Biochemical characterization and identification of catalytic residues in alpha-glucuronidase from Bacillus stearothermophilus T-6

Use of xylosidase 3C from Segatella baroniae to discriminate xylan non-reducing terminus substitution characteristics

OBJECTIVE

New characterized carbohydrate-active enzymes are needed for use as tools to discriminate complex carbohydrate structural features. Fungal glycoside hydrolase family 3 (GH3) β-xylosidases have been shown to be useful for the structural elucidation of glucuronic acid (GlcA) and arabinofuranose (Araf) substituted oligoxylosides. A homolog of these GH3 fungal enzymes from the bacterium Segatella baroniae (basonym Prevotella bryantii), Xyl3C, has been previously characterized, but those studies did not address important functional specificity features. In an interest to utilize this enzyme for laboratory methods intended to discriminate the structure of the non-reducing terminus of substituted xylooligosaccharides, we have further characterized this GH3 xylosidase.

RESULTS

In addition to verification of basic functional characteristics of this xylosidase we have determined its mode of action as it relates to non-reducing end xylose release from GlcA and Araf substituted oligoxylosides. Xyl3C cleaves xylose from the non-reducing terminus of β-1,4-xylan until occurrence of a penultimate substituted xylose. If this substitution is O2 linked, then Xyl3C removes the non-reducing xylose to leave the substituted xylose as the new non-reducing terminus. However, if the substitution is O3 linked, Xyl3C does not hydrolyze, thus leaving the substitution one-xylose (penultimate) from the non-reducing terminus. Hence, Xyl3C enables discrimination between O2 and O3 linked substitutions on the xylose penultimate to the non-reducing end. These findings are contrasted using a homologous enzyme also from S. baroniae, Xyl3B, which is found to yield a penultimate substituted nonreducing terminus regardless of which GlcA or Araf substitution exists.

Glycomic profiling identifies key-structural differences in three arabinoxylan fractions from sugarcane culms

Sugarcane is an important food and bioenergy crop, and although the residual biomass is potentially available for biorefinery and biofuels production the complex plant cell wall matrix requires pretreatment prior to enzymatic hydrolysis. Arabinoxylans require multiple enzymes for xylose backbone and saccharide side-branch hydrolysis to release xylooligosaccharides and pentoses. The effect of arabinoxylan structure on xylooligosaccharide release by combinations of up to five xylanolytic enzymes was studied using three arabinoxylan fractions extracted from sugarcane culms by sodium chlorite, DMSO and alkaline treatments. Reducing sugar release and LC-MS detection with chemometric analysis identified different xylooligosaccharide profiles between extracts following enzyme treatments. The position and degree of side-branch decorations are determinants of enzyme activity and xylooligosaccharide diversity with the alkaline and post‑sodium chlorite extracts as the most accessible and most recalcitrant, respectively, indicating acetyl substituents as a major recalcitrance factor. The complex xylooligosaccharide profile with the DMSO extract suggests regions with different levels of branching. Chemometric analysis identified GH10 xylanase hydrolysis products that act as substrates for other enzymes, such as α-glucuronidase. The strategy reported here can identify specific enzyme combinations to overcome barriers for biomass processing such as pretreatment selection, recalcitrance to enzyme digestion and optimization of reducing sugar release.

Plastid Transformation: New Challenges in the Circular Economy Era

In a circular economy era the transition towards renewable and sustainable materials is very urgent. The development of bio-based solutions, that can ensure technological circularity in many priority areas (e.g., agriculture, biotechnology, ecology, green industry, etc.), is very strategic. The agricultural and fishing industry wastes represent important feedstocks that require the development of sustainable and environmentally-friendly industrial processes to produce and recover biofuels, chemicals and bioactive molecules. In this context, the replacement, in industrial processes, of chemicals with enzyme-based catalysts assures great benefits to humans and the environment. In this review, we describe the potentiality of the plastid transformation technology as a sustainable and cheap platform for the production of recombinant industrial enzymes, summarize the current knowledge on the technology, and display examples of cellulolytic enzymes already produced. Further, we illustrate several types of bacterial auxiliary and chitinases/chitin deacetylases enzymes with high biotechnological value that could be manufactured by plastid transformation.

Cross-utilization of β-galactosides and cellobiose in Geobacillus stearothermophilus

Strains of the Gram-positive, thermophilic bacterium Geobacillus stearothermophilus possess elaborate systems for the utilization of hemicellulolytic polysaccharides, including xylan, arabinan, and galactan. These systems have been studied extensively in strains T-1 and T-6, representing microbial models for the utilization of soil polysaccharides, and many of their components have been characterized both biochemically and structurally. Here, we characterized routes by which G. stearothermophilus utilizes mono- and disaccharides such as galactose, cellobiose, lactose, and galactosyl-glycerol. The G. stearothermophilus genome encodes a phosphoenolpyruvate carbohydrate phosphotransferase system (PTS) for cellobiose. We found that the cellobiose-PTS system is induced by cellobiose and characterized the corresponding GH1 6-phospho-β-glucosidase, Cel1A. The bacterium also possesses two transport systems for galactose, a galactose-PTS system and an ABC galactose transporter. The ABC galactose transport system is regulated by a three-component sensing system. We observed that both systems, the sensor and the transporter, utilize galactose-binding proteins that also bind glucose with the same affinity. We hypothesize that this allows the cell to control the flux of galactose into the cell in the presence of glucose. Unexpectedly, we discovered that G. stearothermophilus T-1 can also utilize lactose and galactosyl-glycerol via the cellobiose-PTS system together with a bifunctional 6-phospho-β-gal/glucosidase, Gan1D. Growth curves of strain T-1 growing in the presence of cellobiose, with either lactose or galactosyl-glycerol, revealed initially logarithmic growth on cellobiose and then linear growth supported by the additional sugars. We conclude that Gan1D allows the cell to utilize residual galactose-containing disaccharides, taking advantage of the promiscuity of the cellobiose-PTS system.

