Tailored catalysts for plant cell-wall degradation: redesigning the exo/endo preference of Cellvibrio japonicus arabinanase 43A

Exo- and Endo-1,5-α-L-Arabinanases and Prebiotic Arabino-Oligosaccharides Production

There is growing interest in pentose-based prebiotic oligosaccharides as alternatives to traditional hexose-based prebiotics. Among these, arabino-oligosaccharides (AOS), derived from the enzymatic hydrolysis of arabinan polymers, have gained significant attention. AOS can selectively stimulate the growth of beneficial gut bacteria, including Bifidobacterium and Bacteroides species, and contribute to health-benefit functions such as blood sugar control, positioning AOS as a promising synbiotic candidate. For the industrial production of AOS, the development of efficient enzymatic processes is essential, with exo- and endo-1,5-α-L-arabinanases (exo- and endo-ABNs) playing a crucial catalytic role. Most ABNs belong to the glycoside hydrolase (GH) family 43, characterized by a five-bladed β-propeller fold structure. These enzymes hydrolyze internal α-1,5-L-arabinofuranosidic linkages, producing AOS with varying degrees of polymerization. Some ABNs GH43 were known to exhibit exo-type hydrolytic modes of action, producing specific AOS products such as arabinotriose. Additionally, exo-ABNs from GH93, which feature a six-bladed β-propeller fold, exclusively release arabinobiose through their exo-type catalytic mechanism. This review represents the first comprehensive analysis of exo- and endo-ABNs, offering scientific insights into their biotechnological potential for AOS production. It systematically compares enzyme classification, structural differences, catalytic mechanisms, paving the way for innovative applications in health, food, and pharmaceutical industries.

Arabinan hydrolysis by GH43 enzymes of Hungateiclostridium clariflavum and the potential synergistic mechanisms

Glycoside hydrolase family 43 (GH43) represents a major source of arabinan- and arabinoxylan-active enzymes. Interestingly, some microbes remarkably enriched GH genes of this family, with the reason unknown. Hungateiclostridium clariflavum DSM 19,732 is an efficient lignocellulose degrader, which harbors up to 7 GH43 genes in its genome. We cloned three of the seven GH43 genes, and found that Abn43A is a unique endoarabinanase, which unprecedently showed approximately two times larger activity on sugar beet arabinan (116.8 U/mg) than that on linear arabinan, and it is efficient in arabinooligosaccharide production. Abn43B is an exoarabinanase which directly releases arabinose from linear arabinan. Abn43C is an α-L-arabinofuranosidase which is capable of splitting the arabinose side-chains from arabinooligosaccharides, arabinoxylooligosaccharides, and arabinoxylan. Most importantly, the three GH43 enzymes synergized in hydrolyzing arabinan. Compared to Abn43B alone, a supplement of Abn43A increased the arabinose production from linear arabinan by 150%, reaching 0.44 g/g arabinan. Moreover, an addition of Abn43C to Abn43A and Abn43B boosted the arabinose production from sugar beet arabinan by 15 times, reaching 0.262 g/g arabinan. Our work suggested the intensified functions of multiple GH43 enzymes toward arabinan degradation in H. clariflavum, and a potential synergetic mechanism among the three GH43 enzymes is suggested. KEY POINTS: • Endoarabinanase GH43A prefers branched substrate to linear one • Exoarabinanase GH43B can directly release arabinose from linear arabinan • The three GH43 enzymes synergized in arabinan hydrolysis.

Active site dynamics and catalytic mechanism in arabinan hydrolysis catalyzed by GH43 endo-arabinanase from QM/MM molecular dynamics simulation and potential energy surface

The endo-1,5-α-L-arabinanases, belonging to glycoside hydrolase family 43 (GH43), catalyse the hydrolysis of α-1,5-arabinofuranosidic bonds in arabinose-containing polysaccharides. These enzymes are proposed targets for industrial and medical applications. Here, molecular dynamics (MD), potential energy surface and free energy (potential of mean force) simulations are undertaken using hybrid quantum mechanical/molecular mechanical (QM/MM) potentials to understand the active site dynamics, catalytic mechanism and the electrostatic influence of active site residues of the GH43 endo-arabinanase from G. stearothermophilus. The calculated results give support to the single-displacement mechanism proposed for the inverting GH43 enzymes: first a proton is transferred from the general acid E201 to the substrate, followed by a nucleophilic attack by water, activated by the general base D27, on the anomer carbon. A conformational change (²E ↔E₃ ↔ ⁴E) in the -1 sugar ring is observed involving a transition state featuring an oxocarbenium ion character. Residues D87, K106, H271 are highlighted as potential targets for future mutation experiments in order to increase the efficiency of the reaction. To our knowledge, this is the first QM/MM study providing molecular insights into the glycosidic bond hydrolysis of a furanoside substrate by an inverting GH in solution.Communicated by Ramaswamy H. Sarma.

Two distinct catalytic pathways for GH43 xylanolytic enzymes unveiled by X-ray and QM/MM simulations

Xylanolytic enzymes from glycoside hydrolase family 43 (GH43) are involved in the breakdown of hemicellulose, the second most abundant carbohydrate in plants. Here, we kinetically and mechanistically describe the non-reducing-end xylose-releasing exo-oligoxylanase activity and report the crystal structure of a native GH43 Michaelis complex with its substrate prior to hydrolysis. Two distinct calcium-stabilized conformations of the active site xylosyl unit are found, suggesting two alternative catalytic routes. These results are confirmed by QM/MM simulations that unveil the complete hydrolysis mechanism and identify two possible reaction pathways, involving different transition state conformations for the cleavage of xylooligosaccharides. Such catalytic conformational promiscuity in glycosidases is related to the open architecture of the active site and thus might be extended to other exo-acting enzymes. These findings expand the current general model of catalytic mechanism of glycosidases, a main reaction in nature, and impact on our understanding about their interaction with substrates and inhibitors.

