mM) and C2 (1.9 ± 0.4 mM). In contrast, k_cat value for C3 (18 ± 3.9 s^-1) was higher compared to C2 (4.5 ± 0.7 s^-1). The purified recombinant enzyme displayed a low activity toward acyl chain substrates containing eight or more carbon atoms. Recombinant AoAXEC catalyzed the release of acetic acid from wheat arabinoxylan. However, no activity was detected on methyl esters of ferulic, p-coumaric, caffeic, or sinapic acids. Additionally, the liberation of phenolic acids, such as ferulic acid, from wheat arabinoxylan was not exhibited by the recombinant protein.

Functional Validation of Two Fungal Subfamilies in Carbohydrate Esterase Family 1 by Biochemical Characterization of Esterases From Uncharacterized Branches

The fungal members of Carbohydrate Esterase family 1 (CE1) from the CAZy database include both acetyl xylan esterases (AXEs) and feruloyl esterases (FAEs). AXEs and FAEs are essential auxiliary enzymes to unlock the full potential of feedstock. They are being used in many biotechnology applications including food and feed, pulp and paper, and biomass valorization. AXEs catalyze the hydrolysis of acetyl group from xylan, while FAEs release ferulic and other hydroxycinnamic acids from xylan and pectin. Previously, we reported a phylogenetic analysis for the fungal members of CE1, establishing five subfamilies (CE1_SF1–SF5). Currently, the characterized AXEs are in the subfamily CE1_SF1, whereas CE1_SF2 contains mainly characterized FAEs. These two subfamilies are more related to each other than to the other subfamilies and are predicted to have evolved from a common ancestor, but target substrates with a different molecular structure. In this study, four ascomycete enzymes from CE1_SF1 and SF2 were heterologously produced in Pichia pastoris and characterized with respect to their biochemical properties and substrate preference toward different model and plant biomass substrates. The selected enzymes from CE1_SF1 only exhibited AXE activity, whereas the one from CE1_SF2 possessed dual FAE/AXE activity. This dual activity enzyme also showed broad substrate specificity toward model substrates for FAE activity and efficiently released both acetic acid and ferulic acid (∼50%) from wheat arabinoxylan and wheat bran which was pre-treated with a commercial xylanase. These fungal AXEs and FAEs also showed promising biochemical properties, e.g., high stability over a wide pH range and retaining more than 80% of their residual activity at pH 6.0–9.0. These newly characterized fungal AXEs and FAEs from CE1 have high potential for biotechnological applications. In particular as an additional ingredient for enzyme cocktails to remove the ester-linked decorations which enables access for the backbone degrading enzymes. Among these novel enzymes, the dual FAE/AXE activity enzyme also supports the evolutionary relationship of CE1_SF1 and SF2.

Functional Expression and Characterization of Acetyl Xylan Esterases CE Family 7 from Lactobacillus antri and Bacillus halodurans

Acetyl xylan esterase (AXE; E.C. 3.1.1.72) is one of the accessory enzymes for xylan degradation, which can remove the terminal acetate residues from xylan polymers. In this study, two genes encoding putative AXEs (LaAXE and BhAXE) were cloned from Lactobacillus antri DSM 16041 and Bacillus halodurans C-125, and constitutively expressed in Escherichia coli. They possess considerable activities towards various substrates such as p-nitrophenyl acetate, 4-methylumbelliferyl acetate, glucose pentaacetate, and 7-amino cephalosporanic acid. LaAXE and BhAXE showed the highest activities at pH 7.0 and 8.0 at 50°C, respectively. These enzymes are AXE members of carbohydrate esterase (CE) family 7 with the cephalosporine-C deacetylase activity for the production of antibiotics precursors. The simultaneous treatment of LaAXE with Thermotoga neapolitana β-xylanase showed 1.44-fold higher synergistic degradation of beechwood xylan than the single treatment of xylanase, whereas BhAXE showed no significant synergism. It was suggested that LaAXE can deacetylate beechwood xylan and enhance the successive accessibility of xylanase towards the resulting substrates. The novel LaAXE originated from a lactic acid bacterium will be utilized for the enzymatic production of D-xylose and xylooligosaccharides.

