Overproduction and characterization of seleno-methionine xylanase T-6

Spectroscopic FTIR and NMR study of the interactions of sugars with proteins

FTIR and NMR spectra were measured in parallel for specific two-components mixtures of various proteins with different sugar molecules, such as arabinose, glucose, and sucrose. In the FTIR spectra of arabinose with some of these proteins, the bands assigned to the vibrational modes of the CH and COH groups disappeared, and new ones, related to an arabinose-protein CN mode, appeared. Similar changes were observed in the FTIR spectra of lyophilized mixtures of arabinose with different amino acids. In additional FTIR spectra, measured for other protein-sugar mixtures, the bands correlated to the ring modes of arabinose, in the range 1150-1000 cm^-1, disappeared, and two new very strong narrow bands became dominant, indicating ring opening or some kind of arabinose decomposition. Contrary to the prevailing opinion that complexes between sugars and proteins are formed mainly by hydrogen bonds, the IR and NMR spectra of the sugar-protein mixtures studied here suggest that significant chemical reactions also take place between the interacting sugar and the protein. Two types of sugar-protein chemical reactions can be distinguished on the basis of these IR spectra, leading to the formation of a new CN bond and to the decomposition of sugar skeletal bonds. The new IR bands suggest that the latter reaction results in the formation of new bonds, which are related to new polyether moieties. These results highlight the often ignored non-specific chemical reactions that take place between sugars and proteins, and demonstrate that the simultaneous application of FTIR and NMR spectroscopic analyses can detect and further characterize these types of sugar-protein interactions.

Genetic toolbox for controlled expression of functional proteins in Geobacillus spp

Species of genus Geobacillus are thermophilic bacteria and play an ever increasing role as hosts for biotechnological applications both in academia and industry. Here we screened a number of Geobacillus strains to determine which industrially relevant carbon sources they can utilize. One of the strains, G. thermoglucosidasius C56-YS93, was then chosen to develop a toolbox for controlled gene expression over a wide range of levels. It includes a library of semi-synthetic constitutive promoters (76-fold difference in expression levels) and an inducible promoter from the xylA gene. A library of synthetic in silico designed ribosome binding sites was also created for further tuning of translation. The PxylA was further used to successfully express native and heterologous xylanases in G. thermoglucosidasius. This toolbox enables fine-tuning of gene expression in Geobacillus species for metabolic engineering approaches in production of biochemicals and heterologous proteins.

Embryo Microinjection of Selenomethionine Reduces Hatchability and Modifies Oxidant Responsive Gene Expression in Zebrafish

In previous studies we demonstrated that exposure to selenomethionine (SeMet) causes developmental toxicities in zebrafish (Danio rerio). The objectives of this study were to establish a dose-response relationship for developmental toxicities in zebrafish after embryo microinjection of Se (8, 16 or 32 μg/g dry mass of eggs) in the form of SeMet, and to investigate potential underlying mechanism(s) of SeMet-induced developmental toxicities. A dose-dependent increase in frequencies of mortality and total deformities, and reduced hatchability were observed in zebrafish exposed to excess Se via embryo microinjection. The egg Se concentration causing 20% mortality was then used to investigate transcript abundance of proteins involved in antioxidant protection and methylation. Excess Se exposure modified gene expression of oxidant-responsive transcription factors (nuclear factor erythroid 2-related factor nrf2a and nrf2b), and enzymes involved in cellular methylation (methionine adenosyltransferase mat1a and mat2ab) in zebrafish larvae. Notably, excess Se exposure up-regulated transcript abundance of aryl hydrocarbon receptor 2 (ahr2), a signalling pathway involved in the toxicity of dioxin-related compounds. Our findings suggest that oxidative stress or modification of methylation, or a combination of these mechanisms, might be responsible for Se-induced developmental toxicities in fishes.