Structural and functional characterization of a bifunctional GH30-7 xylanase B from the filamentous fungus Talaromyces cellulolyticus

Glucuronoxylanases are endo-xylanases and members of the glycoside hydrolase family 30 subfamilies 7 (GH30-7) and 8 (GH30-8). Unlike for the well-studied GH30-8 enzymes, the structural and functional characteristics of GH30-7 enzymes remain poorly understood. Here, we report the catalytic properties and three-dimensional structure of GH30-7 xylanase B (Xyn30B) identified from the cellulolytic fungus Talaromyces cellulolyticus Xyn30B efficiently degraded glucuronoxylan to acidic xylooligosaccharides (XOSs), including an α-1,2-linked 4-O-methyl-d-glucuronosyl substituent (MeGlcA). Rapid analysis with negative-mode electrospray-ionization multistage MS (ESI(-)-MS ⁿ ) revealed that the structures of the acidic XOS products are the same as those of the hydrolysates (MeGlcA²Xyl _n , n > 2) obtained with typical glucuronoxylanases. Acidic XOS products were further degraded by Xyn30B, releasing first xylobiose and then xylotetraose and xylohexaose as transglycosylation products. This hydrolase reaction was unique to Xyn30B, and the substrate was cleaved at the xylobiose unit from its nonreducing end, indicating that Xyn30B is a bifunctional enzyme possessing both endo-glucuronoxylanase and exo-xylobiohydrolase activities. The crystal structure of Xyn30B was determined as the first structure of a GH30-7 xylanase at 2.25 Å resolution, revealing that Xyn30B is composed of a pseudo-(α/β)₈-catalytic domain, lacking an α6 helix, and a small β-rich domain. This structure and site-directed mutagenesis clarified that Arg⁴⁶, conserved in GH30-7 glucuronoxylanases, is a critical residue for MeGlcA appendage-dependent xylan degradation. The structural comparison between Xyn30B and the GH30-8 enzymes suggests that Asn⁹³ in the β2-α2 loop is involved in xylobiohydrolase activity. In summary, our findings indicate that Xyn30B is a bifunctional endo- and exo-xylanase.

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.

EcXyl43 β-xylosidase: molecular modeling, activity on natural and artificial substrates, and synergism with endoxylanases for lignocellulose deconstruction

Biomass hydrolysis constitutes a bottleneck for the biotransformation of lignocellulosic residues into bioethanol and high-value products. The efficient deconstruction of polysaccharides to fermentable sugars requires multiple enzymes acting concertedly. GH43 β-xylosidases are among the most interesting enzymes involved in hemicellulose deconstruction into xylose. In this work, the structural and functional properties of β-xylosidase EcXyl43 from Enterobacter sp. were thoroughly characterized. Molecular modeling suggested a 3D structure formed by a conserved N-terminal catalytic domain linked to an ancillary C-terminal domain. Both domains resulted essential for enzymatic activity, and the role of critical residues, from the catalytic and the ancillary modules, was confirmed by mutagenesis. EcXyl43 presented β-xylosidase activity towards natural and artificial substrates while arabinofuranosidase activity was only detected on nitrophenyl α-L-arabinofuranoside (pNPA). It hydrolyzed xylobiose and purified xylooligosaccharides (XOS), up to degree of polymerization 6, with higher activity towards longer XOS. Low levels of activity on commercial xylan were also observed, mainly on the soluble fraction. The addition of EcXyl43 to GH10 and GH11 endoxylanases increased the release of xylose from xylan and pre-treated wheat straw. Additionally, EcXyl43 exhibited high efficiency and thermal stability under its optimal conditions (40 °C, pH 6.5), with a half-life of 58 h. Therefore, this enzyme could be a suitable additive for hemicellulases in long-term hydrolysis reactions. Because of its moderate inhibition by monomeric sugars but its high inhibition by ethanol, EcXyl43 could be particularly more useful in separate hydrolysis and fermentation (SHF) than in simultaneous saccharification and co-fermentation (SSCF) or consolidated bioprocessing (CBP).

GH43 endo-arabinanase from Bacillus licheniformis: Structure, activity and unexpected synergistic effect on cellulose enzymatic hydrolysis

The hydrolysis of the plant biomass provides many interesting opportunities for the generation of building blocks for the green chemistry industrial applications. An important progress has been made for the hydrolysis of the cellulosic component of the biomass while, for the hemicellulosic components, the advances are less straightforward. Here, we describe the cloning, expression and biochemical and structural characterization of BlAbn1, a GH43 arabinanase from Bacillus licheniformis. This enzyme is selective for linear arabinan and efficiently hydrolyzes this substrate, with a specific activity of 127 U/mg. The enzyme has optimal conditions for activity at pH 8.0 and 45 °C and its activity is only partially dependent of a bound calcium ion since 70% of the maximal activity is preserved even when 1 mM EDTA is added to the reaction medium. BlAbn1 crystal structure revealed a typical GH43 fold and narrow active site, which explains the selectivity for linear substrates. Unexpectedly, the enzyme showed a synergic effect with the commercial cocktail Accellerase 1500 on cellulose hydrolysis. Scanning Electron Microscopy, Solid-State NMR and relaxometry data indicate that the enzyme weakens the interaction between cellulose fibers in filter paper, thus providing an increased access to the cellulases of the cocktail.