Characterization of Acetylxylan Esterase from White-Rot Fungus Irpex lacteus

The carbohydrate esterase family 1 (CE1) in CAZy contains acetylxylan esterases (AXEs) and feruloyl esterases (FAEs). Here we cloned a gene coding for an AXE belonging to CE1 from Irpex lacteus (IlAXE1). IlAXE1 was heterologously expressed in Pichia pastoris, and the recombinant enzyme was purified and characterized. IlAXE1 hydrolyzed p-nitrophenyl acetate, α-naphthyl acetate and 4-methylumbelliferyl acetate, however, it did not show any activity on ethyl ferulate and methyl p-coumarate. We also examined the activity on partially acetylated and feruloylated xylan extracted from corncob by hydrothermal reaction. Similarly, ferulic and p-coumaric acids were not liberated, and acetic acid was only detected in the reaction mixture. The results indicated that IlAXE1 is an acetylxylan esterase actually reacted to acetyl xylan. However, since IlAXE1 was unable to completely release acetic acid esterifying xylopyranosyl residues, it is assumed that acetyl groups exhibiting resistance to deacetylation by IlAXE1 are present in corn cob xylan.

Characterization of a feruloyl esterase from Aspergillus terreus facilitates the division of fungal enzymes from Carbohydrate Esterase family 1 of the carbohydrate-active enzymes (CAZy) database

Feruloyl esterases (FAEs) are accessory enzymes for plant biomass degradation, which catalyse hydrolysis of carboxylic ester linkages between hydroxycinnamic acids and plant cell‐wall carbohydrates. They are a diverse group of enzymes evolved from, e.g. acetyl xylan esterases (AXEs), lipases and tannases, thus complicating their classification and prediction of function by sequence similarity. Recently, an increasing number of fungal FAEs have been biochemically characterized, owing to their potential in various biotechnological applications and multitude of candidate FAEs in fungal genomes. However, only part of the fungal FAEs are included in Carbohydrate Esterase family 1 (CE1) of the carbohydrate‐active enzymes (CAZy) database. In this work, we performed a phylogenetic analysis that divided the fungal members of CE1 into five subfamilies of which three contained characterized enzymes with conserved activities. Conservation within one of the subfamilies was confirmed by characterization of an additional CE1 enzyme from Aspergillus terreus. Recombinant A. terreus FaeD (AtFaeD) showed broad specificity towards synthetic methyl and ethyl esters, and released ferulic acid from plant biomass substrates, demonstrating its true FAE activity and interesting features as potential biocatalyst. The subfamily division of the fungal CE1 members enables more efficient selection of candidate enzymes for biotechnological processes.

Carbohydrate Esterases: An Overview

Carbohydrate esterases are a group of enzymes which release acyl or alkyl groups attached by ester linkage to carbohydrates. The CAZy database, which classifies enzymes that assemble, modify, and break down carbohydrates and glycoconjugates, classifies all carbohydrate esterases into 16 families. This chapter is an overview of the research for nearly 50 years around the main groups of carbohydrate esterases dealing with the degradation of polysaccharides, their main biochemical and molecular traits, as well as its application for the synthesis of high added value esters.

Crystal Structure and Substrate Specificity Modification of Acetyl Xylan Esterase from Aspergillus luchuensis

Acetyl xylan esterase (AXE) catalyzes the hydrolysis of the acetyl bonds present in plant cell wall polysaccharides. Here, we determined the crystal structure of AXE from Aspergillus luchuensis (AlAXEA), providing the three-dimensional structure of an enzyme in the Esterase_phb family. AlAXEA shares its core α/β-hydrolase fold structure with esterases in other families, but it has an extended central β-sheet at both its ends and an extra loop. Structural comparison with a ferulic acid esterase (FAE) from Aspergillus niger indicated that AlAXEA has a conserved catalytic machinery: a catalytic triad (Ser119, His259, and Asp202) and an oxyanion hole (Cys40 and Ser120). Near the catalytic triad of AlAXEA, two aromatic residues (Tyr39 and Trp160) form small pockets at both sides. Homology models of fungal FAEs in the same Esterase_phb family have wide pockets at the corresponding sites because they have residues with smaller side chains (Pro, Ser, and Gly). Mutants with site-directed mutations at Tyr39 showed a substrate specificity similar to that of the wild-type enzyme, whereas those with mutations at Trp160 acquired an expanded substrate specificity. Interestingly, the Trp160 mutants acquired weak but significant type B-like FAE activity. Moreover, the engineered enzymes exhibited ferulic acid-releasing activity from wheat arabinoxylan.IMPORTANCE Hemicelluloses in the plant cell wall are often decorated by acetyl and ferulic acid groups. Therefore, complete and efficient degradation of plant polysaccharides requires the enzymes for cleaving the side chains of the polymer. Since the Esterase_phb family contains a wide array of fungal FAEs and AXEs from fungi and bacteria, our study will provide a structural basis for the molecular mechanism of these industrially relevant enzymes in biopolymer degradation. The structure of the Esterase_phb family also provides information for bacterial polyhydroxyalkanoate depolymerases that are involved in biodegradation of thermoplastic polymers.

