Cellobiohydrolase B, a second exo-cellobiohydrolase from the cellulolytic bacterium Cellulomonas fimi

Chitin-Active Lytic Polysaccharide Monooxygenases Are Rare in Cellulomonas Species

Cellulomonas flavigena is a saprotrophic bacterium that encodes, within its genome, four predicted lytic polysaccharide monooxygenases (LPMOs) from Auxiliary Activity family 10 (AA10). We showed previously that three of these cleave the plant polysaccharide cellulose by oxidation at carbon-1 (J. Li, L. Solhi, E.D. Goddard-Borger, Y. Mattieu et al., Biotechnol Biofuels 14:29, 2021, https://doi.org/10.1186/s13068-020-01860-3). Here, we present the biochemical characterization of the fourth C. flavigena AA10 member (CflaLPMO10D) as a chitin-active LPMO. Both the full-length CflaLPMO10D-Carbohydrate-Binding Module family 2 (CBM2) and catalytic module-only proteins were produced in Escherichia coli using the native general secretory (Sec) signal peptide. To quantify chitinolytic activity, we developed a high-performance anion-exchange chromatography-pulsed amperometric detection (HPAEC-PAD) method as an alternative to the established hydrophilic interaction liquid ion chromatography coupled with UV detection (HILIC-UV) method for separation and detection of released oxidized chito-oligosaccharides. Using this method, we demonstrated that CflaLPMO10D is strictly active on the β-allomorph of chitin, with optimal activity at pH 5 to 6 and a preference for ascorbic acid as the reducing agent. We also demonstrated the importance of the CBM2 member for both mediating enzyme localization to substrates and prolonging LPMO activity. Together with previous work, the present study defines the distinct substrate specificities of the suite of C. flavigena AA10 members. Notably, a cross-genome survey of AA10 members indicated that chitinolytic LPMOs are, in fact, rare among Cellulomonas bacteria. IMPORTANCE Species from the genus Cellulomonas have a long history of study due to their roles in biomass recycling in nature and corresponding potential as sources of enzymes for biotechnological applications. Although Cellulomonas species are more commonly associated with the cleavage and utilization of plant cell wall polysaccharides, here, we show that C. flavigena produces a unique lytic polysaccharide monooxygenase with activity on β-chitin, which is found, for example, in arthropods. The limited distribution of orthologous chitinolytic LPMOs suggests adaptation of individual cellulomonads to specific nutrient niches present in soil ecosystems. This research provides new insight into the biochemical specificity of LPMOs in Cellulomonas species and related bacteria, and it raises new questions about the physiological function of these enzymes.

High cellulolytic potential of the Ktedonobacteria lineage revealed by genome-wide analysis of CAZymes

Traditionally, filamentous fungi and actinomycetes are well-known cellulolytic microorganisms that have been utilized in the commercial production of cellulase enzyme cocktails for industrial-scale degradation of plant biomass. Noticeably, the Ktedonobacteria lineage (phylum Chloroflexi) with actinomycetes-like morphology was identified and exhibited diverse carbohydrate utilization or degradation abilities. In this study, we performed genome-wide profiling of carbohydrate-active enzymes (CAZymes) in the filamentous Ktedonobacteria lineage. Numerous CAZymes (153-290 CAZymes, representing 63-131 glycoside hydrolases (GHs) per genome), including complex mixtures of endo- and exo-cellulases, were predicted in 15 available Ktedonobacteria genomes. Of note, 4-28 CAZymes were predicted to be extracellular enzymes, whereas 3-29 CAZymes were appended with carbohydrate-binding modules (CBMs) that may promote their binding to insoluble carbohydrate substrates. This number far exceeded other Chloroflexi lineages and were comparable to the cellulolytic actinomycetes. Six multi-modular extracellular GHs were cloned from the thermophilic Thermosporothrix hazakensis SK20-1^T strain and heterologously expressed. The putative endo-glucanases of ThazG5-1, ThazG9, and ThazG12 exhibited strong cellulolytic activity, whereas the putative exo-glucanases ThazG6 and ThazG48 formed weak but observable halos on carboxymethyl cellulose plates, indicating their potential biotechnological application. The purified recombinant ThazG12 had near-neutral pH (optimal 6.0), high thermostability (60°C), and broad specificity against soluble and insoluble polysaccharide substrates. It also represented described a novel thermostable bacterial β-1,4-glucanase in the GH12 family. Together, this research revealed the underestimated cellulolytic potential of the Ktedonobacteria lineage and highlighted its potential biotechnological utility as a promising microbial resource for the discovery of industrially useful cellulases.

Four cellulose-active lytic polysaccharide monooxygenases from Cellulomonas species

BACKGROUND

The discovery of lytic polysaccharide monooxygenases (LPMOs) has fundamentally changed our understanding of microbial lignocellulose degradation. Cellulomonas bacteria have a rich history of study due to their ability to degrade recalcitrant cellulose, yet little is known about the predicted LPMOs that they encode from Auxiliary Activity Family 10 (AA10).