Broad Analysis of Vicinal Disulfides: Occurrences, Conformations with Cis or with Trans Peptides, and Functional Roles Including Sugar Binding

Vicinal disulfides between sequence-adjacent cysteine residues are very rare and rather startling structural features which play a variety of functional roles. Typically discussed as an isolated curiosity, they have never received a general treatment covering both cis and trans forms. Enabled by the growing database of high-resolution structures, required deposition of diffraction data, and improved methods for discriminating reliable from dubious cases, we identify and describe distinct protein families with reliably genuine examples of cis or trans vicinal disulfides and discuss their conformations, conservation, and functions. No cis-trans interconversions and only one case of catalytic redox function are seen. Some vicinal disulfides are essential to large, functionally coupled motions, whereas most form the centers of tightly packed internal regions. Their most widespread biological role is providing a rigid hydrophobic contact surface under the undecorated side of a sugar or multiring ligand, contributing an important aspect of binding specificity.

The Mechanism by Which Arabinoxylanases Can Recognize Highly Decorated Xylans

The enzymatic degradation of plant cell walls is an important biological process of increasing environmental and industrial significance. Xylan, a major component of the plant cell wall, consists of a backbone of β-1,4-xylose (Xylp) units that are often decorated with arabinofuranose (Araf) side chains. A large penta-modular enzyme, CtXyl5A, was shown previously to specifically target arabinoxylans. The mechanism of substrate recognition displayed by the enzyme, however, remains unclear. Here we report the crystal structure of the arabinoxylanase and the enzyme in complex with ligands. The data showed that four of the protein modules adopt a rigid structure, which stabilizes the catalytic domain. The C-terminal non-catalytic carbohydrate binding module could not be observed in the crystal structure, suggesting positional flexibility. The structure of the enzyme in complex with Xylp-β-1,4-Xylp-β-1,4-Xylp-[α-1,3-Araf]-β-1,4-Xylp showed that the Araf decoration linked O₃ to the xylose in the active site is located in the pocket (−2* subsite) that abuts onto the catalytic center. The −2* subsite can also bind to Xylp and Arap, explaining why the enzyme can utilize xylose and arabinose as specificity determinants. Alanine substitution of Glu⁶⁸, Tyr⁹², or Asn¹³⁹, which interact with arabinose and xylose side chains at the −2* subsite, abrogates catalytic activity. Distal to the active site, the xylan backbone makes limited apolar contacts with the enzyme, and the hydroxyls are solvent-exposed. This explains why CtXyl5A is capable of hydrolyzing xylans that are extensively decorated and that are recalcitrant to classic endo-xylanase attack.

Polysaccharide degradation systems of the saprophytic bacterium Cellvibrio japonicus

Study of recalcitrant polysaccharide degradation by bacterial systems is critical for understanding biological processes such as global carbon cycling, nutritional contributions of the human gut microbiome, and the production of renewable fuels and chemicals. One bacterium that has a robust ability to degrade polysaccharides is the Gram-negative saprophyte Cellvibrio japonicus. A bacterium with a circuitous history, C. japonicus underwent several taxonomy changes from an initially described Pseudomonas sp. Most of the enzymes described in the pre-genomics era have also been renamed. This review aims to consolidate the biochemical, structural, and genetic data published on C. japonicus and its remarkable ability to degrade cellulose, xylan, and pectin substrates. Initially, C. japonicus carbohydrate-active enzymes were studied biochemically and structurally for their novel polysaccharide binding and degradation characteristics, while more recent systems biology approaches have begun to unravel the complex regulation required for lignocellulose degradation in an environmental context. Also included is a discussion for the future of C. japonicus as a model system, with emphasis on current areas unexplored in terms of polysaccharide degradation and emerging directions for C. japonicus in both environmental and biotechnological applications.

Carbohydrate-active enzymes: sequences, shapes, contortions and cells

The enzyme-catalysed degradation of oligo and polysaccharides is of considerable interest in many fields ranging from the fundamental–understanding the intrinsic chemical beauty–through to the applied, including diverse practical applications in medicine and biotechnology. Carbohydrates are the most stereochemically-complex biopolymer, and myriad different natural polysaccharides have led to evolution of multifaceted enzyme consortia for their degradation. The glycosidic bonds that link sugar monomers are among the most chemically-stable, yet enzymatically-labile, bonds in the biosphere. That glycoside hydrolases can achieve a rate enhancement (kcat/kuncat) >1017-fold provides testament to their remarkable proficiency and the sophistication of their catalysis reaction mechanisms. The last two decades have seen significant advances in the discovery of new glycosidase sequences, sequence-based classification into families and clans, 3D structures and reaction mechanisms, providing new insights into enzymatic catalysis. New impetus to these studies has been provided by the challenges inherent in plant and microbial polysaccharide degradation, both in the context of environmentally-sustainable routes to foods and biofuels, and increasingly in human nutrition. Study of the reaction mechanism of glycoside hydrolases has also inspired the development of enzyme inhibitors, both as mechanistic probes and increasingly as therapeutic agents. We are on the cusp of a new era where we are learning how to dovetail powerful computational techniques with structural and kinetic data to provide an unprecedented view of conformational details of enzyme action.

