Molecular cloning of the gene encoding thermostable endo-1,5-alpha-L-arabinase of Bacillus thermodenitrificans TS-3 and its expression in Bacillus subtilis

Production of thermostable endo-1,5-α-L-arabinanase in Pichia pastoris for enzymatically releasing functional oligosaccharides from sugar beet pulp

Sugar beet pulp is an agricultural processing residue that is a rich source of the cell wall polysaccharide arabinan. Functional oligosaccharides, specifically feruloylated arabino-oligosaccharides (FAOs), can be isolated from sugar beet pulp through selective action by endo-arabinanase (glycoside hydrolase family 43). This study aimed to develop yeast (Pichia pastoris) as an efficient, eukaryotic platform to produce a thermophilic endo-1,5-α-L-arabinanase (TS-ABN) for extracting FAOs from sugar beet pulp. Recombinant TS-ABN was secreted into yeast culture medium at a yield of ~ 80 mg/L, and the protein exhibited specific enzyme activity, pH and temperature optimum, and thermostability comparable to those of the native enzyme. Treatment of sugar beet pulp with Pichia-secreted TS-ABN released FAOs recovered by hydrophobic chromatography at 1.52% (w/w). The isolated FAOs averaged seven arabinose residues per ferulic acid, and treatment of T84 human colon epithelial cells significantly increased expression of two key tight junction-related proteins-zonula occludens-1 and occludin-in a dose-dependent manner. This research establishes a biochemical platform for utilizing sugar beet pulp to produce value-added bioproducts with potential nutraceutical applications.

Structures of endo-1,5-α-L-arabinanase mutants from Bacillus thermodenitrificans TS-3 in complex with arabino-oligosaccharides

The thermostable endo-1,5-α-L-arabinanase from Bacillus thermodenitrificans TS-3 (ABN-TS) hydrolyzes the α-1,5-L-arabinofuranoside linkages of arabinan. In this study, the crystal structures of inactive ABN-TS mutants, D27A and D147N, were determined in complex with arabino-oligosaccharides. The crystal structures revealed that ABN-TS has at least six subsites in the deep V-shaped cleft formed across one face of the propeller structure. The structural features indicate that substrate recognition is profoundly influenced by the remote subsites as well as by the subsites surrounding the active center. The `open' structure of the substrate-binding cleft of the endo-acting ABN-TS is suitable for the random binding of several sugar units in polymeric substrates.

An Acid-Adapted Endo-α-1,5-L-arabinanase for Pectin Releasing

An arabinanase gene was cloned by overlap-PCR from Penicillium sp. Y702 and expressed in Pichia pastoris. The recombinant enzyme was named AbnC702 with 20 U/mg of endo-arabinanase activity toward linear α-1,5-L-arabinan. The optimal pH and temperature of AbnC702 were 5.0 and 50 °C, respectively. The recombinant AbnC702 was highly stable at pH 5.0-7.0 and 50 °C. It could retain about 72.3 % of maximum specific activity at pH 5.0 after incubation for 2.5 h, which indicated AbnC702 was an acid-adapted enzyme. The K _m and V _max values were 24.8 ± 4.7 mg/ml and 88.5 ± 5.6 U/mg, respectively. A three-dimensional structure of AbnC702 was made by homology modeling, and the counting of acidic/basic amino residues within the region of 10 Å around the active site, as well the hydrogen bonds within the area of 5 Å around the active site, might theoretically interpret the acid adaptability of AbnC702. Analysis of hydrolysis products by thin layer chromatography (TLC) combined with high-performance liquid chromatography (HPLC) verified that the recombinant AbnC702 was an endo-1,5-α-L-arabinanase, which yielded arabinobiose and arabinotriose as major products. AbnC702 was applied in pectin extraction from apple pomace with synergistic action of α-L-arabinofuranosidase.

Characterization of abnZ2 (yxiA1) and abnZ3 (yxiA3) in Paenibacillus polymyxa, encoding two novel endo-1,5-α-l-arabinanases

Background

Protopectinases which were consisted of various different enzymes can promote the solubilization of protopectin from the plant cell and can be applied in the protein industry extraction. The genome sequence of Paenibacillus polymyxa Z6 that produces a protopectinases complex was partially determined. Two new genes, yxiA1 and yxiA3, were identified as uncharacterized protein in the P. polymyxa genome. And, they were classified as the member of the glycoside hydrolase family 43 (GH43) according to the primary protein sequence.

Results

The two genes were cloned and expressed in Escherichia coli BL21 (DE3). And, the results indicated that the product of yxiA1 and yxiA3 were two endo-α-1,5-l-arabinanases. Thus, the two genes were renamed as abnZ2 (yxiA1) and abnZ3 (yxiA3). Recombinant AbnZ2 had optimal activity at pH 6.0 and 35°C. And, AbnZ3 had optimal activity at pH 6.0 and 30°C. However, unlike most reported endo-arabinanases, the specific activity of AbnZ3 remained 48.7% of maximum at 5°C, which meant AbnZ3 was an excellent cold-adapted enzyme.