Thermophilic Degradation of Hemicellulose, a Critical Feedstock in the Production of Bioenergy and Other Value-Added Products

Renewable fuels have gained importance as the world moves toward diversifying its energy portfolio. A critical step in the biomass-to-bioenergy initiative is deconstruction of plant cell wall polysaccharides to their unit sugars for subsequent fermentation to fuels. To acquire carbon and energy for their metabolic processes, diverse microorganisms have evolved genes encoding enzymes that depolymerize polysaccharides to their carbon/energy-rich building blocks. The microbial enzymes mostly target the energy present in cellulose, hemicellulose, and pectin, three major forms of energy storage in plants. In the effort to develop bioenergy as an alternative to fossil fuel, a common strategy is to harness microbial enzymes to hydrolyze cellulose to glucose for fermentation to fuels. However, the conversion of plant biomass to renewable fuels will require both cellulose and hemicellulose, the two largest components of the plant cell wall, as feedstock to improve economic feasibility. Here, we explore the enzymes and strategies evolved by two well-studied bacteria to depolymerize the hemicelluloses xylan/arabinoxylan and mannan. The sets of enzymes, in addition to their applications in biofuels and value-added chemical production, have utility in animal feed enzymes, a rapidly developing industry with potential to minimize adverse impacts of animal agriculture on the environment.

Towards enzymatic breakdown of complex plant xylan structures: State of the art

Significant progress over the past few years has been achieved in the enzymology of microbial degradation and saccharification of plant xylan, after cellulose being the most abundant natural renewable polysaccharide. Several new types of xylan depolymerizing and debranching enzymes have been described in microorganisms. Despite the increasing variety of known glycoside hydrolases and carbohydrate esterases, some xylan structures still appear quite recalcitrant. This review focuses on the mode of action of different types of depolymerizing endoxylanases and their cooperation with β-xylosidase and accessory enzymes in breakdown of complex highly branched xylan structures. Emphasis is placed on the enzymatic hydrolysis of alkali-extracted deesterified polysaccharide as well as acetylated xylan isolated from plant cell walls under non-alkaline conditions. It is also shown how the combination of selected endoxylanases and debranching enzymes can determine the nature of prebiotic xylooligosaccharides or lead to complete hydrolysis of the polysaccharide. The article also highlights the possibility for discovery of novel xylanolytic enzymes, construction of multifunctional chimeric enzymes and xylanosomes in parallel with increasing knowledge on the fine structure of the polysaccharide.

Structure-function relationships in Gan42B, an intracellular GH42 β-galactosidase from Geobacillus stearothermophilus

Geobacillus stearothermophilus T-6 is a Gram-positive thermophilic soil bacterium that contains a battery of degrading enzymes for the utilization of plant cell-wall polysaccharides, including xylan, arabinan and galactan. A 9.4 kb gene cluster has recently been characterized in G. stearothermophilus that encodes a number of galactan-utilization elements. A key enzyme of this degradation system is Gan42B, an intracellular GH42 β-galactosidase capable of hydrolyzing short β-1,4-galactosaccharides into galactose units, making it of high potential for various biotechnological applications. The Gan42B monomer is made up of 686 amino acids, and based on sequence homology it was suggested that Glu323 is the catalytic nucleophile and Glu159 is the catalytic acid/base. In the current study, the detailed three-dimensional structure of wild-type Gan42B (at 2.45 Å resolution) and its catalytic mutant E323A (at 2.50 Å resolution), as determined by X-ray crystallography, are reported. These structures demonstrate that the three-dimensional structure of the Gan42B monomer generally correlates with the overall fold observed for GH42 proteins, consisting of three main domains: an N-terminal TIM-barrel domain, a smaller mixed α/β domain, and the smallest all-β domain at the C-terminus. The two catalytic residues are located in the TIM-barrel domain in a pocket-like active site such that their carboxylic functional groups are about 5.3 Å from each other, consistent with a retaining mechanism. The crystal structure demonstrates that Gan42B is a homotrimer, resembling a flowerpot in general shape, in which each monomer interacts with the other two to form a cone-shaped tunnel cavity in the centre. The cavity is ∼35 Å at the wide opening and ∼5 Å at the small opening and ∼40 Å in length. The active sites are situated at the interfaces between the monomers, so that every two neighbouring monomers participate in the formation of each of the three active sites of the trimer. They are located near the small opening of the cone tunnel, all facing the centre of the cavity. The biological relevance of this trimeric structure is supported by independent results obtained from gel-permeation chromatography. These data and their comparison to the structural data of related GH42 enzymes are used for a more general discussion concerning structure-activity aspects in this GH family.

Novel thermophilic hemicellulases for the conversion of lignocellulose for second generation biorefineries

The biotransformation of lignocellulose biomasses into fermentable sugars is a very complex procedure including, as one of the most critical steps, the (hemi) cellulose hydrolysis by specific enzymatic cocktails. We explored here, the potential of stable glycoside hydrolases from thermophilic organisms, so far not used in commercial enzymatic preparations, for the conversion of glucuronoxylan, the major hemicellulose of several energy crops. Searches in the genomes of thermophilic bacteria led to the identification, efficient production, and detailed characterization of novel xylanase and α-glucuronidase from Alicyclobacillus acidocaldarius (GH10-XA and GH67-GA, respectively) and a α-glucuronidase from Caldicellulosiruptor saccharolyticus (GH67-GC). Remarkably, GH10-XA, if compared to other thermophilic xylanases from this family, coupled good specificity on beechwood xylan and the best stability at 65 °C (3.5 days). In addition, GH67-GC was the most stable α-glucuronidases from this family and the first able to hydrolyse both aldouronic acid and aryl-α-glucuronic acid substrates. These enzymes, led to the very efficient hydrolysis of beechwood xylan by using 7- to 9-fold less protein (concentrations <0.3 μM) and in much less reaction time (2h vs 12h) if compared to other known biotransformations catalyzed by thermophilic enzymes. In addition, remarkably, together with a thermophilic β-xylosidase, they catalyzed the production of xylose from the smart cooking pre-treated biomass of one of the most promising energy crops for second generation biorefineries. We demonstrated that search by the CAZy Data Bank of currently available genomes and detailed enzymatic characterization of recombinant enzymes allow the identification of glycoside hydrolases with novel and interesting properties and applications.