RESULTS

Here, we present the comprehensive biochemical characterization of three AA10 LPMOs from Cellulomonas flavigena (CflaLPMO10A, CflaLPMO10B, and CflaLPMO10C) and one LPMO from Cellulomonas fimi (CfiLPMO10). We demonstrate that these four enzymes oxidize insoluble cellulose with C1 regioselectivity and show a preference for substrates with high surface area. In addition, CflaLPMO10B, CflaLPMO10C, and CfiLPMO10 exhibit limited capacity to perform mixed C1/C4 regioselective oxidative cleavage. Thermostability analysis indicates that these LPMOs can refold spontaneously following denaturation dependent on the presence of copper coordination. Scanning and transmission electron microscopy revealed substrate-specific surface and structural morphological changes following LPMO action on Avicel and phosphoric acid-swollen cellulose (PASC). Further, we demonstrate that the LPMOs encoded by Cellulomonas flavigena exhibit synergy in cellulose degradation, which is due in part to decreased autoinactivation.

CONCLUSIONS

Together, these results advance understanding of the cellulose utilization machinery of historically important Cellulomonas species beyond hydrolytic enzymes to include lytic cleavage. This work also contributes to the broader mapping of enzyme activity in Auxiliary Activity Family 10 and provides new biocatalysts for potential applications in biomass modification.

Convergent evolution of processivity in bacterial and fungal cellulases

Some cellulases exhibit “processivity”: the ability to degrade crystalline cellulose through successive hydrolytic catalytic reactions without the release of the enzyme from the substrate surface. We previously observed the movement of fungal processive cellulases by high-speed atomic force microscopy, and here, we use the same technique to directly observe the processive movement of bacterial cellobiohydrolases settling a long-standing controversy. Although fungal and bacterial processive cellulases have completely different protein folds, they have evolved to acquire processivity through the same strategy of adding subsites to extend the substrate-binding site and forming a tunnel-like active site by increasing the number of loops covering the active site. This represents an example of protein-level convergent evolution to acquire the same functions from different ancestors.

Cellulose is the most abundant biomass on Earth, and many microorganisms depend on it as a source of energy. It consists mainly of crystalline and amorphous regions, and natural degradation of the crystalline part is highly dependent on the degree of processivity of the degrading enzymes (i.e., the extent of continuous hydrolysis without detachment from the substrate cellulose). Here, we report high-speed atomic force microscopic (HS-AFM) observations of the movement of four types of cellulases derived from the cellulolytic bacteria Cellulomonas fimi on various insoluble cellulose substrates. The HS-AFM images clearly demonstrated that two of them (CfCel6B and CfCel48A) slide on crystalline cellulose. The direction of processive movement of CfCel6B is from the nonreducing to the reducing end of the substrate, which is opposite that of processive cellulase Cel7A of the fungus Trichoderma reesei (TrCel7A), whose movement was first observed by this technique, while CfCel48A moves in the same direction as TrCel7A. When CfCel6B and TrCel7A were mixed on the same substrate, “traffic accidents” were observed, in which the two cellulases blocked each other’s progress. The processivity of CfCel6B was similar to those of fungal family 7 cellulases but considerably higher than those of fungal family 6 cellulases. The results indicate that bacteria utilize family 6 cellulases as high-processivity enzymes for efficient degradation of crystalline cellulose, whereas family 7 enzymes have the same function in fungi. This is consistent with the idea of convergent evolution of processive cellulases in fungi and bacteria to achieve similar functionality using different protein foldings.

Characterisation of novel biomass degradation enzymes from the genome of Cellulomonas fimi

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Identified over 90 putative polysaccharide degrading ORFs in C. fimi genome.

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Cloned 14 putative cellulolytic ORFs as BioBricks, screened them for activity.

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Partially purified AfsB, BxyF, BxyH and XynF and characterised them further.

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BxyH proved highly temperature and alkaline pH tolerant.

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BioBricks are an easy method for screening genes for specific activities.

Recent analyses of genome sequences belonging to cellulolytic bacteria have revealed many genes potentially coding for cellulosic biomass degradation enzymes. Annotation of these genes however, is based on few biochemically characterised examples. Here we present a simple strategy based on BioBricks for the rapid screening of candidate genes expressed in Escherichia coli. As proof of principle we identified over 70 putative biomass degrading genes from bacterium Cellulomonas fimi, expressing a subset of these in BioBrick format. Six novel genes showed activity in E. coli. Four interesting enzymes were characterised further. α-l-arabinofuranosidase AfsB, β-xylosidases BxyF and BxyH and multi-functional β-cellobiosidase/xylosidase XynF were partially purified to determine their optimum pH, temperature and kinetic parameters. One of these enzymes, BxyH, was unexpectedly found to be highly active at strong alkaline pH and at temperatures as high as 100 °C. This report demonstrates a simple method of quickly screening and characterising putative genes as BioBricks.