Processivity, Substrate Positioning, and Binding: The Role of Polar Residues in a Family 18 Glycoside Hydrolase

The enzymatic degradation of recalcitrant polysaccharides such as cellulose (β-1,4-linked glucose) and chitin (β-1,4-linked N-acetylglucosamine) by glycoside hydrolases (GHs) is of significant biological and economical importance. In nature, depolymerization is primarily accomplished by processive GHs, which remain attached to the substrate between subsequent hydrolytic reactions. Recent computational efforts have suggested that the processive ability of a GH is directly linked to the ligand binding free energy. The contribution of individual aromatic residues in the active site of these enzymes has been extensively studied. In this study, we offer the first experimental evidence confirming correlation of binding free energy and degree of processivity and evidence that polar residues are essential for maintaining processive ability. Exchanging Thr(276) with Ala in substrate binding subsite -2 in the processive ChiA of Serratia marcescens results in a decrease in both the enthalpy (2.6 and 3.8 kcal/mol) and free energy (0.5 and 2.2 kcal/mol) for the binding to the substrate (GlcNAc)6 and the inhibitor allosamidin, respectively, compared to that of the wild type. Moreover, the initial apparent processivity as measured by [(GlcNAc)2]/[GlcNAc] ratios (17.1 ± 0.4) and chitin degradation efficiency (20%) are greatly reduced for ChiA-T276A versus those of the wild type (30.1 ± 1.5 and 75%, respectively). Mutation of Arg(172) to Ala reduces the level of recognition and positioning of the substrate into the active site. Molecular dynamics simulations indicate ChiA-R172A behaves like the wild type, but the dynamics of ChiA-T276A are greatly influenced by mutation, which is reflective of their influence on processivity.

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.

Structure-based conversion of the coenzyme requirement of a short-chain dehydrogenase/reductase involved in bacterial alginate metabolism

The alginate-assimilating bacterium, Sphingomonas sp. strain A1, degrades the polysaccharides to monosaccharides through four alginate lyase reactions. The resultant monosaccharide, which is nonenzymatically converted to 4-deoxy-L-erythro-5-hexoseulose uronate (DEH), is further metabolized to 2-keto-3-deoxy-D-gluconate by NADPH-dependent reductase A1-R in the short-chain dehydrogenase/reductase (SDR) family. A1-R-deficient cells produced another DEH reductase, designated A1-R', with a preference for NADH. Here, we show the identification of a novel NADH-dependent DEH reductase A1-R' in strain A1, structural determination of A1-R' by x-ray crystallography, and structure-based conversion of a coenzyme requirement in SDR enzymes, A1-R and A1-R'. A1-R' was purified from strain A1 cells and enzymatically characterized. Except for the coenzyme requirement, there was no significant difference in enzyme characteristics between A1-R and A1-R'. Crystal structures of A1-R' and A1-R'·NAD(+) complex were determined at 1.8 and 2.7 Å resolutions, respectively. Because of a 64% sequence identity, overall structures of A1-R' and A1-R were similar, although a difference in the coenzyme-binding site (particularly the nucleoside ribose 2' region) was observed. Distinct from A1-R, A1-R' included a negatively charged, shallower binding site. These differences were caused by amino acid residues on the two loops around the site. The A1-R' mutant with the two A1-R-typed loops maintained potent enzyme activity with specificity for NADPH rather than NADH, demonstrating that the two loops determine the coenzyme requirement, and loop exchange is a promising method for conversion of coenzyme requirement in the SDR family.

GH52 xylosidase from Geobacillus stearothermophilus: characterization and introduction of xylanase activity by site‑directed mutagenesis of Tyr509

A xylosidase gene, gsxyn, was cloned from the deep-sea thermophilic Geobacillus stearothermophilus, which consisted of 2,118 bp and encoded a protein of 705 amino acids with a calculated molecular mass of 79.8 kDa. The GSxyn of glycoside hydrolase family 52 (GH52) displayed its maximum activity at 70 °C and pH 5.5. The K m and k cat values of GSxyn for ρNPX were 0.48 mM and 36.64 s−1, respectively. Interestingly, a new exo-xylanase activity was introduced into GSxyn by mutating the tyrosine509 into glutamic acid, whereas the resultant enzyme variant, Y509E, retained the xylosidase activity. The optimum xylanase activity of theY509E mutant displayed at pH 6.5 and 50 °C, and retained approximately 45 % of its maximal activity at 55 °C, pH 6.5 for 60 min. The K m and k cat values of the xylanase activity of Y509E mutant for beechwood xylan were 5.10 mg/ml and 22.53 s−1, respectively. The optimum xylosidase activity of theY509E mutant displayed at pH 5.5 and 60 °C. The K m and k cat values of the xylosidase activity of Y509E mutant for ρNPX were 0.51 mM and 22.53 s−1, respectively. This report demonstrated that GH52 xylosidase has provided a platform for generating bifunctional enzymes for industrially significant and complex substrates, such as plant cell wall.

Crystal structure and characterization of the glycoside hydrolase family 62 α-L-arabinofuranosidase from Streptomyces coelicolor

α-L-arabinofuranosidase, which belongs to the glycoside hydrolase family 62 (GH62), hydrolyzes arabinoxylan but not arabinan or arabinogalactan. The crystal structures of several α-L-arabinofuranosidases have been determined, although the structures, catalytic mechanisms, and substrate specificities of GH62 enzymes remain unclear. To evaluate the substrate specificity of a GH62 enzyme, we determined the crystal structure of α-L-arabinofuranosidase, which comprises a carbohydrate-binding module family 13 domain at its N terminus and a catalytic domain at its C terminus, from Streptomyces coelicolor. The catalytic domain was a five-bladed β-propeller consisting of five radially oriented anti-parallel β-sheets. Sugar complex structures with l-arabinose, xylotriose, and xylohexaose revealed five subsites in the catalytic cleft and an l-arabinose-binding pocket at the bottom of the cleft. The entire structure of this GH62 family enzyme was very similar to that of glycoside hydrolase 43 family enzymes, and the catalytically important acidic residues found in family 43 enzymes were conserved in GH62. Mutagenesis studies revealed that Asp(202) and Glu(361) were catalytic residues, and Trp(270), Tyr(461), and Asn(462) were involved in the substrate-binding site for discriminating the substrate structures. In particular, hydrogen bonding between Asn(462) and xylose at the nonreducing end subsite +2 was important for the higher activity of substituted arabinofuranosyl residues than that for terminal arabinofuranoses.