The GH67 α-glucuronidase of Paenibacillus curdlanolyticus B-6 removes hexenuronic acid groups and facilitates biodegradation of the model xylooligosaccharide hexenuronosyl xylotriose

4-O-Methylglucuronic acid (MeGlcA) side groups attached to the xylan backbone through α-1,2 linkages are converted to hexenuronic acid (HexA) during alkaline pulping. α-Glucuronidase (EC 3.2.1.139) hydrolyzes 1,2-linked MeGlcA from xylooligosaccharides. To determine whether α-glucuronidase can also hydrolyze HexA-decorated xylooligosaccharides, a gene encoding α-glucuronidase (AguA) was cloned from Paenibacillus curdlanolyticus B-6. The purified protein degraded hexenuronosyl xylotriose (ΔX3), a model substrate prepared from kraft pulp. AguA released xylotriose and HexA from ΔX3, but the Vmax and kcat values for ΔX3 were lower than those for MeGlcA, indicating that HexA side groups may affect the hydrolytic activity. To explore the potential for biological bleaching, ΔX3 degradation was performed using intracellular extract from P. curdlanolyticus B-6. The intracellular extract, with synergistic α-glucuronidase and β-xylosidase activities, degraded ΔX3 to xylose and HexA. These results indicate that α-glucuronidase can be used to remove HexA from ΔX3 derived from pulp, reducing the need for chemical treatments in the pulping process.

Preliminary crystallographic analysis of Xyn52B2, a GH52 β-D-xylosidase from Geobacillus stearothermophilus T6

Geobacillus stearothermophilus T6 is a thermophilic bacterium that possesses an extensive hemicellulolytic system, including over 40 specific genes that are dedicated to this purpose. For the utilization of xylan, the bacterium uses an extracellular xylanase which degrades xylan to decorated xylo-oligomers that are imported into the cell. These oligomers are hydrolyzed by side-chain-cleaving enzymes such as arabinofuranosidases, acetylesterases and a glucuronidase, and finally by an intracellular xylanase and a number of β-xylosidases. One of these β-xylosidases is Xyn52B2, a GH52 enzyme that has already proved to be useful for various glycosynthesis applications. In addition to its demonstrated glycosynthase properties, interest in the structural aspects of Xyn52B2 stems from its special glycoside hydrolase family, GH52, the structures and mechanisms of which are only starting to be resolved. Here, the cloning, overexpression, purification and crystallization of Xyn52B2 are reported. The most suitable crystal form that has been obtained belonged to the orthorhombic P212121 space group, with average unit-cell parameters a = 97.7, b = 119.1, c = 242.3 Å. Several X-ray diffraction data sets have been collected from flash-cooled crystals of this form, including the wild-type enzyme (3.70 Å resolution), the E335G catalytic mutant (2.95 Å resolution), a potential mercury derivative (2.15 Å resolution) and a selenomethionine derivative (3.90 Å resolution). These data are currently being used for detailed three-dimensional structure determination of the Xyn52B2 protein.

Structure-specificity relationships in Abp, a GH27 β-L-arabinopyranosidase from Geobacillus stearothermophilus T6

L-Arabinose sugar residues are relatively abundant in plants and are found mainly in arabinan polysaccharides and in other arabinose-containing polysaccharides such as arabinoxylans and pectic arabinogalactans. The majority of the arabinose units in plants are present in the furanose form and only a small fraction of them are present in the pyranose form. The L-arabinan-utilization system in Geobacillus stearothermophilus T6, a Gram-positive thermophilic soil bacterium, has recently been characterized, and one of the key enzymes was found to be an intracellular β-L-arabinopyranosidase (Abp). Abp, a GH27 enzyme, was shown to remove β-L-arabinopyranose residues from synthetic substrates and from the native substrates sugar beet arabinan and larch arabinogalactan. The Abp monomer is made up of 448 amino acids, and based on sequence homology it was suggested that Asp197 is the catalytic nucleophile and Asp255 is the catalytic acid/base. In the current study, the detailed three-dimensional structure of wild-type Abp (at 2.28 Å resolution) and its catalytic mutant Abp-D197A with (at 2.20 Å resolution) and without (at 2.30 Å resolution) a bound L-arabinose product are reported as determined by X-ray crystallography. These structures demonstrate that the three-dimensional structure of the Abp monomer correlates with the general fold observed for GH27 proteins, consisting of two main domains: an N-terminal TIM-barrel domain and a C-terminal all-β domain. The two catalytic residues are located in the TIM-barrel domain, such that their carboxylic functional groups are about 5.9 Å from each other, consistent with a retaining mechanism. An isoleucine residue (Ile67) located at a key position in the active site is shown to play a critical role in the substrate specificity of Abp, providing a structural basis for the high preference of the enzyme towards arabinopyranoside over galactopyranoside substrates. The crystal structure demonstrates that Abp is a tetramer made up of two `open-pincers' dimers, which clamp around each other to form a central cavity. The four active sites of the Abp tetramer are situated on the inner surface of this cavity, all opening into the central space of the cavity. The biological relevance of this tetrameric structure is supported by independent results obtained from size-exclusion chromatography (SEC), dynamic light-scattering (DLS) and small-angle X-ray scattering (SAXS) experiments. These data and their comparison to the structural data of related GH27 enzymes are used for a more general discussion concerning structure-selectivity aspects in this glycoside hydrolase (GH) family.

Insight into glycoside hydrolases for debranched xylan degradation from extremely thermophilic bacterium Caldicellulosiruptor lactoaceticus