Effect of Different Lignocellulosic Diets on Bacterial Microbiota and Hydrolytic Enzyme Activities in the Gut of the Cotton Boll Weevil (Anthonomus grandis)

Cotton boll weevils, Anthonomus grandis, are omnivorous coleopteran that can feed on diets with different compositions, including recalcitrant lignocellulosic materials. We characterized the changes in the prokaryotic community structure and the hydrolytic activities of A. grandis larvae fed on different lignocellulosic diets. A. grandis larvae were fed on three different artificial diets: cottonseed meal (CM), Napier grass (NG) and corn stover (CS). Total DNA was extracted from the gut samples for amplification and sequencing of the V3-V4 hypervariable region of the 16S rRNA gene. Proteobacteria and Firmicutes dominated the gut microbiota followed by Actinobacteria, Spirochaetes and a small number of unclassified phyla in CM and NG microbiomes. In the CS feeding group, members of Spirochaetes were the most prevalent, followed by Proteobacteria and Firmicutes. Bray-Curtis distances showed that the samples from the CS community were clearly separated from those samples of the CM and NG diets. Gut extracts from all three diets exhibited endoglucanase, xylanase, β-glucosidase and pectinase activities. These activities were significantly affected by pH and temperature across different diets. We observed that the larvae reared on a CM showed significantly higher activities than larvae reared on NG and CS. We demonstrated that the intestinal bacterial community structure varies depending on diet composition. Diets with more variable and complex compositions, such as CS, showed higher bacterial diversity and richness than the two other diets. In spite of the detected changes in composition and diversity, we identified a core microbiome shared between the three different lignocellulosic diets. These results suggest that feeding with diets of different lignocellulosic composition could be a viable strategy to discover variants of hemicellulose and cellulose breakdown systems.

C/N ratio drives soil actinobacterial cellobiohydrolase gene diversity

Cellulose accounts for approximately half of photosynthesis-fixed carbon; however, the ecology of its degradation in soil is still relatively poorly understood. The role of actinobacteria in cellulose degradation has not been extensively investigated despite their abundance in soil and known cellulose degradation capability. Here, the diversity and abundance of the actinobacterial glycoside hydrolase family 48 (cellobiohydrolase) gene in soils from three paired pasture-woodland sites were determined by using terminal restriction fragment length polymorphism (T-RFLP) analysis and clone libraries with gene-specific primers. For comparison, the diversity and abundance of general bacteria and fungi were also assessed. Phylogenetic analysis of the nucleotide sequences of 80 clones revealed significant new diversity of actinobacterial GH48 genes, and analysis of translated protein sequences showed that these enzymes are likely to represent functional cellobiohydrolases. The soil C/N ratio was the primary environmental driver of GH48 community compositions across sites and land uses, demonstrating the importance of substrate quality in their ecology. Furthermore, mid-infrared (MIR) spectrometry-predicted humic organic carbon was distinctly more important to GH48 diversity than to total bacterial and fungal diversity. This suggests a link between the actinobacterial GH48 community and soil organic carbon dynamics and highlights the potential importance of actinobacteria in the terrestrial carbon cycle.

Molecular and biochemical analyses of CbCel9A/Cel48A, a highly secreted multi-modular cellulase by Caldicellulosiruptor bescii during growth on crystalline cellulose

During growth on crystalline cellulose, the thermophilic bacterium Caldicellulosiruptor bescii secretes several cellulose-degrading enzymes. Among these enzymes is CelA (CbCel9A/Cel48A), which is reported as the most highly secreted cellulolytic enzyme in this bacterium. CbCel9A/Cel48A is a large multi-modular polypeptide, composed of an N-terminal catalytic glycoside hydrolase family 9 (GH9) module and a C-terminal GH48 catalytic module that are separated by a family 3c carbohydrate-binding module (CBM3c) and two identical CBM3bs. The wild-type CbCel9A/Cel48A and its truncational mutants were expressed in Bacillus megaterium and Escherichia coli, respectively. The wild-type polypeptide released twice the amount of glucose equivalents from Avicel than its truncational mutant that lacks the GH48 catalytic module. The truncational mutant harboring the GH9 module and the CBM3c was more thermostable than the wild-type protein, likely due to its compact structure. The main hydrolytic activity was present in the GH9 catalytic module, while the truncational mutant containing the GH48 module and the three CBMs was ineffective in degradation of either crystalline or amorphous cellulose. Interestingly, the GH9 and/or GH48 catalytic modules containing the CBM3bs form low-density particles during hydrolysis of crystalline cellulose. Moreover, TM3 (GH9/CBM3c) and TM2 (GH48 with three CBM3 modules) synergistically hydrolyze crystalline cellulose. Deletion of the CBM3bs or mutations that compromised their binding activity suggested that these CBMs are important during hydrolysis of crystalline cellulose. In agreement with this observation, seven of nine genes in a C. bescii gene cluster predicted to encode cellulose-degrading enzymes harbor CBM3bs. Based on our results, we hypothesize that C. bescii uses the GH48 module and the CBM3bs in CbCel9A/Cel48A to destabilize certain regions of crystalline cellulose for attack by the highly active GH9 module and other endoglucanases produced by this hyperthermophilic bacterium.