Structure of the bovine COPI δ subunit μ homology domain at 2.15 Å resolution

The heptameric COPI coat (coatomer) plays an essential role in vesicular transport in the early secretory system of eukaryotic cells. While the structures of some of the subunits have been determined, that of the δ-COP subunit has not been reported to date. The δ-COP subunit is part of a subcomplex with structural similarity to tetrameric clathrin adaptors (APs), where δ-COP is the structural homologue of the AP μ subunit. Here, the crystal structure of the μ homology domain (MHD) of δ-COP (δ-MHD) obtained by phasing using a combined SAD-MR method is presented at 2.15 Å resolution. The crystallographic asymmetric unit contains two monomers that exhibit short sections of disorder, which may allude to flexible regions of the protein. The δ-MHD is composed of two subdomains connected by unstructured linkers. Comparison between this structure and those of known MHD domains from the APs shows significant differences in the positions of specific loops and β-sheets, as well as a more general change in the relative positions of the protein subdomains. The identified difference may be the major source of cargo-binding specificity. Finally, the crystal structure is used to analyze the potential effect of the I422T mutation in δ-COP previously reported to cause a neurodegenerative phenotype in mice.

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.

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.

Assembly of xylanases into designer cellulosomes promotes efficient hydrolysis of the xylan component of a natural recalcitrant cellulosic substrate

In nature, the complex composition and structure of the plant cell wall pose a barrier to enzymatic degradation. Nevertheless, some anaerobic bacteria have evolved for this purpose an intriguing, highly efficient multienzyme complex, the cellulosome, which contains numerous cellulases and hemicellulases. The rod-like cellulose component of the plant cell wall is embedded in a colloidal blend of hemicelluloses, a major component of which is xylan. In order to enhance enzymatic degradation of the xylan component of a natural complex substrate (wheat straw) and to study the synergistic action among different xylanases, we have employed a variation of the designer cellulosome approach by fabricating a tetravalent complex that includes the three endoxylanases of Thermobifida fusca (Xyn10A, Xyn10B, and Xyn11A) and an Xyl43A β-xylosidase from the same bacterium. Here, we describe the conversion of Xyn10A and Xyl43A to the cellulosomal mode. The incorporation of the Xyl43A enzyme together with the three endoxylanases into a common designer cellulosome served to enhance the level of reducing sugars produced during wheat straw degradation. The enhanced synergistic action of the four xylanases reflected their immediate juxtaposition in the complex, and these tetravalent xylanolytic designer cellulosomes succeeded in degrading significant (~25%) levels of the total xylan component of the wheat straw substrate. The results suggest that the incorporation of xylanases into cellulosome complexes is advantageous for efficient decomposition of recalcitrant cellulosic substrates--a distinction previously reserved for cellulose-degrading enzymes.

IMPORTANCE

Xylanases are important enzymes for our society, due to their variety of industrial applications. Together with cellulases and other glycoside hydrolases, xylanases may also provide cost-effective conversion of plant-derived cellulosic biomass into soluble sugars en route to biofuels as an alternative to fossil fuels. Xylanases are commonly found in multienzyme cellulosome complexes, produced by anaerobic bacteria, which are considered to be among the most efficient systems for degradation of cellulosic biomass. Using a designer cellulosome approach, we have incorporated the entire xylanolytic system of the bacterium Thermobifida fusca into defined artificial cellulosome complexes. The combined action of these designer cellulosomes versus that of the wild-type free xylanase system was then compared. Our data demonstrated that xylanolytic designer cellulosomes displayed enhanced synergistic activities on a natural recalcitrant wheat straw substrate and could thus serve in the development of advanced systems for improved degradation of lignocellulosic material.