Caldicellulosiruptor lactoaceticus 6A, an anaerobic and extremely thermophilic bacterium, uses natural xylan as carbon source. The encoded genes of C. lactoaceticus 6A for glycoside hydrolase (GH) provide a platform for xylan degradation. The GH family 10 xylanase (Xyn10A) and GH67 α-glucuronidase (Agu67A) from C. lactoaceticus 6A were heterologously expressed, purified and characterized. Both Xyn10A and Agu67A are predicted as intracellular enzymes as no signal peptides identified. Xyn10A and Agu67A had molecular weight of 47.0 kDa and 80.0 kDa respectively as determined by SDS-PAGE, while both appeared as homodimer when analyzed by gel filtration. Xyn10A displayed the highest activity at 80 °C and pH 6.5, as 75 °C and pH 6.5 for Agu67A. Xyn10A had good stability at 75 °C, 80 °C, and pH 4.5-8.5, respectively, and was sensitive to various metal ions and reagents. Xyn10A possessed hydrolytic activity towards xylo-oligosaccharides (XOs) and beechwood xylan. At optimum conditions, the specific activity of Xyn10A was 44.6 IU/mg with beechwood xylan as substrate, and liberated branched XOs, xylobiose, and xylose. Agu67A was active on branched XOs with methyl-glucuronic acids (MeGlcA) sub-chains, and primarily generated XOs equivalents and MeGlcA. The specific activity of Agu67A was 1.3 IU/mg with aldobiouronic acid as substrate. The synergistic action of Xyn10A and Agu67A was observed with MeGlcA branched XOs and xylan as substrates, both backbone and branched chain of substrates were degraded, and liberated xylose, xylobiose, and MeGlcA. The synergism of Xyn10A and Agu67A provided not only a thermophilic method for natural xylan degradation, but also insight into the mechanisms for xylan utilization of C. lactoaceticus.

Cloning, purification and preliminary crystallographic analysis of Ara127N, a GH127 β-L-arabinofuranosidase from Geobacillus stearothermophilus T6

The L-arabinan utilization system of Geobacillus stearothermophilus T6 is composed of five transcriptional units that are clustered within a 38 kb DNA segment. One of the transcriptional units contains 11 genes, the last gene of which (araN) encodes a protein, Ara127N, that belongs to the newly established GH127 family. Ara127N shares 44% sequence identity with the recently characterized HypBA1 protein from Bifidobacterium longum and thus is likely to function similarly as a β-L-arabinofuranosidase. β-L-Arabinofuranosidases are enzymes that hydrolyze β-L-arabinofuranoside linkages, the less common form of such linkages, a unique enzymatic activity that has been identified only recently. The interest in the structure and mode of action of Ara127N therefore stems from its special catalytic activity as well as its membership of the new GH127 family, the structure and mechanism of which are only starting to be resolved. Ara127N has recently been cloned, overexpressed, purified and crystallized. Two suitable crystal forms have been obtained: one (CTP form) belongs to the monoclinic space group P21, with unit-cell parameters a = 104.0, b = 131.2, c = 107.6 Å, β = 112.0°, and the other (RB form) belongs to the orthorhombic space group P212121, with unit-cell parameters a = 65.5, b = 118.1, c = 175.0 Å. A complete X-ray diffraction data set has been collected to 2.3 Å resolution from flash-cooled crystals of the wild-type enzyme (RB form) at -173°C using synchrotron radiation. A selenomethionine derivative of Ara127N has also been prepared and crystallized for multi-wavelength anomalous diffraction (MAD) experiments. Crystals of selenomethionine Ara127N appeared to be isomorphous to those of the wild type (CTP form) and enabled the measurement of a three-wavelength MAD diffraction data set at the selenium absorption edge. These data are currently being used for detailed three-dimensional structure determination of the Ara127N protein.

Multiple regulatory mechanisms control the expression of the Geobacillus stearothermophilus gene for extracellular xylanase

Geobacillus stearothermophilus T-6 produces a single extracellular xylanase (Xyn10A) capable of producing short, decorated xylo-oligosaccharides from the naturally branched polysaccharide, xylan. Gel retardation assays indicated that the master negative regulator, XylR, binds specifically to xylR operators in the promoters of xylose and xylan-utilization genes. This binding is efficiently prevented in vitro by xylose, the most likely molecular inducer. Expression of the extracellular xylanase is repressed in medium containing either glucose or casamino acids, suggesting that carbon catabolite repression plays a role in regulating xynA. The global transcriptional regulator CodY was shown to bind specifically to the xynA promoter region in vitro, suggesting that CodY is a repressor of xynA. The xynA gene is located next to an uncharacterized gene, xynX, that has similarity to the NIF3 (Ngg1p interacting factor 3)-like protein family. XynX binds specifically to a 72-bp fragment in the promoter region of xynA, and the expression of xynA in a xynX null mutant appeared to be higher, indicating that XynX regulates xynA. The specific activity of the extracellular xylanase increases over 50-fold during early exponential growth, suggesting cell density regulation (quorum sensing). Addition of conditioned medium to fresh and low cell density cultures resulted in high expression of xynA, indicating that a diffusible extracellular xynA density factor is present in the medium. The xynA density factor is heat-stable, sensitive to proteases, and was partially purified using reverse phase liquid chromatography. Taken together, these results suggest that xynA is regulated by quorum-sensing at low cell densities.

Preliminary crystallographic analysis of a double mutant of the acetyl xylo-oligosaccharide esterase Axe2 in its dimeric form

Xylans are polymeric sugars constituting a significant part of the plant cell wall. They are usually substituted with acetyl side groups attached at positions 2 or 3 of the xylose backbone units. Acetylxylan esterases are part of the hemicellulolytic system of many microorganisms which utilize plant biomass for growth. These enzymes hydrolyze the ester linkages of the xylan acetyl groups and thus improve the accessibility of main-chain-hydrolyzing enzymes and their ability to break down the sugar backbone units. The acetylxylan esterases are therefore critically important for those microorganisms and as such could be used for a wide range of biotechnological applications. The structure of an acetylxylan esterase (Axe2) isolated from the thermophilic bacterium Geobacillus stearothermophilus T6 has been determined, and it has been demonstrated that the wild-type enzyme is present as a unique torus-shaped octamer in the crystal and in solution. In order to understand the functional origin of this unique oligomeric structure, a series of rational noncatalytic, site-specific mutations have been made on Axe2. Some of these mutations led to a different dimeric form of the protein, which showed a significant reduction in catalytic activity. One of these double mutants, Axe2-Y184F-W190P, has recently been overexpressed, purified and crystallized. The best crystals obtained belonged to the orthorhombic space group P212121, with unit-cell parameters a = 71.1, b = 106.0, c = 378.6 Å. A full diffraction data set to 2.3 Å resolution has been collected from a flash-cooled crystal of this type at 100 K using synchrotron radiation. This data set is currently being used for the three-dimensional structure analysis of the Axe2-Y184F-W190P mutant in its dimeric form.