Genomics review of holocellulose deconstruction by aspergilli

SUMMARY

Biomass is constructed of dense recalcitrant polymeric materials: proteins, lignin, and holocellulose, a fraction constituting fibrous cellulose wrapped in hemicellulose-pectin. Bacteria and fungi are abundant in soil and forest floors, actively recycling biomass mainly by extracting sugars from holocellulose degradation. Here we review the genome-wide contents of seven Aspergillus species and unravel hundreds of gene models encoding holocellulose-degrading enzymes. Numerous apparent gene duplications followed functional evolution, grouping similar genes into smaller coherent functional families according to specialized structural features, domain organization, biochemical activity, and genus genome distribution. Aspergilli contain about 37 cellulase gene models, clustered in two mechanistic categories: 27 hydrolyze and 10 oxidize glycosidic bonds. Within the oxidative enzymes, we found two cellobiose dehydrogenases that produce oxygen radicals utilized by eight lytic polysaccharide monooxygenases that oxidize glycosidic linkages, breaking crystalline cellulose chains and making them accessible to hydrolytic enzymes. Among the hydrolases, six cellobiohydrolases with a tunnel-like structural fold embrace single crystalline cellulose chains and cooperate at nonreducing or reducing end termini, splitting off cellobiose. Five endoglucanases group into four structural families and interact randomly and internally with cellulose through an open cleft catalytic domain, and finally, seven extracellular β-glucosidases cleave cellobiose and related oligomers into glucose. Aspergilli contain, on average, 30 hemicellulase and 7 accessory gene models, distributed among 9 distinct functional categories: the backbone-attacking enzymes xylanase, mannosidase, arabinase, and xyloglucanase, the short-side-chain-removing enzymes xylan α-1,2-glucuronidase, arabinofuranosidase, and xylosidase, and the accessory enzymes acetyl xylan and feruloyl esterases.

Plant-polysaccharide-degrading enzymes from Basidiomycetes

SUMMARY

Basidiomycete fungi subsist on various types of plant material in diverse environments, from living and dead trees and forest litter to crops and grasses and to decaying plant matter in soils. Due to the variation in their natural carbon sources, basidiomycetes have highly varied plant-polysaccharide-degrading capabilities. This topic is not as well studied for basidiomycetes as for ascomycete fungi, which are the main sources of knowledge on fungal plant polysaccharide degradation. Research on plant-biomass-decaying fungi has focused on isolating enzymes for current and future applications, such as for the production of fuels, the food industry, and waste treatment. More recently, genomic studies of basidiomycete fungi have provided a profound view of the plant-biomass-degrading potential of wood-rotting, litter-decomposing, plant-pathogenic, and ectomycorrhizal (ECM) basidiomycetes. This review summarizes the current knowledge on plant polysaccharide depolymerization by basidiomycete species from diverse habitats. In addition, these data are compared to those for the most broadly studied ascomycete genus, Aspergillus, to provide insight into specific features of basidiomycetes with respect to plant polysaccharide degradation.