The genome sequences of Cellulomonas fimi and "Cellvibrio gilvus" reveal the cellulolytic strategies of two facultative anaerobes, transfer of "Cellvibrio gilvus" to the genus Cellulomonas, and proposal of Cellulomonas gilvus sp. nov

Actinobacteria in the genus Cellulomonas are the only known and reported cellulolytic facultative anaerobes. To better understand the cellulolytic strategy employed by these bacteria, we sequenced the genome of the Cellulomonas fimi ATCC 484(T). For comparative purposes, we also sequenced the genome of the aerobic cellulolytic "Cellvibrio gilvus" ATCC 13127(T). An initial analysis of these genomes using phylogenetic and whole-genome comparison revealed that "Cellvibrio gilvus" belongs to the genus Cellulomonas. We thus propose to assign "Cellvibrio gilvus" to the genus Cellulomonas. A comparative genomics analysis between these two Cellulomonas genome sequences and the recently completed genome for Cellulomonas flavigena ATCC 482(T) showed that these cellulomonads do not encode cellulosomes but appear to degrade cellulose by secreting multi-domain glycoside hydrolases. Despite the minimal number of carbohydrate-active enzymes encoded by these genomes, as compared to other known cellulolytic organisms, these bacteria were found to be proficient at degrading and utilizing a diverse set of carbohydrates, including crystalline cellulose. Moreover, they also encode for proteins required for the fermentation of hexose and xylose sugars into products such as ethanol. Finally, we found relatively few significant differences between the predicted carbohydrate-active enzymes encoded by these Cellulomonas genomes, in contrast to previous studies reporting differences in physiological approaches for carbohydrate degradation. Our sequencing and analysis of these genomes sheds light onto the mechanism through which these facultative anaerobes degrade cellulose, suggesting that the sequenced cellulomonads use secreted, multidomain enzymes to degrade cellulose in a way that is distinct from known anaerobic cellulolytic strategies.

Sequence, structure, and evolution of cellulases in glycoside hydrolase family 48

Currently, the cost of cellulase enzymes remains a key economic impediment to commercialization of biofuels. Enzymes from glycoside hydrolase family 48 (GH48) are a critical component of numerous natural lignocellulose-degrading systems. Although computational mining of large genomic data sets is a promising new approach for identifying novel cellulolytic activities, current computational methods are unable to distinguish between cellulases and enzymes with different substrate specificities that belong to the same protein family. We show that by using a robust computational approach supported by experimental studies, cellulases and non-cellulases can be effectively identified within a given protein family. Phylogenetic analysis of GH48 showed non-monophyletic distribution, an indication of horizontal gene transfer. Enzymatic function of GH48 proteins coded by horizontally transferred genes was verified experimentally, which confirmed that these proteins are cellulases. Computational and structural studies of GH48 enzymes identified structural elements that define cellulases and can be used to computationally distinguish them from non-cellulases. We propose that the structural element that can be used for in silico discrimination between cellulases and non-cellulases belonging to GH48 is an ω-loop located on the surface of the molecule and characterized by highly conserved rare amino acids. These markers were used to screen metagenomics data for "true" cellulases.

Biodiversity characterization of cellulolytic bacteria present on native Chaco soil by comparison of ribosomal RNA genes

Sequence analysis of the 16S ribosomal RNA gene was used to study bacterial diversity of a pristine forest soil and of two cultures of the same soil enriched with cellulolytic bacteria. Our analysis revealed high bacterial diversity in the native soil sample, evidencing at least 10 phyla, in which Actinobacteria, Proteobacteria and Acidobacteria accounted for more than 76% of all sequences. In both enriched samples, members of Proteobacteria were the most frequently represented. The majority of bacterial genera in both enriched samples were identified as Brevundimonas and Caulobacter, but members of Devosia, Sphingomonas, Variovorax, Acidovorax, Pseudomonas, Xanthomonas, Stenotrophomonas, Achromobacter and Delftia were also found. In addition, it was possible to identify cellulolytic taxa such as Acidothermus, Micromonospora, Streptomyces, Paenibacillus and Pseudomonas, which indicates that this ecosystem could be an attractive source for study of novel enzymes for cellulose degradation.

The noncellulosomal family 48 cellobiohydrolase from Clostridium phytofermentans ISDg: heterologous expression, characterization, and processivity

Family 48 glycoside hydrolases (cellobiohydrolases) are among the most important cellulase components for crystalline cellulose hydrolysis mediated by cellulolytic bacteria. Open reading frame (Cphy_3368) of Clostridium phytofermentans ISDg encodes a putative family 48 glycoside hydrolase (CpCel48) with a family 3 cellulose-binding module. CpCel48 was successfully expressed as two soluble intracellular forms with or without a C-terminal His-tag in Escherichia coli and as a secretory active form in Bacillus subtilis. It was found that calcium ion enhanced activity and thermostability of the enzyme. CpCel48 had high activities of 15.1 U micromol(-1) on Avicel and 35.9 U micromol(-1) on regenerated amorphous cellulose (RAC) with cellobiose as a main product and cellotriose and cellotetraose as by-products. By contrast, it had very weak activities on soluble cellulose derivatives (e.g., carboxymethyl cellulose (CMC)) and did not significantly decrease the viscosity of the CMC solution. Cellotetraose was the smallest oligosaccharide substrate for CpCel48. Since processivity is a key characteristic for cellobiohydrolases, the new initial false/right attack model was developed for estimation of processivity by considering the enzyme's substrate specificity, the crystalline structure of homologous Cel48 enzymes, and the configuration of cellulose chains. The processivities of CpCel48 on Avicel and RAC were estimated to be approximately 3.5 and 6.0, respectively. Heterologous expression of secretory active cellobiohydrolase in B. subtilis is an important step for developing recombinant cellulolytic B. subtilis strains for low-cost production of advanced biofuels from cellulosic materials in a single step.