Mechanistic strategies for catalysis adopted by evolutionary distinct family 43 arabinanases

Arabinanases (ABNs, EC 3.2.1.99) are promising catalysts for environmentally friendly biomass conversion into energy and chemicals. These enzymes catalyze the hydrolysis of the α-1,5-linked L-arabinofuranoside backbone of plant cell wall arabinans releasing arabino-oligosaccharides and arabinose, the second most abundant pentose in nature. In this work, new findings about the molecular mechanisms governing activation, functional differentiation, and catalysis of GH43 ABNs are presented. Biophysical, mutational, and biochemical studies with the hyperthermostable two-domain endo-acting ABN from Thermotoga petrophila (TpABN) revealed how some GH43 ABNs are activated by calcium ions via hyperpolarization of the catalytically relevant histidine and the importance of the ancillary domain for catalysis and conformational stability. On the other hand, the two GH43 ABNs from rumen metagenome, ARN2 and ARN3, presented a calcium-independent mechanism in which sodium is the most likely substituent for calcium ions. The crystal structure of the two-domain endo-acting ARN2 showed that its ability to efficiently degrade branched substrates is due to a larger catalytic interface with higher accessibility than that observed in other ABNs with preference for linear arabinan. Moreover, crystallographic characterization of the single-domain exo-acting ARN3 indicated that its cleavage pattern producing arabinose is associated with the chemical recognition of the reducing end of the substrate imposed by steric impediments at the aglycone-binding site. By structure-guided rational design, ARN3 was converted into a classical endo enzyme, confirming the role of the extended Arg(203)-Ala(230) loop in determining its action mode. These results reveal novel molecular aspects concerning the functioning of GH43 ABNs and provide new strategies for arabinan degradation.

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.

Activation of a GH43 β-xylosidase by divalent metal cations: slow binding of divalent metal and high substrate specificity

RS223-BX of glycoside hydrolase family 43 is a β-d-xylosidase that is strongly activated (k(cat)/K(m) as much as 116-fold) by the addition of divalent metal cations, Ca(2+), Co(2+), Fe(2+), Mg(2+), Mn(2+) and Ni(2+). Slow activation by Mg(2+) was demonstrated (k(on) 0.013 s(-1) mM(-1), k(off) 0.008 s(-1)) at pH 7.0 and 25 °C. k(off) and k(on) values are independent of Mg(2+) concentration, but k(off) and k(on) are slower in the presence of increasing levels of substrate 4-nitrophenyl-β-D-xylopyranoside. The kinetics strongly suggest that M(2+) binds to the enzyme rapidly, forming E M(2+), followed by slow isomerization to the activated enzyme, E* M(2+). Moderately high values of kcat (7-30 s(-1)) were found for M(2+)-activated RS223-BX acting on xylobiose (natural substrate) at pH 7.0 and 25 °C. Certain M(2+)-activated RS223-BX exhibit the highest reported values of k(cat)/K(m) of any β-xylosidase acting on natural substrates: for example, at pH 7.0 and 25°C, xylobiose (Mn(2+), 190 s(-1) mM(-1)), xylotriose (Ca(2+), 150 s(-1) mM(-1)) and xylotetraose (Ca(2+), 260 s(-1) mM(-1)). There is potential for the enzyme to add value to industrial saccharification operations at low substrate and high d-glucose and high d-xylose concentrations.

Detailed modes of action and biochemical characterization of endo-arabinanase from Bacillus licheniformis DSM13

An endo-arabinanase (BLABNase) gene from Bacillus licheniformis DSM13 was cloned and expressed in Escherichia coli, and the biochemical properties of its encoded enzyme were characterized. The BLABNase gene consists of a single open reading frame of 987 nucleotides that encodes 328 amino acids with a predicted molecular mass of about 36 kDa. BLABNase exhibited the highest activity against debranched α-(1,5)-arabinan in 50 mM sodium acetate buffer (pH 6.0) at 55°C. Enzymatic characterization revealed that BLABNase hydrolyzes debranched or linear arabinans with a much higher activity than branched arabinan from sugar beet. Enzymatic hydrolysis pattern analyses demonstrated BLABNase to be a typical endo-(1,5)-α-S-arabinanase (EC 3.2.1.99) that randomly cleaves the internal α-(1,5)-linked L-arabinofuranosyl residues of a branchless arabinan backbone to release arabinotriose mainly, although a small amount of arabino-oligosaccharide intermediates is also liberated. Our results indicated that BLABNase acts preferentially along with the oligosaccharides longer than arabinopentaose, thus enabling the enzymatic production of various arabino-oligosaccharides.

Divalent metal activation of a GH43 β-xylosidase

Depolymerization of xylan, a major fraction of lignocellulosic biomass, releases xylose which can be converted into transportation fuels and chemical feedstocks. A requisite enzyme for the breakdown of xylan is β-xylosidase. A gene encoding the 324-amino acid β-xylosidase, RS223-BX, was cloned from an anaerobic mixed microbial culture. This glycoside hydrolase belongs to family 43. Unlike other GH43 enzymes, RS223-BX can be strongly activated by exogenously supplied Ca(2+), Co(2+), Fe(2+), Mg(2+), Mn(2+) and Ni(2+) (e.g., 28-fold by Mg(2+)) and it is inhibited by Cu(2+) or Zn(2+). Sedimentation equilibrium centrifugation experiments indicated that the divalent metal cations mediate multimerization of the enzyme from a dimeric to a tetrameric state, which have equal catalytic activity on an active-site basis. Compared to the determined active sites of other GH43 β-xylosidases, the predicted active site of RS223-BX contains two additional amino acids with carboxylated side chains that provide potential sites for divalent metal cations to reside. Thus, the divalent metal cations likely occupy the active site and participate in the catalytic mechanism. RS223-BX accepts as substrate xylobiose, arabinobiose, 4-nitrophenyl-β-D-xylopyranoside, and 4-nitrophenyl-α-L-arabinofuranoside. Additionally, the enzyme has good pH and temperature stabilities and a large K(i) for D-glucose (1.3 M), favorable properties for performance in saccharification reactors.