Purification, crystallization and preliminary crystallographic analysis of Gan1D, a GH1 6-phospho-β-galactosidase from Geobacillus stearothermophilus T1

Geobacillus stearothermophilus T1 is a Gram-positive thermophilic soil bacterium that contains an extensive system for the utilization of plant cell-wall polysaccharides, including xylan, arabinan and galactan. The bacterium uses a number of extracellular enzymes that break down the high-molecular-weight polysaccharides into short oligosaccharides, which enter the cell and are further hydrolyzed into sugar monomers by dedicated intracellular glycoside hydrolases. The interest in the biochemical characterization and structural analysis of these proteins originates mainly from the wide range of their potential biotechnological applications. Studying the different hemicellulolytic utilization systems in G. stearothermophilus T1, a new galactan-utilization gene cluster was recently identified, which encodes a number of proteins, one of which is a GH1 putative 6-phospho-β-galactosidase (Gan1D). Gan1D has recently been cloned, overexpressed, purified and crystallized as part of its comprehensive structure-function study. The best crystals obtained for this enzyme belonged to the triclinic space group P1, with average crystallographic unit-cell parameters of a = 67.0, b = 78.1, c = 92.1 Å, α = 102.4, β = 93.5, γ = 91.7°. A full diffraction data set to 1.33 Å resolution has been collected for the wild-type enzyme, as measured from flash-cooled crystals at 100 K, using synchrotron radiation. These data are currently being used for the detailed three-dimensional crystal structure analysis of Gan1D.

A unique octameric structure of Axe2, an intracellular acetyl-xylooligosaccharide esterase from Geobacillus stearothermophilus

Geobacillus stearothermophilus T6 is a thermophilic, Gram-positive soil bacterium that possesses an extensive and highly regulated hemicellulolytic system, allowing the bacterium to efficiently degrade high-molecular-weight polysaccharides such as xylan, arabinan and galactan. As part of the xylan-degradation system, the bacterium uses a number of side-chain-cleaving enzymes, one of which is Axe2, a 219-amino-acid intracellular serine acetylxylan esterase that removes acetyl side groups from xylooligosaccharides. Bioinformatic analyses suggest that Axe2 belongs to the lipase GDSL family and represents a new family of carbohydrate esterases. In the current study, the detailed three-dimensional structure of Axe2 is reported, as determined by X-ray crystallography. The structure of the selenomethionine derivative Axe2-Se was initially determined by single-wavelength anomalous diffraction techniques at 1.70 Å resolution and was used for the structure determination of wild-type Axe2 (Axe2-WT) and the catalytic mutant Axe2-S15A at 1.85 and 1.90 Å resolution, respectively. These structures demonstrate that the three-dimensional structure of the Axe2 monomer generally corresponds to the SGNH hydrolase fold, consisting of five central parallel β-sheets flanked by two layers of helices (eight α-helices and five 310-helices). The catalytic triad residues, Ser15, His194 and Asp191, are lined up along a substrate channel situated on the concave surface of the monomer. Interestingly, the Axe2 monomers are assembled as a `doughnut-shaped' homo-octamer, presenting a unique quaternary structure built of two staggered tetrameric rings. The eight active sites are organized in four closely situated pairs, which face the relatively wide internal cavity. The biological relevance of this octameric structure is supported by independent results obtained from gel-filtration, TEM and SAXS experiments. These data and their comparison to the structural data of related hydrolases are used for a more general discussion focusing on the structure-function relationships of enzymes of this category.

Crystallization and preliminary crystallographic analysis of GanB, a GH42 intracellular β-galactosidase from Geobacillus stearothermophilus

Geobacillus stearothermophilus T-6 is a Gram-positive thermophilic soil bacterium that contains a multi-enzyme system for the utilization of plant cell-wall polysaccharides, including xylan, arabinan and galactan. The bacterium uses a number of endo-acting extracellular enzymes that break down the high-molecular-weight polysaccharides into decorated oligosaccharides. These oligosaccharides enter the cell and are further hydrolyzed into sugar monomers by a set of intracellular glycoside hydrolases. One of these intracellular degrading enzymes is GanB, a glycoside hydrolase family 42 β-galactosidase capable of hydrolyzing short β-1,4-galactosaccharides to galactose. GanB and related enzymes therefore play an important part in the hemicellulolytic utilization system of many microorganisms which use plant biomass for growth. The interest in the biochemical characterization and structural analysis of these enzymes stems from their potential biotechnological applications. GanB from G. stearothermophilus T-6 has recently been cloned, overexpressed, purified, biochemically characterized and crystallized in our laboratory as part of its complete structure-function study. The best crystals obtained for this enzyme belong to the primitive orthorhombic space group P2₁2₁2₁, with average crystallographic unit-cell parameters of a=71.84, b=181.35, c=196.57 Å. Full diffraction data sets to 2.45 and 2.50 Å resolution have been collected for both the wild-type enzyme and its E323A nucleophile catalytic mutant, respectively, as measured from flash-cooled crystals at 100 K using synchrotron radiation. These data are currently being used for the full three-dimensional crystal structure determination of GanB.

Crystallization and preliminary crystallographic analysis of Abp, a GH27 β-L-arabinopyranosidase from Geobacillus stearothermophilus

Geobacillus stearothermophilus T-6 is a thermophilic soil bacterium that possesses an extensive system for the utilization of hemicellulose. The bacterium produces a small number of endo-acting extracellular enzymes that cleave high-molecular-weight hemicellulolytic polymers into short decorated oligosaccharides, which are further hydrolysed into the respective sugar monomers by a battery of intracellular glycoside hydrolases. One of these intracellular processing enzymes is β-L-arabinopyranosidase (Abp), which is capable of removing β-L-arabinopyranose residues from naturally occurring arabino-polysaccharides. As arabino-polymers constitute a significant part of the hemicellulolytic content of plant biomass, their efficient enzymatic degradation presents an important challenge for many potential biotechnological applications. This aspect has led to an increasing interest in the biochemical characterization and structural analysis of this and related hemicellulases. Abp from G. stearothermophilus T-6 has recently been cloned, overexpressed, purified, biochemically characterized and crystallized in our laboratory, as part of its complete structure-function study. The best crystals obtained for this enzyme belonged to the primitive orthorhombic space group P2(1)2(1)2(1), with average unit-cell parameters a = 107.7, b = 202.2, c = 287.3 Å. Full diffraction data sets to 2.3 Å resolution have been collected for both the wild-type enzyme and its D197A catalytic mutant from flash-cooled crystals at 100 K, using synchrotron radiation. These data are currently being used for a high-resolution three-dimensional structure determination of Abp.