Contribution of a xylan-binding module to the degradation of a complex cellulosic substrate by designer cellulosomes

Conversion of components of the Thermobifida fusca free-enzyme system to the cellulosomal mode using the designer cellulosome approach can be employed to discover the properties and inherent advantages of the cellulosome system. In this article, we describe the conversion of the T. fusca xylanases Xyn11A and Xyn10B and their synergistic interaction in the free state or within designer cellulosome complexes in order to enhance specific degradation of hatched wheat straw as a model for a complex cellulosic substrate. Endoglucanase Cel5A from the same bacterium and its recombinant dockerin-containing chimera were also studied for their combined effect, together with the xylanases, on straw degradation. Synergism was demonstrated when Xyn11A was combined with Xyn10B and/or Cel5A, and approximately 1.5-fold activity enhancements were achieved by the designer cellulosome complexes compared to the free wild-type enzymes. These improvements in activity were due to both substrate-targeting and proximity effects among the enzymes contained in the designer cellulosome complexes. The intrinsic cellulose/xylan-binding module (XBM) of Xyn11A appeared to be essential for efficient substrate degradation. Indeed, only designer cellulosomes in which the XBM was maintained as a component of Xyn11A achieved marked enhancement in activity compared to the combination of wild-type enzymes. Moreover, integration of the XBM in designer cellulosomes via a dockerin module (separate from the Xyn11A catalytic module) failed to enhance activity, suggesting a role in orienting the parent xylanase toward its preferred polysaccharide component of the complex wheat straw substrate. The results provide novel mechanistic insight into the synergistic activity of designer cellulosome components on natural plant cell wall substrates.

High-resolution Crystal Structure of a Truncated ColE7 Translocation Domain: Implications for Colicin Transport Across Membranes

ColE7 is a nuclease-type colicin released from Escherichia coli to kill sensitive bacterial cells by degrading the nucleic acid molecules in their cytoplasm. ColE7 is classified as one of the group A colicins, since the N-terminal translocation domain (T-domain) of the nuclease-type colicins interact with specific membrane-bound or periplasmic Tol proteins during protein import. Here, we show that if the N-terminal tail of ColE7 is deleted, ColE7 (residues 63-576) loses its bactericidal activity against E.coli. Moreover, TolB protein interacts directly with the T-domain of ColE7 (residues 1-316), but not with the N-terminal deleted T-domain (residues 60-316), as detected by co-immunoprecipitation experiments, confirming that the N-terminal tail is required for ColE7 interactions with TolB. The crystal structure of the N-terminal tail deleted ColE7 T-domain was determined by the multi-wavelength anomalous dispersion method at a resolution of 1.7 angstroms. The structure of the ColE7 T-domain superimposes well with the T-domain of ColE3 and TR-domain of ColB, a group A Tol-dependent colicin and a group B TonB-dependent colicin, respectively. The structural resemblance of group A and B colicins implies that the two groups of colicins may share a mechanistic connection during cellular import.

Matching fusion protein systems for affinity analysis of two interacting families of proteins: the cohesin-dockerin interaction

Cellulosomes are multi-enzyme complexes that orchestrate the efficient degradation of cellulose and related plant cell wall polysaccharides. The complex is maintained by the high-affinity protein-protein interaction between two complementary modules: the cohesin and the dockerin. In order to characterize the interaction between different cohesins and dockerins, we have developed matching fusion-protein systems, which harbor either the cohesin or the dockerin component. For this purpose, corresponding plasmid cassettes were designed, which encoded for the following carrier proteins: (i) a thermostable xylanase with an appended His-tag; and (ii) a highly stable cellulose-binding module (CBM). The resultant xylanase-dockerin and CBM-cohesin fusion products exhibited high expression levels of soluble protein. The expressed, affinity-purified proteins were extremely stable, and the functionality of the cohesin or dockerin component was retained. The fusion protein system was used to establish a sensitive and reliable, semi-quantitative enzyme-linked affinity assay for determining multiple samples of cohesin-dockerin interactions in microtiter plates. A variety of cohesin-dockerin systems, which had been examined previously using other methodologies, were revisited applying the affinity-based enzyme assay, the results of which served to verify the validity of the approach.