Hallmarks of processivity in glycoside hydrolases from crystallographic and computational studies of the Serratia marcescens chitinases

Degradation of recalcitrant polysaccharides in nature is typically accomplished by mixtures of processive and nonprocessive glycoside hydrolases (GHs), which exhibit synergistic activity wherein nonprocessive enzymes provide new sites for productive attachment of processive enzymes. GH processivity is typically attributed to active site geometry, but previous work has demonstrated that processivity can be tuned by point mutations or removal of single loops. To gain additional insights into the differences between processive and nonprocessive enzymes that give rise to their synergistic activities, this study reports the crystal structure of the catalytic domain of the GH family 18 nonprocessive endochitinase, ChiC, from Serratia marcescens. This completes the structural characterization of the co-evolved chitinolytic enzymes from this bacterium and enables structural analysis of their complementary functions. The ChiC catalytic module reveals a shallow substrate-binding cleft that lacks aromatic residues vital for processivity, a calcium-binding site not previously seen in GH18 chitinases, and, importantly, a displaced catalytic acid (Glu-141), suggesting flexibility in the catalytic center. Molecular dynamics simulations of two processive chitinases (ChiA and ChiB), the ChiC catalytic module, and an endochitinase from Lactococcus lactis show that the nonprocessive enzymes have more flexible catalytic machineries and that their bound ligands are more solvated and flexible. These three features, which relate to the more dynamic on-off ligand binding processes associated with nonprocessive action, correlate to experimentally measured differences in processivity of the S. marcescens chitinases. These newly defined hallmarks thus appear to be key dynamic metrics in determining processivity in GH enzymes complementing structural insights.

Introducing endo-xylanase activity into an exo-acting arabinofuranosidase that targets side chains

The degradation of the plant cell wall by glycoside hydrolases is central to environmentally sustainable industries. The major polysaccharides of the plant cell wall are cellulose and xylan, a highly decorated β-1,4-xylopyranose polymer. Glycoside hydrolases displaying multiple catalytic functions may simplify the enzymes required to degrade plant cell walls, increasing the industrial potential of these composite structures. Here we test the hypothesis that glycoside hydrolase family 43 (GH43) provides a suitable scaffold for introducing additional catalytic functions into enzymes that target complex structures in the plant cell wall. We report the crystal structure of Humicola insolens AXHd3 (HiAXHd3), a GH43 arabinofuranosidase that hydrolyses O3-linked arabinose of doubly substituted xylans, a feature of the polysaccharide that is recalcitrant to degradation. HiAXHd3 displays an N-terminal five-bladed β-propeller domain and a C-terminal β-sandwich domain. The interface between the domains comprises a xylan binding cleft that houses the active site pocket. Substrate specificity is conferred by a shallow arabinose binding pocket adjacent to the deep active site pocket, and through the orientation of the xylan backbone. Modification of the rim of the active site introduces endo-xylanase activity, whereas the resultant enzyme variant, Y166A, retains arabinofuranosidase activity. These data show that the active site of HiAXHd3 is tuned to hydrolyse arabinofuranosyl or xylosyl linkages, and it is the topology of the distal regions of the substrate binding surface that confers specificity. This report demonstrates that GH43 provides a platform for generating bespoke multifunctional enzymes that target industrially significant complex substrates, exemplified by the plant cell wall.

The structure and function of an arabinan-specific alpha-1,2-arabinofuranosidase identified from screening the activities of bacterial GH43 glycoside hydrolases

Reflecting the diverse chemistry of plant cell walls, microorganisms that degrade these composite structures synthesize an array of glycoside hydrolases. These enzymes are organized into sequence-, mechanism-, and structure-based families. Genomic data have shown that several organisms that degrade the plant cell wall contain a large number of genes encoding family 43 (GH43) glycoside hydrolases. Here we report the biochemical properties of the GH43 enzymes of a saprophytic soil bacterium, Cellvibrio japonicus, and a human colonic symbiont, Bacteroides thetaiotaomicron. The data show that C. japonicus uses predominantly exo-acting enzymes to degrade arabinan into arabinose, whereas B. thetaiotaomicron deploys a combination of endo- and side chain-cleaving glycoside hydrolases. Both organisms, however, utilize an arabinan-specific α-1,2-arabinofuranosidase in the degradative process, an activity that has not previously been reported. The enzyme can cleave α-1,2-arabinofuranose decorations in single or double substitutions, the latter being recalcitrant to the action of other arabinofuranosidases. The crystal structure of the C. japonicus arabinan-specific α-1,2-arabinofuranosidase, CjAbf43A, displays a five-bladed β-propeller fold. The specificity of the enzyme for arabinan is conferred by a surface cleft that is complementary to the helical backbone of the polysaccharide. The specificity of CjAbf43A for α-1,2-l-arabinofuranose side chains is conferred by a polar residue that orientates the arabinan backbone such that O2 arabinose decorations are directed into the active site pocket. A shelflike structure adjacent to the active site pocket accommodates O3 arabinose side chains, explaining how the enzyme can target O2 linkages that are components of single or double substitutions.