Hydrolysis of wheat arabinoxylan by two acetyl xylan esterases from Chaetomium thermophilum

The thermophilic filamentous ascomycete Chaetomium thermophilum produces functionally diverse hemicellulases when grown on hemicellulose as carbon source. Acetyl xylan esterase (EC 3.1.1.72) is an important accessory enzyme in hemicellulose biodegradation. Although the genome of C. thermophilum has been sequenced, its carbohydrate esterases are not annotated yet. We applied peptide pattern recognition (PPR) tool for sequence analysis of the C. thermophilum genome, and 11 carbohydrate esterase genes were discovered. Furthermore, we cloned and heterologously expressed two putative acetyl xylan esterase genes, CtAxeA and CtAxeB, in Pichia pastoris. The recombinant proteins, rCtAxeA and rCtAxeB, released acetic acids from p-nitrophenyl acetate and water-insoluble wheat arabinoxylan. These results indicate that CtAxeA and CtAxeB are true acetyl xylan esterases. For both recombinant esterases, over 93 % of the initial activity was retained after 24 h of incubation at temperatures up to 60 °C, and over 90 % of the initial activity was retained after 24 h of incubation in different buffers from pH 4.0 to 9.0 at 4 and 50 °C. The overall xylose yield from wheat arabinoxylan hydrolysis was 8 % with xylanase treatment and increased to 34 % when xylanase was combined with rCtAxeA and rCtAxeB. In sum, the present study first report the biochemical characterization of two acetyl xylan esterases from C. thermophilum, which are efficient in hydrolyzing hemicellulose with potential application in biomass bioconversion to high value chemicals or biofuels.

A combined approach for improving alkaline acetyl xylan esterase production in Pichia pastoris, and effects of glycosylation on enzyme secretion, activity and stability

High level expression of axe1, a gene previously cloned from Volvariella volvacea that encodes an acetyl xylan esterase with two potential N-linked glycosylation sites, has been achieved in Pichia pastoris using a codon-optimized axe1 synthesized by the primer extension PCR procedure. The GC content of the codon-optimized axe1 was 48.62% compared with 55.49% in the native gene. Using the codon-optimized construct, AXE1 expression in P. pastoris was increased from an undetectable level to 136.45 U/ml six days after induction of yeast cultures grown in BMMY medium. A further increase (to 463 U/ml) was achieved when conditions for yeast culture were optimized as follows: 2.8% methanol, 0.63% casamino acids, and pH 8.0. This latter value represented a 3.4-fold and 246-fold increase in the enzyme levels recorded in non-optimized P. pastoris cultures and in rice straw-grown cultures of V. volvacea, respectively. N-linked glycosylation played an essential role in AXE1 secretion but had only a slight effect on the catalytic activity and stability of the recombinant enzyme.

Functional expression of a thermophilic glucuronyl esterase from Sporotrichum thermophile: identification of the nucleophilic serine

A glucuronyl esterase (GE) from the thermophilic fungus Sporotrichum thermophile, belonging to the carbohydrate esterase family 15 (CE-15), was functionally expressed in the methylotrophic yeast Pichia pastoris. The putative GE gene ge2 from the genomic DNA was successfully cloned in frame with the sequence for the Saccharomyces cerevisiae alpha-factor secretion signal under the transcriptional control of the alcohol oxidase (AOX1) promoter and integrated in P. pastoris X-33 to confirm that the encoded enzyme StGE2 exhibits esterase activity. The enzyme was active on substrates containing glucuronic acid methyl ester, showing optimal activity at pH 7.0 and 55 degrees C. The esterase displayed broad pH range stability between 4-10 and temperature stability up to 50 degrees C, rendering StGE2 a strong candidate for future biotechnological applications that require robust biocatalysts. ClustalW alignment of StGE2 with characterized GEs and selected homologous sequences, members of CE-15 family, revealed a novel consensus sequence G-C-S-R-X-G that features the characteristic serine residue involved in the generally conserved catalytic mechanism of the esterase family. The putative serine has been mutated, and the corresponding enzyme has been expressed in P. pastoris to prove that the candidate nucleophilic residue is responsible for catalyzing the enzymatic reaction.

Expression, purification, and analyses of glycosylation and disulfide bonds of Stereum purpureum endopolygalacturonase I in Pichia pastoris

We have succeeded in the expression of Stereum purpureum endopolygalacturonase I (EndoPG I) using the Pichia expression system and in purification of the three kinds of recombinant EndoPG I, which have one to three sugar chains by using CM52 column chromatography. The sugar chains which were added to EndoPG I were the M8, M9, and/or M10 high-mannose type. The results of LC-MS analysis showed that recombinant EndoPG Is were randomly glycosylated at four N-glycosylation sites. From the thermal denaturation curves of the recombinant enzymes, it was suggested that EndoPG I differing in thermal stability was included in the sample after purification. Therefore, we investigated the disulfide bonds of recombinant EndoPG I by LC-MS analysis. As a result, peptides without a second or third disulfide bond were detected. This result is the first indicating that there are incomplete enzymes in terms of disulfide bonds in the Pichia expression system.