Functional asymmetry in cohesin binding belies inherent symmetry of the dockerin module: insight into cellulosome assembly revealed by systematic mutagenesis

The cellulosome is an intricate multi-enzyme complex, known for its efficient degradation of recalcitrant cellulosic substrates. Its supramolecular architecture is determined by the high-affinity intermodular cohesin-dockerin interaction. The dockerin module comprises a calcium-binding, duplicated 'F-hand' loop-helix motif that bears striking similarity to the EF-hand loop-helix-loop motif of eukaryotic calcium-binding proteins. In the present study, we demonstrate by progressive truncation and alanine scanning of a representative type-I dockerin module from Clostridium thermocellum, that only one of the repeated motifs is critical for high-affinity cohesin binding. The results suggest that the near-symmetry in sequence and structure of the repeated elements of the dockerin is not essential to cohesin binding. The first calcium-binding loop can be deleted entirely, with almost full retention of binding. Likewise, significant deletion of the second repeated segment can be achieved, provided that its calcium-binding loop remains intact. Essentially the same conclusion was verified by systematically mutating the highly conserved residues in the calcium-binding loop. Mutations in one of the calcium-binding loops failed to disrupt cohesin recognition and binding, whereas a single mutation in both loops served to reduce the affinity significantly. The results are mutually compatible with recent crystal structures of the type-I cohesin-dockerin heterodimer, which demonstrate that the dockerin can bind in an equivalent manner to its cohesin counterpart through either its first or second repeated motif. The observed plasticity in cohesin-dockerin binding may facilitate cellulosome assembly in vivo or, alternatively, provide a conformational switch that promotes access of the tethered cellulosomal enzymes to their polysaccharide substrates.

Cellulases of mesophilic microorganisms: cellulosome and noncellulosome producers

The cellulolytic activity of mesophilic bacteria and fungi is described, with special emphasis on the large extracellular enzyme complex called the cellulosome. The cellulosome is composed of a scaffolding protein, which is attached to various cellulolytic and hemicellulolytic enzymes, and this complex allows the organisms to degrade plant cell walls very efficently. The enzymes include a variety of cellulases, hemicellulases, and pectinases that work synergistically to degrade complex cell-wall molecules.

Bioconversion of lignocellulose materials

One of the most economically viable processes for the bioconversion of many lignocellulosic waste is represented by white rot fungi. Phanerochaete chrysosporium is one of the important commercially cultivated fungi which exhibit varying abilities to utilize different lignocellulosic as growth substrate. Examination of the lignocellulolytic enzyme profiles of the two organisms Phanerochaete chrysosporium and Rhizopus stolonifer show this diversity to be reflected in qualitative variation in the major enzymatic determinants (ie cellulase, xylanase, ligninase and etc) required for substrate bioconversion. For example P. chrysosporium which is cultivated on highly lignified substrates such as wood (or) sawdust, produces two extracellular enzymes which have associated with lignin deploymerization. (Mn peroxidase and lignin peroxidase). Conversely Rhizopus stolonifer which prefers high cellulose and low lignin containg substrates produce a family of cellulolytic enzymes including at least cellobiohydrolases and β-glucosidases, but very low level of recognized lignin degrading enzymes.

Characterization of a beta-N-acetylhexosaminidase and a beta-N-acetylglucosaminidase/beta-glucosidase from Cellulomonas fimi

The gram-positive soil bacterium Cellulomonas fimi is shown to produce at least two intracellular beta-N-acetylglucosaminidases, a family 20 beta-N-acetylhexosaminidase (Hex20), and a novel family 3-beta-N-acetylglucosaminidase/beta-glucosidase (Nag3), through screening of a genomic expression library, cloning of genes and analysis of their sequences. Nag3 exhibits broad substrate specificity for substituents at the C2 position of the glycone: kcat/Km values at 25 degrees C were 0.066 s(-1) x mM(-1) and 0.076 s(-1) x mM(-1) for 4'-nitrophenyl beta-N-acetyl-D-glucosaminide and 4'-nitrophenyl beta-D-glucoside, respectively. The first glycosidase with this broad specificity to be described, Nag3, suggests an interesting evolutionary link between beta-N-acetylglucosaminidases and beta-glucosidases of family 3. Reaction by a double-displacement mechanism was confirmed for Nag3 through the identification of a glycosyl-enzyme species trapped with the slow substrate 2',4'-dinitrophenyl 2-deoxy-2-fluoro-beta-D-glucopyranoside. Hex20 requires the acetamido group at C2 of the substrate, being unable to cleave beta-glucosides, since its mechanism involves an oxazolinium ion intermediate. However, it is broad in its specificity for the D-glucosyl/D-galactosyl configuration of the glycone: Km and kcat values were 53 microM and 482.3 s(-1) for 4'-nitrophenyl beta-N-acetyl-D-glucosaminide and 66 microM and 129.1 s(-1) for 4'-nitrophenyl beta-N-acetyl-D-galactosaminide.