Crystallization and preliminary crystallographic analysis of Axe2, an acetylxylan esterase from Geobacillus stearothermophilus

Acetylxylan esterases are part of the hemi-cellulolytic system of many microorganisms which utilize plant biomass for growth. Xylans, which are polymeric sugars that constitute a significant part of the plant biomass, are usually substituted with acetyl side groups attached at position 2 or 3 of the xylose backbone units. Acetylxylan esterases hydrolyse the ester linkages of the xylan acetyl groups and thus improve the ability of main-chain hydrolysing enzymes to break down the sugar backbone units. As such, these enzymes play an important part in the hemi-cellulolytic utilization system of many microorganisms that use plant biomass for growth. Interest in the biochemical characterization and structural analysis of these enzymes stems from their numerous potential biotechnological applications. An acetylxylan esterase (Axe2) of this type from Geobacillus stearothermophilus T-6 has recently been cloned, overexpressed, purified, biochemically characterized and crystallized. One of the crystal forms obtained (RB1) belonged to the tetragonal space group I422, with unit-cell parameters a = b = 110.2, c = 213.1 Å. A full diffraction data set was collected to 1.85 Å resolution from flash-cooled crystals of the wild-type enzyme at 100 K using synchrotron radiation. A selenomethionine derivative of Axe2 has also been prepared and crystallized for single-wavelength anomalous diffraction experiments. The crystals of the selenomethionine-derivatized Axe2 appeared to be isomorphous to those of the wild-type enzyme and enabled the measurement of a full 1.85 Å resolution diffraction data set at the selenium absorption edge and a full 1.70 Å resolution data set at a remote wavelength. These data are currently being used for three-dimensional structure determination of the Axe2 protein.

Isolation of α-glucuronidase enzyme from a rumen metagenomic library

α-Glucuronidase enzymes play an essential role in the full enzymatic hydrolysis of hemicellulose. Up to this point, all genes encoding α-glucuronidase enzymes have been cloned from individual, pure culture strains. Using a high-throughput screening strategy, we have isolated the first α-glucuronidase gene (rum630-AG) from a mixed population of microorganisms. The gene was subcloned into a prokaryotic vector, and the enzyme was overexpressed and biochemically characterized. The RUM630-AG enzyme had optimum activity at pH 6.5 and 40 °C. When birchwood xylan was used as substrate, the RUM630-AG functioned synergistically with an endoxylanase enzyme to hydrolyze the substrate.

Isolation and characterization of a novel GH67 α-glucuronidase from a mixed culture

Hemicelluloses represent a large reservoir of carbohydrates that can be utilized for renewable products. Hydrolysis of hemicellulose into simple sugars is inhibited by its various chemical substituents. The glucuronic acid substituent is removed by the enzyme α-glucuronidase. A gene (deg75-AG) encoding a putative α-glucuronidase enzyme was isolated from a culture of mixed compost microorganisms. The gene was subcloned into a prokaryotic vector, and the enzyme was overexpressed and biochemically characterized. The DEG75-AG enzyme had optimum activity at 45 °C. Unlike other α-glucuronidases, the DEG75-AG had a more basic pH optimum of 7–8. When birchwood xylan was used as substrate, the addition of DEG75-AG increased hydrolysis twofold relative to xylanase alone.

A new family of carbohydrate esterases is represented by a GDSL hydrolase/acetylxylan esterase from Geobacillus stearothermophilus

Acetylxylan esterases hydrolyze the ester linkages of acetyl groups at positions 2 and/or 3 of the xylose moieties in xylan and play an important role in enhancing the accessibility of xylanases to the xylan backbone. The hemicellulolytic system of the thermophilic bacterium Geobacillus stearothermophilus T-6 comprises a putative acetylxylan esterase gene, axe2. The gene product belongs to the GDSL hydrolase family and does not share sequence homology with any of the carbohydrate esterases in the CAZy Database. The axe2 gene is induced by xylose, and the purified gene product completely deacetylates xylobiose peracetate (fully acetylated) and hydrolyzes the synthetic substrates 2-naphthyl acetate, 4-nitrophenyl acetate, 4-methylumbelliferyl acetate, and phenyl acetate. The pH profiles for k(cat) and k(cat)/K(m) suggest the existence of two ionizable groups affecting the binding of the substrate to the enzyme. Using NMR spectroscopy, the regioselectivity of Axe2 was directly determined with the aid of one-dimensional selective total correlation spectroscopy. Methyl 2,3,4-tri-O-acetyl-β-d-xylopyranoside was rapidly deacetylated at position 2 or at positions 3 and 4 to give either diacetyl or monoacetyl intermediates, respectively; methyl 2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside was initially deacetylated at position 6. In both cases, the complete hydrolysis of the intermediates occurred at a much slower rate, suggesting that the preferred substrate is the peracetate sugar form. Site-directed mutagenesis of Ser-15, His-194, and Asp-191 resulted in complete inactivation of the enzyme, consistent with their role as the catalytic triad. Overall, our results show that Axe2 is a serine acetylxylan esterase representing a new carbohydrate esterase family.