Xylanases, xylanase families and extremophilic xylanases

Xylanases are hydrolytic enzymes which randomly cleave the beta 1,4 backbone of the complex plant cell wall polysaccharide xylan. Diverse forms of these enzymes exist, displaying varying folds, mechanisms of action, substrate specificities, hydrolytic activities (yields, rates and products) and physicochemical characteristics. Research has mainly focused on only two of the xylanase containing glycoside hydrolase families, namely families 10 and 11, yet enzymes with xylanase activity belonging to families 5, 7, 8 and 43 have also been identified and studied, albeit to a lesser extent. Driven by industrial demands for enzymes that can operate under process conditions, a number of extremophilic xylanases have been isolated, in particular those from thermophiles, alkaliphiles and acidiphiles, while little attention has been paid to cold-adapted xylanases. Here, the diverse physicochemical and functional characteristics, as well as the folds and mechanisms of action of all six xylanase containing families will be discussed. The adaptation strategies of the extremophilic xylanases isolated to date and the potential industrial applications of these enzymes will also be presented.

Cohesin-dockerin interaction in cellulosome assembly: a single Asp-to-Asn mutation disrupts high-affinity cohesin-dockerin binding

The cohesive cellulosome complex is sustained by the high-affinity cohesin-dockerin interaction. In previous work, we demonstrated that a single Thr-to-Leu replacement in the Clostridium thermocellum dockerin component differentiates between non-recognition and high-affinity recognition by the interspecies rival cohesin from C. cellulolyticum. In this report, we show that a single Asp-to-Asn substitution on the cohesin counterpart also disrupts normal recognition of the dockerin. The Asp34 carboxyl group of the cohesin appears to play a central role in the resultant hydrogen-bonding network as an acceptor of two crucial hydrogen bonds from Ser45 of the dockerin domain. The results underscore the fragile nature of the intermolecular contact interactions that maintain this very high-affinity protein--protein interaction.

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.

DNA binding and cleavage by the periplasmic nuclease Vvn: a novel structure with a known active site

The Vibrio vulnificus nuclease, Vvn, is a non-specific periplasmic nuclease capable of digesting DNA and RNA. The crystal structure of Vvn and that of Vvn mutant H80A in complex with DNA were resolved at 2.3 A resolution. Vvn has a novel mixed alpha/beta topology containing four disulfide bridges, suggesting that Vvn is not active under reducing conditions in the cytoplasm. The overall structure of Vvn shows no similarity to other endonucleases; however, a known 'betabetaalpha-metal' motif is identified in the central cleft region. The crystal structure of the mutant Vvn-DNA complex demonstrates that Vvn binds mainly at the minor groove of DNA, resulting in duplex bending towards the major groove by approximately 20 degrees. Only the DNA phosphate backbones make hydrogen bonds with Vvn, suggesting a structural basis for its sequence-independent recognition of DNA and RNA. Based on the enzyme-substrate and enzyme-product structures observed in the mutant Vvn-DNA crystals, a catalytic mechanism is proposed. This structural study suggests that Vvn hydrolyzes DNA by a general single-metal ion mechanism, and indicates how non-specific DNA-binding proteins may recognize DNA.

The cellulosome system of Acetivibrio cellulolyticus includes a novel type of adaptor protein and a cell surface anchoring protein