Engineering the Exo-loop of Trichoderma reesei Cellobiohydrolase, Cel7A. A comparison with Phanerochaete chrysosporium Cel7D

The exo-loop of Trichoderma reesei cellobiohydrolase Cel7A forms the roof of the active site tunnel at the catalytic centre. Mutants were designed to study the role of this loop in crystalline cellulose degradation. A hydrogen bond to substrate made by a tyrosine at the tip of the loop was removed by the Y247F mutation. The mobility of the loop was reduced by introducing a new disulphide bridge in the mutant D241C/D249C. The tip of the loop was deleted in mutant Delta(G245-Y252). No major structural disturbances were observed in the mutant enzymes, nor was the thermostability of the enzyme affected by the mutations. The Y247F mutation caused a slight k(cat) reduction on 4-nitrophenyl lactoside, but only a small effect on cellulose hydrolysis. Deletion of the tip of the loop increased both k(cat) and K(M) and gave reduced product inhibition. Increased activity was observed on amorphous cellulose, while only half the original activity remained on crystalline cellulose. Stabilisation of the exo-loop by the disulphide bridge enhanced the activity on both amorphous and crystalline cellulose. The ratio Glc(2)/(Glc(3)+Glc(1)) released from cellulose, which is indicative of processive action, was highest with Tr Cel7A wild-type enzyme and smallest with the deletion mutant on both substrates. Based on these data it seems that the exo-loop of Tr Cel7A has evolved to facilitate processive crystalline cellulose degradation, which does not require significant conformational changes of this loop.

Exo-mode of action of cellobiohydrolase Cel48C from Paenibacillus sp. BP-23. A unique type of cellulase among Bacillales

Sequence analysis of a Paenibacillus sp. BP-23 recombinant clone coding for a previously described endoglucanase revealed the presence of an additional truncated ORF with homology to family 48 glycosyl hydrolases. The corresponding 3509-bp DNA fragment was isolated after gene walking and cloned in Escherichia coli Xl1-Blue for expression and purification. The encoded enzyme, a cellulase of 1091 amino acids with a deduced molecular mass of 118 kDa and a pI of 4.85, displayed a multidomain organization bearing a canonical family 48 catalytic domain, a bacterial type 3a cellulose-binding module, and a putative fibronectin-III domain. The cloned cellulase, unique among Bacillales and designated Cel48C, was purified through affinity chromatography using its ability to bind Avicel. Maximum activity was achieved at 45 degrees C and pH 6.0 on acid-swollen cellulose, bacterial microcrystalline cellulose, Avicel and cellodextrins, whereas no activity was found on carboxy methyl cellulose, cellobiose, cellotriose, pNP-glycosides or 4-methylumbeliferyl alpha-d-glucoside. Cellobiose was the major product of cellulose hydrolysis, identifying Cel48C as a processive cellobiohydrolase. Although no chromogenic activity was detected from pNP-glycosides, TLC analysis revealed the release of p-nitrophenyl-glycosides and cellodextrins from these substrates, suggesting that Cel48C acts from the reducing ends of the sugar chain. Presence of such a cellobiohydrolase in Paenibacillus sp. BP-23 would contribute to widen up its range of action on natural cellulosic substrates.

Kinetic parameters and mode of action of the cellobiohydrolases produced by Talaromyces emersonii

Three forms of cellobiohydrolase (EC 3.2.1.91), CBH IA, CBH IB and CBH II, were isolated to apparent homogeneity from culture filtrates of the aerobic fungus Talaromyces emersonii. The three enzymes are single sub-unit glycoproteins, and unlike most other fungal cellobiohydrolases are characterised by noteworthy thermostability. The kinetic properties and mode of action of each enzyme against polymeric and small soluble oligomeric substrates were investigated in detail. CBH IA, CBH IB and CBH II catalyse the hydrolysis of microcrystalline cellulose, albeit to varying extents. Hydrolysis of a soluble cellulose derivative (CMC) and barley 1,3;1,4-beta-D-glucan was not observed. Cellobiose (G2) is the main reaction product released by CBH IA, CBH IB, and CBH II from microcrystalline cellulose. All three CBHs are competitively inhibited by G2; inhibition constant values (K(i)) of 2.5 and 0.18 mM were obtained for CBH IA and CBH IB, respectively (4-nitrophenyl-beta-cellobioside as substrate), while a K(i) of 0.16 mM was determined for CBH II (2-chloro-4-nitrophenyl-beta-cellotrioside as substrate). Bond cleavage patterns were determined for each CBH on 4-methylumbelliferyl derivatives of beta-cellobioside and beta-cellotrioside (MeUmbG(n)). While the Tal. emersonii CBHs share certain properties with their counterparts from Trichoderma reesei, Humicola insolens and other fungal sources, distinct differences were noted.