The L-Arabinan utilization system of Geobacillus stearothermophilus

Geobacillus stearothermophilus T-6 is a thermophilic soil bacterium that has a 38-kb gene cluster for the utilization of arabinan, a branched polysaccharide that is part of the plant cell wall. The bacterium encodes a unique three-component regulatory system (araPST) that includes a sugar-binding lipoprotein (AraP), a histidine sensor kinase (AraS), and a response regulator (AraT) and lies adjacent to an ATP-binding cassette (ABC) arabinose transport system (araEGH). The lipoprotein (AraP) specifically bound arabinose, and gel mobility shift experiments showed that the response regulator, AraT, binds to a 139-bp fragment corresponding to the araE promoter region. Taken together, the results showed that the araPST system appeared to sense extracellular arabinose and to activate a specific ABC transporter for arabinose (AraEGH). The promoter regions of the arabinan utilization genes contain a 14-bp inverted repeat motif resembling an operator site for the arabinose repressor, AraR. AraR was found to bind specifically to these sequences, and binding was efficiently prevented in the presence of arabinose, suggesting that arabinose is the molecular inducer of the arabinan utilization system. The expression of the arabinan utilization genes was reduced in the presence of glucose, indicating that regulation is also mediated via a catabolic repression mechanism. The cluster also encodes a second putative ABC sugar transporter (AbnEFJ) whose sugar-binding lipoprotein (AbnE) was shown to interact specifically with linear and branched arabino-oligosaccharides. The final degradation of the arabino-oligosaccharides is likely carried out by intracellular enzymes, including two α-l-arabinofuranosidases (AbfA and AbfB), a β-l-arabinopyranosidase (Abp), and an arabinanase (AbnB), all of which are encoded in the 38-kb cluster.

Crystallization and preliminary crystallographic analysis of the glycoside hydrolase family 115 α-glucuronidase from Streptomyces pristinaespiralis

α-Glucuronidase from Streptomyces pristinaespiralis (SpGlcA115A) is composed of a single-chain peptide containing a catalytic domain belonging to glycosyl hydrolase family 115, a novel family of hemicellulolytic α-glucuronidases. The enzyme catalyzes the hydrolysis of α-linked 4-O-methylglucuronosyl and glucuronosyl residues from both polymeric xylans and oligosaccharides. SpGlcA115A was crystallized at 293 K using the sitting-drop vapour-diffusion method. The crystals belonged to space group R3 and diffracted to a resolution of 1.9 Å.

A novel family of hemicellulolytic alpha-glucuronidase

Investigation of the xylanolytic enzyme system of the xylose-fermenting yeast Pichia stipitis resulted in the discovery of an extracellular alpha-glucuronidase efficiently debranching hardwood glucuronoxylan. This activity is not exhibited by more extensively investigated alpha-glucuronidases of glycoside hydrolase (GH) family 67, operating on substrates in which the uronic acid is linked to the non-reducing xylopyranosyl residues of main chain fragments. The N-terminus of the purified enzyme corresponded exactly to the P. stipitis gene ABN67901 coding for a protein of unknown function. BLAST search revealed the presence of similar genes in genomes of other microorganisms. These results lead to the emergence of a new family of alpha-glucuronidases.

A two-component system regulates the expression of an ABC transporter for xylo-oligosaccharides in Geobacillus stearothermophilus

Geobacillus stearothermophilus T-6 utilizes an extensive and highly regulated hemicellulolytic system. The genes comprising the xylanolytic system are clustered in a 39.7-kb chromosomal segment. This segment contains a 6-kb transcriptional unit (xynDCEFG) coding for a potential two-component system (xynDC) and an ATP-binding cassette (ABC) transport system (xynEFG). The xynD promoter region contains a 16-bp inverted repeat resembling the operator site for the xylose repressor, XylR. XylR was found to bind specifically to this sequence, and binding was efficiently prevented in vitro in the presence of xylose. The ABC transport system was shown to comprise an operon of three genes (xynEFG) that is transcribed from its own promoter. The nonphosphorylated fused response regulator, His6-XynC, bound to a 220-bp fragment corresponding to the xynE operator. DNase I footprinting analysis showed four protected zones that cover the -53 and the +34 regions and revealed direct repeat sequences of a GAAA-like motif. In vitro transcriptional assays and quantitative reverse transcription-PCR demonstrated that xynE transcription is activated 140-fold in the presence of 1.5 microM XynC. The His6-tagged sugar-binding lipoprotein (XynE) of the ABC transporter interacted with different xylosaccharides, as demonstrated by isothermal titration calorimetry. The change in the heat capacity of binding (DeltaCp) for XynE with xylotriose suggests a stacking interaction in the binding site that can be provided by a single Trp residue and a sugar moiety. Taken together, our data show that XynEFG constitutes an ABC transport system for xylo-oligosaccharides and that its transcription is negatively regulated by XylR and activated by the response regulator XynC, which is part of a two-component sensing system.

Effect of dimer dissociation on activity and thermostability of the alpha-glucuronidase from Geobacillus stearothermophilus: dissecting the different oligomeric forms of family 67 glycoside hydrolases

The oligomeric organization of enzymes plays an important role in many biological processes, such as allosteric regulation, conformational stability and thermal stability. alpha-Glucuronidases are family 67 glycosidases that cleave the alpha-1,2-glycosidic bond between 4-O-methyl-D-glucuronic acid and xylose units as part of an array of hemicellulose-hydrolyzing enzymes. Currently, two crystal structures of alpha-glucuronidases are available, those from Geobacillus stearothermophilus (AguA) and from Cellvibrio japonicus (GlcA67A). Both enzymes are homodimeric, but surprisingly their dimeric organization is different, raising questions regarding the significance of dimerization for the enzymes' activity and stability. Structural comparison of the two enzymes suggests several elements that are responsible for the different dimerization organization. Phylogenetic analysis shows that the alpha-glucuronidases AguA and GlcA67A can be classified into two distinct subfamilies of bacterial alpha-glucuronidases, where the dimer-forming residues of each enzyme are conserved only within its own subfamily. It seems that the different dimeric forms of AguA and GlcA67A represent the two alternative dimeric organizations of these subfamilies. To study the biological significance of the dimerization in alpha-glucuronidases, we have constructed a monomeric form of AguA by mutating three of its interface residues (W328E, R329T, and R665N). The activity of the monomer was significantly lower than the activity of the wild-type dimeric AguA, and the optimal temperature for activity of the monomer was around 35 degrees C, compared to 65 degrees C of the wild-type enzyme. Nevertheless, the melting temperature of the monomeric protein, 72.9 degrees C, was almost identical to that of the wild-type, 73.4 degrees C. It appears that the dimerization of AguA is essential for efficient catalysis and that the dissociation into monomers results in subtle conformational changes in the structure which indirectly influence the active site region and reduce the activity. Structural and mechanistic explanations for these effects are discussed.