A scaffoldin gene cluster was identified in the mesophilic cellulolytic anaerobe Acetivibrio cellulolyticus. The previously described scaffoldin gene, cipV, encodes an N-terminal family 9 glycoside hydrolase, a family 3b cellulose-binding domain, seven cohesin domains, and a C-terminal dockerin. The gene immediately downstream of cipV was sequenced and designated scaB. The protein encoded by this gene has 942 amino acid residues and a calculated molecular weight of 100,358 and includes an N-terminal signal peptide, four type II cohesions, and a C-terminal dockerin. ScaB cohesins 1 and 2 are very closely linked. Similar, but not identical, 39-residue Thr-rich linker segments separate cohesin 2 from cohesin 3 and cohesin 3 from cohesin 4, and an 84-residue Thr-rich linker connects the fourth cohesin to a C-terminal dockerin. The scaC gene downstream of scaB codes for a 1,237-residue polypeptide that includes a signal peptide, three cohesins, and a C-terminal S-layer homology (SLH) module. A long, ca. 550-residue linker separates the third cohesin and the SLH module of ScaC and is characterized by an 18-residue Pro-Thr-Ala-Ser-rich segment that is repeated 27 times. The calculated molecular weight of the mature ScaC polypeptide (excluding the signal peptide) is 124,162. The presence of the cohesins and the conserved SLH module implies that ScaC acts as an anchoring protein. The ScaC cohesins are on a separate branch of the phylogenetic tree that is close to, but distinct from, the type I cohesins. Affinity blotting with representative recombinant probes revealed the following specific intermodular interactions: (i) an expressed CipV cohesin binds selectively to an enzyme-borne dockerin, (ii) a representative ScaB cohesin binds to the CipV band of the cell-free supernatant fraction, and (iii) a ScaC cohesin binds to the ScaB dockerin. The experimental evidence thus indicates that CipV acts as a primary (enzyme-recognizing) scaffoldin, and the protein was also designated ScaA. In addition, ScaB is thought to assume the role of an adaptor protein, which connects the primary scaffoldin (ScaA) to the cohesin-containing anchoring scaffoldin (ScaC). The cellulosome system of A. cellulolyticus thus appears to exhibit a special type of organization that reflects the function of the ScaB adaptor protein. The intercalation of three multiple cohesin-containing scaffoldins results in marked amplification of the number of enzyme subunits per cellulosome unit. At least 96 enzymes can apparently be incorporated into an individual A. cellulolyticus cellulosome. The role of such amplified enzyme incorporation and the resultant proximity of the enzymes within the cellulosome complex presumably contribute to the enhanced synergistic action and overall efficient digestion of recalcitrant forms of cellulose. Comparison of the emerging organization of the A. cellulolyticus cellulosome with the organizations in other cellulolytic bacteria revealed the diversity of the supramolecular architecture.

Crystal Structures of KDOP Synthase in Its Binary Complexes with the Substrate Phosphoenolpyruvate and with a Mechanism-Based Inhibitor†,‡

The crystal structures of 3-deoxy-D-manno-2-octulosonate-8-phosphate synthase (KDOPS) from Escherichia coli complexed with the substrate phosphoenolpyruvate (PEP) and with a mechanism-based inhibitor (K(d) = 0.4 microM) were determined by molecular replacement using X-ray diffraction data to 2.8 and 2.3 A resolution, respectively. Both the KDOPS.PEP and KDOPS.inhibitor complexes crystallize in the cubic space group I23 with cell constants a = b = c = 117.9 and 117.6 A, respectively, and one subunit per asymmetric unit. The two structures are nearly identical, and superposition of their Calpha atoms indicates an rms difference of 0.41 A. The PEP in the KDOPS.PEP complex is anchored to the enzyme in a conformation that blocks its si face and leaves its re face largely devoid of contacts. This results from KDOPS's selective choice of a PEP conformer in which the phosphate group of PEP is extended toward the si face. Furthermore, the structure reveals that the bridging (P-O-C) oxygen atom and the carboxylate group of PEP are not strongly hydrogen-bonded to the enzyme. The resulting high degree of negative charge on the carboxylate group of PEP would then suggest that the condensation step between PEP and D-arabinose-5-phosphate (A5P) should proceed in a stepwise fashion through the intermediacy of a transient oxocarbenium ion at C2 of PEP. The molecular structural results are discussed in light of the chemically similar but mechanistically distinct reaction that is catalyzed by the enzyme 3-deoxy-D-arabino-2-heptulosonate-7-phosphate synthase and in light of the preferred enzyme-bound states of the substrate A5P.