Structure and function of an arabinoxylan-specific xylanase

The enzymatic degradation of plant cell walls plays a central role in the carbon cycle and is of increasing environmental and industrial significance. The enzymes that catalyze this process include xylanases that degrade xylan, a β-1,4-xylose polymer that is decorated with various sugars. Although xylanases efficiently hydrolyze unsubstituted xylans, these enzymes are unable to access highly decorated forms of the polysaccharide, such as arabinoxylans that contain arabinofuranose decorations. Here, we show that a Clostridium thermocellum enzyme, designated CtXyl5A, hydrolyzes arabinoxylans but does not attack unsubstituted xylans. Analysis of the reaction products generated by CtXyl5A showed that all the oligosaccharides contain an O3 arabinose linked to the reducing end xylose. The crystal structure of the catalytic module (CtGH5) of CtXyl5A, appended to a family 6 noncatalytic carbohydrate-binding module (CtCBM6), showed that CtGH5 displays a canonical (α/β)(8)-barrel fold with the substrate binding cleft running along the surface of the protein. The catalytic apparatus is housed in the center of the cleft. Adjacent to the -1 subsite is a pocket that could accommodate an l-arabinofuranose-linked α-1,3 to the active site xylose, which is likely to function as a key specificity determinant. CtCBM6, which adopts a β-sandwich fold, recognizes the termini of xylo- and gluco-configured oligosaccharides, consistent with the pocket topology displayed by the ligand-binding site. In contrast to typical modular glycoside hydrolases, there is an extensive hydrophobic interface between CtGH5 and CtCBM6, and thus the two modules cannot function as independent entities.

Mode of action of Chrysosporium lucknowense C1 α-l-arabinohydrolases

The mode of action of four Chrysosporium lucknowense C1 α-L-arabinohydrolases was determined to enable controlled and effective degradation of arabinan. The active site of endoarabinanase Abn1 has at least six subsites, of which the subsites -1 to +2 have to be occupied for hydrolysis. Abn1 was able to hydrolyze a branched arabinohexaose with a double substituted arabinose at subsite -2. The exo acting enzymes Abn2, Abn4 and Abf3 release arabinobiose (Abn2) and arabinose (Abn4 and Abf3) from the non-reducing end of reduced arabinose oligomers. Abn2 binds the two arabinose units only at the subsites -1 and -2. Abf3 prefers small oligomers over large oligomers. It is able to hydrolyze all linkages present in beet arabinan, including the linkages of double substituted residues. Abn4 is more active towards polymeric substrate and releases arabinose monomers from single substituted arabinose residues. Depending on the combination of the enzymes, the C1 arabinohydrolases can be used to effectively release branched arabinose oligomers and/or arabinose monomers.

Adaptation yields a highly efficient xylose-fermenting Zymomonas mobilis strain

Zymomonas mobilis is a superb ethanol producer with productivity exceeding yeast strains by several fold. Although metabolic engineering was successfully applied to expand its substrate range to include xylose, xylose fermentation lagged far behind glucose. In addition, xylose fermentation was often incomplete when its initial concentration was higher than 5%. Improvement of xylose fermentation is therefore necessary. In this work, we applied adaptation to improve xylose fermentation in metabolically engineered strains. As a result of adaptation over 80 days and 30 serial transfers in a medium containing high concentration of xylose, a strain, referred as A3, with markedly improved xylose metabolism was obtained. The strain was able to grow on 10% (w/v) xylose and rapidly ferment xylose to ethanol within 2 days and retained high ethanol yield. Similarly, in mixed glucose-xylose fermentation, a total of 9% (w/v) ethanol was obtained from two doses of 5% glucose and 5% xylose (or a total of 10% glucose and 10% xylose). Further investigation reveals evidence for an altered xylitol metabolism in A3 with reduced xylitol formation. Additionally xylitol tolerance in A3 was increased. Furthermore, xylose isomerase activity was increased by several times in A3, allowing cells to channel more xylose to ethanol than to xylitol. Taken together, these results strongly suggest that altered xylitol metabolism is key to improved xylose metabolism in adapted A3 strain. This work further demonstrates that adaptation and metabolic engineering can be used synergistically for strain improvement.

Circular permutation provides an evolutionary link between two families of calcium-dependent carbohydrate binding modules

The microbial deconstruction of the plant cell wall is a critical biological process, which also provides important substrates for environmentally sustainable industries. Enzymes that hydrolyze the plant cell wall generally contain non-catalytic carbohydrate binding modules (CBMs) that contribute to plant cell wall degradation. Here we report the biochemical properties and crystal structure of a family of CBMs (CBM60) that are located in xylanases. Uniquely, the proteins display broad ligand specificity, targeting xylans, galactans, and cellulose. Some of the CBM60s display enhanced affinity for their ligands through avidity effects mediated by protein dimerization. The crystal structure of vCBM60, displays a β-sandwich with the ligand binding site comprising a broad cleft formed by the loops connecting the two β-sheets. Ligand recognition at site 1 is, exclusively, through hydrophobic interactions, whereas binding at site 2 is conferred by polar interactions between a protein-bound calcium and the O2 and O3 of the sugar. The observation, that ligand recognition at site 2 requires only a β-linked sugar that contains equatorial hydroxyls at C2 and C3, explains the broad ligand specificity displayed by vCBM60. The ligand-binding apparatus of vCBM60 displays remarkable structural conservation with a family 36 CBM (CBM36); however, the residues that contribute to carbohydrate recognition are derived from different regions of the two proteins. Three-dimensional structure-based sequence alignments reveal that CBM36 and CBM60 are related by circular permutation. The biological and evolutionary significance of the mechanism of ligand recognition displayed by family 60 CBMs is discussed.