Conclusions

This paper demonstrated that the gene yxiA1 and yxiA3 were two new endo-arabinanases, and renamed as abnZ2 and abnZ3. Moreover AbnZ3 was an excellent cold-adapted enzyme which could be attractive in fruit juice processing.

Cloning and characterization of a cold-adapted endo-1,5-α-L-arabinanase from Paenibacillus polymyxa and rational design for acidic applicability

AbnZ1, with optimal pH of 6.0 and optimal temperature of 40 °C, is a cold-adapted endo-1,5-α-L-arabinanase encoded by the gene abnZ1 from Paenibacillus polymyxa Z6. The specific activity of AbnZ1 remained 54.1% of maximum at 5 °C. To apply AbnZ1 in acidic conditions, three basic hsitidine (His) residues, His(48), His(218), and His(297), around the catalytic domain were selected as mutation sites, which were replaced with Asp, Glu, Arg, and Lys, respectively, to yield 12 mutants, H48D/E/R/K, H218D/E/R/K, and H297D/E/R/K. The optimum pH of mutant H218D shifted toward the acidic direction by 0.5 unit, and the relative activity was enhanced from 20.4 to 55.7% at pH 5.0. Furthermore, the specific activity of H218D in optimal conditions was 82.6 U/mg versus that of wild type, 73.4 U/mg, and the K(m) decreased from 11.9 to 7.1 mg/mL. This work provided an arabinanase candidate for juice clarification and pectin extraction.

β-xylosidases and α-L-arabinofuranosidases: accessory enzymes for arabinoxylan degradation

Arabinoxylan (AX) is among the most abundant hemicelluloses on earth and one of the major components of feedstocks that are currently investigated as a source for advanced biofuels. As global research into these sustainable biofuels is increasing, scientific knowledge about the enzymatic breakdown of AX advanced significantly over the last decade. This review focuses on the exo-acting AX hydrolases, such as α-arabinofuranosidases and β-xylosidases. It aims to provide a comprehensive overview of the diverse substrate specificities and corresponding structural features found in the different glycoside hydrolase families. A careful review of the available literature reveals a marked difference in activity between synthetically labeled and naturally occurring substrates, often leading to erroneous enzymatic annotations. Therefore, special attention is given to enzymes with experimental evidence on the hydrolysis of natural polymers.

Domain analysis of a modular alpha-L-Arabinofuranosidase with a unique carbohydrate binding strategy from the fiber-degrading bacterium Fibrobacter succinogenes S85

Family 43 glycoside hydrolases (GH43s) are known to exhibit various activities involved in hemicellulose hydrolysis. Thus, these enzymes contribute to efficient plant cell wall degradation, a topic of much interest for biofuel production. In this study, we characterized a unique GH43 protein from Fibrobacter succinogenes S85. The recombinant protein showed α-l-arabinofuranosidase activity, specifically with arabinoxylan. The enzyme is, therefore, an arabinoxylan arabinofuranohydrolase (AXH). The F. succinogenes AXH (FSUAXH1) is a modular protein that is composed of a signal peptide, a GH43 catalytic module, a unique β-sandwich module (XX domain), a family 6 carbohydrate-binding module (CBM6), and F. succinogenes-specific paralogous module 1 (FPm-1). Truncational analysis and site-directed mutagenesis of the protein revealed that the GH43 domain/XX domain constitute a new form of carbohydrate-binding module and that residue Y484 in the XX domain is essential for binding to arabinoxylan, although protein structural analyses may be required to confirm some of the observations. Kinetic studies demonstrated that the Y484A mutation leads to a higher k(cat) for a truncated derivative of FSUAXH1 composed of only the GH43 catalytic module and the XX domain. However, an increase in the K(m) for arabinoxylan led to a 3-fold decrease in catalytic efficiency. Based on the knowledge that most XX domains are found only in GH43 proteins, the evolutionary relationships within the GH43 family were investigated. These analyses showed that in GH43 members with a XX domain, the two modules have coevolved and that the length of a loop within the XX domain may serve as an important determinant of substrate specificity.

Thermostable enzymes as biocatalysts in the biofuel industry

Lignocellulose is the most abundant carbohydrate source in nature and represents an ideal renewable energy source. Thermostable enzymes that hydrolyze lignocellulose to its component sugars have significant advantages for improving the conversion rate of biomass over their mesophilic counterparts. We review here the recent literature on the development and use of thermostable enzymes for the depolymerization of lignocellulosic feedstocks for biofuel production. Furthermore, we discuss the protein structure, mechanisms of thermostability, and specific strategies that can be used to improve the thermal stability of lignocellulosic biocatalysts.