Biochemical and genetic properties of Paenibacillus glycosyl hydrolase having chitosanase activity and discoidin domain

Cells of "Paenibacillus fukuinensis" D2 produced chitosanase into surrounding medium, in the presence of colloidal chitosan or glucosamine. The gene of this enzyme was cloned, sequenced, and subjected to site-directed mutation and deletion analyses. The nucleotide sequence indicated that the chitosanase was composed of 797 amino acids and its molecular weight was 85,610. Unlike conventional family 46 chitosanases, the enzyme has family 8 glycosyl hydrolase catalytic domain, at the amino-terminal side, and discoidin domain at the carboxyl-terminal region. Expression of the cloned gene in Escherichia coli revealed beta-1,4-glucanase function, besides chitosanase activity. Analyses by zymography and immunoblotting suggested that the active enzyme was, after removal of signal peptide, produced from inactive 81-kDa form by proteolysis at the carboxyl-terminal region. Replacements of Glu(115) and Asp(176), highly conserved residues in the family 8 glycosylase region, with Gln and Asn caused simultaneous loss of chitosanase and glucanase activities, suggesting that these residues formed part of the catalytic site. Truncation experiments demonstrated indispensability of an amino-terminal region spanning 425 residues adjacent to the signal peptide.

Characterization of a chitinase gene from Stenotrophomonas maltophilia strain 34S1 and its involvement in biological control

A chitinase gene was cloned on a 2.8-kb DNA fragment from Stenotrophomonas maltophilia strain 34S1 by heterologous expression in Burkholderia cepacia. Sequence analysis of this fragment identified an open reading frame encoding a deduced protein of 700 amino acids. Removal of the signal peptide sequence resulted in a predicted protein that was 68 kDa in size. Analysis of the sequence indicated that the chitinase contained a catalytic domain belonging to family 18 of glycosyl hydrolases. Three putative binding domains, a chitin binding domain, a novel polycystic kidney disease (PKD) domain, and a fibronectin type III domain, were also identified within the sequence. Pairwise comparisons of each domain to the most closely related sequences found in database searches clearly demonstrated variation in gene sources and the species from which related sequences originated. A 51-kDa protein with chitinolytic activity was purified from culture filtrates of S. maltophilia strain 34S1 by hydrophobic interaction chromatography. Although the protein was significantly smaller than the size predicted from the sequence, the N-terminal sequence verified that the first 15 amino acids were identical to the deduced sequence of the mature protein encoded by chiA. Marker exchange mutagenesis of chiA resulted in mutant strain C5, which was devoid of chitinolytic activity and lacked the 51-kDa protein in culture filtrates. Strain C5 was also reduced in the ability to suppress summer patch disease on Kentucky bluegrass, supporting a role for the enzyme in the biocontrol activity of S. maltophilia.

Molecular cloning and sequencing of the gene encoding an exopolygalacturonase of a Bacillus isolate and properties of its recombinant enzyme

An exopolygalacturonase (exo-PGase; EC 3.2.1.82) was found in the culture broth of a Bacillus isolate. The gene encoding the exo-PGase, pehK, was cloned by polymerase chain reaction using mixed primers designed from N-terminal and internal amino acid (aa) sequences of the enzyme (PehK). The determined nucleotide (nt) sequence of pehK revealed a 2940 bp open reading frame (980 aa) that encoded a putative signal sequence (27 aa) and a mature protein (953 aa; 103810 Da). The recombinant enzyme was purified to homogeneity from a culture broth of Bacillus subtilis harboring a pehK-containing plasmid. It had a molecular mass of 105 kDa and a pI value of 5.0. The maximum activity was observed at pH 8 and 55 degrees C in Tris-HCl buffer. The degradation products from polygalacturonic or oligogalacturonic acids were digalacturonic acid, like the exo-PGases, PehX of Erwinia chrysanthemi and PehB of Ralstonia solanacearum. The deduced aa sequence of PehK exhibited moderate homology to those of PehX and PehB with approx. 30% identity for both. High homology was observed in a suitably aligned internal region of the three enzymes (65% identity), and some of the conserved aa residues appeared to form the catalytic core of the enzymes.

Mapping of genes encoding glycoside hydrolases on the chromosome of Cellulomonas fimi

Cellulomonas fimi genomic DNA was digested with HpaI, MunI, HindIII, and NsiI, producing fragments ranging in size from 20 to 1400 kbp that were resolved by pulsed field gel electrophoresis. Genetic and physical linkages were determined by Southern blotting and were used to construct a genome map. Cellulomonas fimi has a single circular chromosome of approx. 4000 kbp. Except for two closely linked genes, cbh6A and cel5A, the genes known to encode glycoside hydrolases are scattered widely on the chromosome.Key words: Cellulomonas fimi, genome map, pulsed field gel electrophoresis, glycoside hydrolases.

Cloning and sequencing of two genes encoding chitinases A and B from Bacillus cereus CH

Two genes encoding chitinases A and B (chiA and chiB) from Bacillus cereus CH were cloned into Escherichia coli XL1-Blue MRF' by using pBluescript II SK+, and their nucleotide sequences were determined. Open reading frames of the chiA and chiB genes encoded distinct polypeptide chains consisting of 360 and 674 amino acid residues, respectively, with calculated molecular sizes of 39 470 and 74 261 Da, respectively. Comparison of the deduced amino acid sequences with those of other bacterial chitinases revealed that chitinase A consisted of a catalytic domain, while chitinase B consisted of three functional domains, a catalytic domain, a fibronectin type III-like domain, and a cellulose-binding domain. The primary structures of these two proteins were not similar to each other.Key words: Bacillus cereus, chitinase, cloning.