New evidence for the role of calcium in the glycosidase reaction of GH43 arabinanases

Endo-1,5-α-L-arabinanases are glycosyl hydrolases that are able to cleave the glycosidic bonds of α-1,5-L-arabinan, releasing arabino-oligosaccharides and L-arabinose. Two extracellular endo-1,5-α-L-arabinanases have been isolated from Bacillus subtilis, BsArb43A and BsArb43B (formally named AbnA and Abn2, respectively). BsArb43B shows low sequence identity with previously characterized 1,5-α-L-arabinanases and is a much larger enzyme. Here we describe the 3D structure of native BsArb43B, biochemical and structure characterization of two BsArb43B mutant proteins (H318A and D171A), and the 3D structure of the BsArb43B D171A mutant enzyme in complex with arabinohexose. The 3D structure of BsArb43B is different from that of other structurally characterized endo-1,5-α-L-arabinanases, as it comprises two domains, an N-terminal catalytic domain, with a 3D fold similar to that observed for other endo-1,5-α-L-arabinanases, and an additional C-terminal domain. Moreover, this work also provides experimental evidence for the presence of a cluster containing a calcium ion in the catalytic domain, and the importance of this calcium ion in the enzymatic mechanism of BsArb43B.

Crystal structure of an Exo-1,5-{alpha}-L-arabinofuranosidase from Streptomyces avermitilis provides insights into the mechanism of substrate discrimination between exo- and endo-type enzymes in glycoside hydrolase family 43

Exo-1,5-α-L-arabinofuranosidases belonging to glycoside hydrolase family 43 have strict substrate specificity. These enzymes hydrolyze only the α-1,5-linkages of linear arabinan and arabino-oligosaccharides in an exo-acting manner. The enzyme from Streptomyces avermitilis contains a core catalytic domain belonging to glycoside hydrolase family 43 and a C-terminal arabinan binding module belonging to carbohydrate binding module family 42. We determined the crystal structure of intact exo-1,5-α-L-arabinofuranosidase. The catalytic module is composed of a 5-bladed β-propeller topologically identical to the other family 43 enzymes. The arabinan binding module had three similar subdomains assembled against one another around a pseudo-3-fold axis, forming a β-trefoil-fold. A sugar complex structure with α-1,5-L-arabinofuranotriose revealed three subsites in the catalytic domain, and a sugar complex structure with α-L-arabinofuranosyl azide revealed three arabinose-binding sites in the carbohydrate binding module. A mutagenesis study revealed that substrate specificity was regulated by residues Asn-159, Tyr-192, and Leu-289 located at the aglycon side of the substrate-binding pocket. The exo-acting manner of the enzyme was attributed to the strict pocket structure of subsite -1, formed by the flexible loop region Tyr-281-Arg-294 and the side chain of Tyr-40, which occupied the positions corresponding to the catalytic glycon cleft of GH43 endo-acting enzymes.

Crystal structure of family 14 polysaccharide lyase with pH-dependent modes of action

The Chlorella virus enzyme vAL-1 (38 kDa), a member of polysaccharide lyase family 14, degrades the Chlorella cell wall by cleaving the glycoside bond of the glucuronate residue (GlcA) through a beta-elimination reaction. The enzyme consists of an N-terminal cell wall-attaching domain (11 kDa) and a C-terminal catalytic module (27 kDa). Here, we show the enzyme characteristics of vAL-1, especially its pH-dependent modes of action, and determine the structure of the catalytic module. vAL-1 also exhibited alginate lyase activity at alkaline pH, and truncation of the N-terminal domain increased the lyase activity by 50-fold at pH 7.0. The truncated form vAL-1(S) released di- to hexasaccharides from alginate at pH 7.0, whereas disaccharides were preferentially generated at pH 10.0. This indicates that vAL-1(S) shows two pH-dependent modes of action: endo- and exotypes. The x-ray crystal structure of vAL-1(S) at 1.2 A resolution showed two antiparallel beta-sheets with a deep cleft showing a beta-jelly roll fold. The structure of GlcA-bound vAL-1(S) at pH 7.0 and 10.0 was determined: GlcA was found to be bound outside and inside the cleft at pH 7.0 and 10.0, respectively. This suggests that the electric charges at the active site greatly influence the binding mode of substrates and regulate endo/exo activity. Site-directed mutagenesis demonstrated that vAL-1(S) has a specific amino acid arrangement distinct from other alginate lyases crucial for catalysis. This is, to our knowledge, the first study in which the structure of a family 14 polysaccharide lyase with two different modes of action has been determined.

Arabinan metabolism during seed development and germination in Arabidopsis

Arabinans are found in the pectic network of many cell walls, where, along with galactan, they are present as side chains of Rhamnogalacturonan l. Whilst arabinans have been reported to be abundant polymers in the cell walls of seeds from a range of plant species, their proposed role as a storage reserve has not been thoroughly investigated. In the cell walls of Arabidopsis seeds, arabinose accounts for approximately 40% of the monosaccharide composition of non-cellulosic polysaccharides of embryos. Arabinose levels decline to approximately 15% during seedling establishment, indicating that cell wall arabinans may be mobilized during germination. Immunolocalization of arabinan in embryos, seeds, and seedlings reveals that arabinans accumulate in developing and mature embryos, but disappear during germination and seedling establishment. Experiments using 14C-arabinose show that it is readily incorporated and metabolized in growing seedlings, indicating an active catabolic pathway for this sugar. We found that depleting arabinans in seeds using a fungal arabinanase causes delayed seedling growth, lending support to the hypothesis that these polymers may help fuel early seedling growth.