Molecular cloning, and characterization of a modular acetyl xylan esterase from the edible straw mushroomVolvariella volvacea

A new Volvariella volvacea gene encoding an acetyl xylan esterase (designated as Vvaxe1) was cloned and expressed in Pichia pastoris. The cDNA contained an ORF of 1047 bp encoding 349 amino acids with a calculated mass of 39 990 Da. VvAXE1 is a modular enzyme consisting of an N-terminal signal peptide, a catalytic domain, and a cellulose-binding domain. The amino acid sequence of the enzyme exhibited a high degree of similarity to cinnamoyl esterase B from Penicillium funiculosum, and acetyl xylan esterases from Aspergillus oryzae, Penicillium purpurogenum, and Aspergillus ficuum. Recombinant acetyl xylan esterase released acetate from several acetylated substrates including beta-d-xylose tetraacetate and acetylated xylan. No activity was detectable on p-nitrophenyl acetate. Enzyme-catalyzed hydrolysis of 4-methylumbelliferyl acetate was maximal at pH 8.0 and 60 degrees C, and reciprocal plots revealed an apparent K(m) value of 307.7 microM and a V(max) value of 24 733 IU micromol(-1) protein. ReAXE1 also exhibited a capacity to bind to Avicel and H(3)PO(4) acid-swollen cellulose.

Analysis of expressed sequence tags from the fungus Aspergillus oryzae cultured under different conditions

We performed random sequencing of cDNAs from nine biologically or industrially important cultures of the industrially valuable fungus Aspergillus oryzae to obtain expressed sequence tags (ESTs). Consequently, 21 446 raw ESTs were accumulated and subsequently assembled to 7589 non-redundant consensus sequences (contigs). Among all contigs, 5491 (72.4%) were derived from only a particular culture. These included 4735 (62.4%) singletons, i.e. lone ESTs overlapping with no others. These data showed that consideration of culture grown under various conditions as cDNA sources enabled efficient collection of ESTs. BLAST searches against the public databases showed that 2953 (38.9%) of the EST contigs showed significant similarities to deposited sequences with known functions, 793 (10.5%) were similar to hypothetical proteins, and the remaining 3843 (50.6%) showed no significant similarity to sequences in the databases. Culture-specific contigs were extracted on the basis of the EST frequency normalized by the total number for each culture condition. In addition, contig sequences were compared with sequence sets in eukaryotic orthologous groups (KOGs), and classified into the KOG functional categories.

Penicillium purpurogenum produces a family 1 acetyl xylan esterase containing a carbohydrate-binding module: characterization of the protein and its gene

At least three acetyl xylan esterases (AXE I, II and III) are secreted by Penicillium purpurogenum. This publication describes more detailed work on AXE I and its gene. AXE I binds cellulose but not xylan; it is glycosylated and inactivated by phenylmethylsulphonyl fluoride, showing that it is a serine esterase. The axe1 gene presents an open reading frame of 1278 bp, including two introns of 68 and 61 bp; it codes for a signal peptide of 31 residues and a mature protein of 351 amino acids (molecular weight 36,693). AXE I has a modular structure: a catalytic module at the amino terminus belonging to family 1 of the carbohydrate esterases, a linker rich in serines and threonines, and a family 1 carboxy terminal carbohydrate binding module (CBM). The CBM is similar to that of AXE from Trichoderma reesei, (with a family 5 catalytic module) indicating that the genes for catalytic modules and CBMs have evolved separately, and that they have been linked by gene fusion. The promoter sequence of axe1 contains several putative sequences for binding of gene expression regulators also found in other family 1 esterase gene promoters. It is proposed that AXE I and II act in succession in xylan degradation; first, xylan is attacked by AXE I and other xylanases possessing CBMs (which facilitate binding to lignocellulose), followed by other enzymes acting mainly on soluble substrates.