Crystal structures of Geobacillus stearothermophilus alpha-glucuronidase complexed with its substrate and products: mechanistic implications

Alpha-glucuronidases cleave the alpha-1,2-glycosidic bond between 4-O-methyl-d-glucuronic acid and short xylooligomers as part of the hemicellulose degradation system. To date, all of the alpha-glucuronidases are classified as family 67 glycosidases, which catalyze the hydrolysis via the investing mechanism. Here we describe several high resolution crystal structures of the alpha-glucuronidase (AguA) from Geobacillus stearothermophilus, in complex with its substrate and products. In the complex of AguA with the intact substrate, the 4-O-methyl-d-glucuronic acid sugar ring is distorted into a half-chair conformation, which is closer to the planar conformation required for the oxocarbenium ion-like transition state structure. In the active site, a water molecule is coordinated between two carboxylic acids, in an appropriate position to act as a nucleophile. From the structural data it is likely that two carboxylic acids, Asp(364) and Glu(392), activate together the nucleophilic water molecule. The loop carrying the catalytic general acid Glu(285) cannot be resolved in some of the structures but could be visualized in its "open" and "closed" (catalytic) conformations in other structures. The protonated state of Glu(285) is presumably stabilized by its proximity to the negative charge of the substrate, representing a new variation of substrate-assisted catalysis mechanism.

Microbial hemicellulases

Hemicellulases are a diverse group of enzymes that hydrolyze hemicelluloses--one of the most abundant groups of polysaccharide in nature. These enzymes have many biotechnological applications and their structure/function relationships are a subject of intense research. During the past year, new high-resolution structures of catalytic and non-catalytic domains of hemicellulases have been elucidated, and, together with biochemical studies, they reveal the principles of catalysis and specificity for these enzymes.

The alpha-glucuronidase, GlcA67A, of Cellvibrio japonicus utilizes the carboxylate and methyl groups of aldobiouronic acid as important substrate recognition determinants

alpha-Glucuronidases are key components of the ensemble of enzymes that degrade the plant cell wall. They hydrolyze the alpha1,2-glycosidic bond between 4-O-methyl-d-glucuronic acid (4-O-MeGlcA) and the xylan or xylooligosaccharide backbone. Here we report the crystal structure of an inactive mutant (E292A) of the alpha-glucuronidase, GlcA67A, from Cellvibrio japonicus in complex with its substrate. The data show that the 4-O-methyl group of the substrate is accommodated within a hydrophobic sheath flanked by Val-210 and Trp-160, whereas the carboxylate moiety is located within a positively charged region of the substrate-binding pocket. The carboxylate side chains of Glu-393 and Asp-365, on the "beta-face" of 4-O-MeGlcA, form hydrogen bonds with a water molecule that is perfectly positioned to mount a nucleophilic attack at the anomeric carbon of the target glycosidic bond, providing further support for the view that, singly or together, these amino acids function as the catalytic base. The capacity of reaction products and product analogues to inhibit GlcA67A shows that the 4-O-methyl group, the carboxylate, and the xylose sugar of aldobiouronic acid all play an important role in substrate binding. Site-directed mutagenesis informed by the crystal structure of enzyme-ligand complexes was used to probe the importance of highly conserved residues at the active site of GlcA67A. The biochemical properties of K288A, R325A, and K360A show that a constellation of three basic amino acids (Lys-288, Arg-325, and Lys-360) plays a critical role in binding the carboxylate moiety of 4-O-MeGlcA. Disruption of the apolar nature of the pocket created by Val-210 (V210N and V210S) has a detrimental effect on substrate binding, although the reduction in affinity is not reflected by an inability to accommodate the 4-O-methyl group. Replacing the two tryptophan residues that stack against the sugar rings of the substrate with alanine (W160A and W543A) greatly reduced activity.

The membrane-bound alpha-glucuronidase from Pseudomonas cellulosa hydrolyzes 4-O-methyl-D-glucuronoxylooligosaccharides but not 4-O-methyl-D-glucuronoxylan

The microbial degradation of xylan is a key biological process. Hardwood 4-O-methyl-D-glucuronoxylans are extensively decorated with 4-O-methyl-D-glucuronic acid, which is cleaved from the polysaccharides by alpha-glucuronidases. In this report we describe the primary structures of the alpha-glucuronidase from Cellvibrio mixtus (C. mixtus GlcA67A) and the alpha-glucuronidase from Pseudomonas cellulosa (P. cellulosa GlcA67A) and characterize P. cellulosa GlcA67A. The primary structures of C. mixtus GlcA67A and P. cellulosa GlcA67A, which are 76% identical, exhibit similarities with alpha-glucuronidases in glycoside hydrolase family 67. The membrane-associated pseudomonad alpha-glucuronidase released 4-O-methyl-D-glucuronic acid from 4-O-methyl-D-glucuronoxylooligosaccharides but not from 4-O-methyl-D-glucuronoxylan. We propose that the role of the glucuronidase, in combination with cell-associated xylanases, is to hydrolyze decorated xylooligosaccharides, generated by extracellular hemicellulases, to xylose and 4-O-methyl-D-glucuronic acid, enabling the pseudomonad to preferentially utilize the sugars derived from these polymers.

The structural basis for catalysis and specificity of the Pseudomonas cellulosa alpha-glucuronidase, GlcA67A

Alpha-glucuronidases, components of an ensemble of enzymes central to the recycling of photosynthetic biomass, remove the alpha-1,2 linked 4-O-methyl glucuronic acid from xylans. The structure of the alpha-glucuronidase, GlcA67A, from Pseudomonas cellulosa reveals three domains, the central of which is a (beta/alpha)(8) barrel housing the catalytic apparatus. Complexes of the enzyme with the individual reaction products, either xylobiose or glucuronic acid, and the ternary complex of both glucuronic acid and xylotriose reveal a "blind" pocket which selects for short decorated xylooligosaccharides substituted with the uronic acid at their nonreducing end, consistent with kinetic data. The catalytic center reveals a constellation of carboxylates; Glu292 is poised to provide protonic assistance to leaving group departure with Glu393 and Asp365 both appropriately positioned to provide base-catalyzed assistance for inverting nucleophilic attack by water.