Structural basis for thermostability of endo-1,5-alpha-L-arabinanase from Bacillus thermodenitrificans TS-3

The crystal structure of a thermostable endo-1,5-alpha-L-arabinanase, ABN-TS, from Bacillus thermodenitrificans TS-3 was determined at 1.9 A to an R-factor of 18.3% and an R-free-factor of 22.5%. The enzyme molecule has a five-bladed beta-propeller fold. The substrate-binding cleft formed across one face of the propeller is open on both sides to allow random binding of several sugar units in the polymeric substrate arabinan. The beta-propeller fold is stabilized through a ring closure. ABN-TS exhibits a new closure-mode involving residues in the N-terminal region: Phe7 to Gly21 exhibit hydrogen bonds and hydrophobic interactions with the first and last blades, and Phe4 links the second and third blades through a hydrogen bond and an aromatic stacking interaction, respectively. The role of the N-terminal region in the thermostability was confirmed with a mutant lacking 16 amino acid residues from the N-terminus of ABN-TS.

Transcriptional regulation of genes encoding arabinan-degrading enzymes in Bacillus subtilis

Bacillus subtilis produces hemicellulases capable of releasing arabinosyl oligomers and arabinose from plant cell walls. In this work, we characterize the transcriptional regulation of three genes encoding arabinan-degrading enzymes that are clustered with genes encoding enzymes that further catabolize arabinose. The abfA gene comprised in the metabolic operon araABDLMNPQ-abfA and the xsa gene located 23 kb downstream most probably encode alpha-L-arabinofuranosidases (EC 3.2.1.55). Here, we show that the abnA gene, positioned immediately upstream from the metabolic operon, encodes an endo-alpha-1,5-arabinanase (EC 3.2.1.99). Furthermore, by in vivo RNA studies, we inferred that abnA and xsa are monocistronic and are transcribed from sigma(A)-like promoters. Transcriptional fusion analysis revealed that the expression of the three arabinases is induced by arabinose and arabinan and is repressed by glucose. The levels of induction by arabinose and arabinan are higher during early postexponential growth, suggesting a temporal regulation. Moreover, the induction mechanism of these genes is mediated through negative control by the key regulator of arabinose metabolism, AraR. Thus, we analyzed AraR-DNA interactions by in vitro quantitative DNase I footprinting and in vivo analysis of single-base-pair substitutions within the promoter regions of xsa and abnA. The results indicate that transcriptional repression of the abfA and xsa genes is achieved by a tightly controlled mechanism but that the regulation of abnA is more flexible. We suggest that the expression of genes encoding extracellular degrading enzymes of arabinose-containing polysaccharides, transport systems, and intracellular enzymes involved in further catabolism is regulated by a coordinate mechanism triggered by arabinose via AraR.

Cloning, characterization, and functional expression of the Klebsiella oxytoca xylodextrin utilization operon (xynTB) in Escherichia coli

Escherichia coli is being developed as a biocatalyst for bulk chemical production from inexpensive carbohydrates derived from lignocellulose. Potential substrates include the soluble xylodextrins (xyloside, xylooligosaccharide) and xylobiose that are produced by treatments designed to expose cellulose for subsequent enzymatic hydrolysis. Adjacent genes encoding xylobiose uptake and hydrolysis were cloned from Klebsiella oxytoca M5A1 and are functionally expressed in ethanologenic E. coli. The xylosidase encoded by xynB contains the COG3507 domain characteristic of glycosyl hydrolase family 43. The xynT gene encodes a membrane protein containing the MelB domain (COG2211) found in Na(+)/melibiose symporters and related proteins. These two genes form a bicistronic operon that appears to be regulated by xylose (XylR) and by catabolite repression in both K. oxytoca and recombinant E. coli. Homologs of this operon were found in Klebsiella pneumoniae, Lactobacillus lactis, E. coli, Clostridium acetobutylicum, and Bacillus subtilis based on sequence comparisons. Based on similarities in protein sequence, the xynTB genes in K. oxytoca appear to have originated from a gram-positive ancestor related to L. lactis. Functional expression of xynB allowed ethanologenic E. coli to metabolize xylodextrins (xylosides) containing up to six xylose residues without the addition of enzyme supplements. 4-O-methylglucuronic acid substitutions at the nonreducing termini of soluble xylodextrins blocked further degradation by the XynB xylosidase. The rate of xylodextrin utilization by recombinant E. coli was increased when a full-length xynT gene was included with xynB, consistent with xynT functioning as a symport. Hydrolysis rates were inversely related to xylodextrin chain length, with xylobiose as the preferred substrate. Xylodextrins were utilized more rapidly by recombinant E. coli than K. oxytoca M5A1 (the source of xynT and xynB). XynB exhibited weak arabinosidase activity, 3% that of xylosidase.