Temporal secretion of a multicellulolytic system in Myxobacter sp. AL-1. Molecular cloning and heterologous expression of cel9 encoding a modular endocellulase clustered in an operon with cel48, an exocellobiohydrolase gene

The Gram-negative soil micro-organism Myxobacter sp. AL-1 possesses at least five extracellular cellulases, the production of which is regulated by the growth cycle. We cloned the complete gene for one of these cellulases, termed cel9, which encoded a 67-kDa modular family 9 endoglycohydrolase, which was produced during the stationary phase of growth and was strongly enhanced by avicel. The predicted product of cel9 matches the structural architecture of family 9 cellulases such as Thermonospora fusca endo/exocellulase E4. Cel9 protein was synthesized in Escherichia coli from a multicopy plasmid and in Bacillus subtilis from the isopropyl thiogalactoside-inducible Pspac promoter and was purified from the culture medium. Thermal stability, optimum pH and temperature dependence of Cel9 were similar when expressed from either source, and were indistinguishable from related cellulases produced by thermophilic bacteria. Downstream from cel9 was found a partial ORF, designated cel48, the deduced product of which was highly similar to bacterial exocellobiohydrolases and processive endoglucanases belonging to family 48 of the glycosyl hydrolases. The cel9 and cel48 genes appear to be arranged as part of an operon.

Cloning, expression and characterization of a family 48 exocellulase, Cel48A, from Thermobifida fusca

The gene for a 104-kDa exocellulase, Cel48A, formerly E6, was cloned from Thermobifida fusca into Escherichia coli and Streptomyces lividans. The DNA sequence revealed a type II cellulose-binding domain at the N-terminus, followed by a FNIII-like domain and ending with a glycosyl hydrolase Family 48 catalytic domain. The enzyme and catalytic domain alone were each expressed in and purified from S. lividans and had very low catalytic activity on swollen cellulose, carboxymethyl cellulose, bacterial microcrystalline cellulose and filter paper. However, in synergistic assays on filter paper, the addition of Cel48A to a balanced mixture of T. fusca endocellulase and exocellulase increased the specific activity from 7.9 to 11.7 micromol cellobiose.min-1.mL-1, more than 15-fold higher than any single enzyme alone. Cel48A retained > 50% of its maximum activity from pH 5 to 9 and from 40 to 60 degrees C. Using SWISSMODEL, the amino-acid sequence of the Cel48Acd was modeled to the known structure of Clostridium cellulolyticum CelF. Family 48 enzymes are remarkably homologous at 35% identity for all their catalytic domains and some of the properties of the 10 members are discussed.

Mannan-degrading enzymes from Cellulomonas fimi

The genes man26a and man2A from Cellulomonas fimi encode mannanase 26A (Man26A) and beta-mannosidase 2A (Man2A), respectively. Mature Man26A is a secreted, modular protein of 951 amino acids, comprising a catalytic module in family 26 of glycosyl hydrolases, an S-layer homology module, and two modules of unknown function. Exposure of Man26A produced by Escherichia coli to C. fimi protease generates active fragments of the enzyme that correspond to polypeptides with mannanase activity produced by C. fimi during growth on mannans, indicating that it may be the only mannanase produced by the organism. A significant fraction of the Man26A produced by C. fimi remains cell associated. Man2A is an intracellular enzyme comprising a catalytic module in a subfamily of family 2 of the glycosyl hydrolases that at present contains only mammalian beta-mannosidases.

amlC, another amylolytic gene maps close to the amlB locus in Streptomyces lividans TK24

The region located upstream of the alpha-amylase gene (amlB) of Streptomyces lividans TK24 (Yin et al., 1997) contains a 2978-bp-long ORF divergent from amlB, and designated amlC. amlC Encodes a 993amino acid (aa) protein with a calculated molecular weight of 107.054kDa. On the basis of sequence similarity as well as enzymatic activity, AmlC is likely to belong to the 1, 4-alpha-D-glucan glucanohydrolase family. amlC is transcribed as a unique 3kb leaderless monocistronic mRNA. Primer extension experiments allowed the identification of promoter sequences that do not resemble the typical eubacterial promoter sequences. amlC was successfully disrupted and was mapped at approx. 700kb from a chromosomal end of S. lividans TK24, 100kb on the right of the amplifiable unit AUD1 (Volff et al., 1996). Nevertheless, amlC disruption seemed to be accompanied by extensive rearrangements of the 2500-kb DraI-II fragment of the chromosome.

Analysis of molecular size distributions of cellulose molecules during hydrolysis of cellulose by recombinant Cellulomonas fimi beta-1,4-glucanases

Four beta-1,4-glucanases (cellulases) of the cellulolytic bacterium Cellulomonas fimi were purified from Escherichia coli cells transformed with recombinant plasmids. Previous analyses using soluble substrates had suggested that CenA and CenC were endoglucanases while CbhA and CbhB resembled the exo-acting cellobiohydrolases produced by cellulolytic fungi. Analysis of molecular size distributions during cellulose hydrolysis by the individual enzymes confirmed these preliminary findings and provided further evidence that endoglucanase CenC has a more processive hydrolytic activity than CenA. The significant differences between the size distributions obtained during hydrolysis of bacterial microcrystalline cellulose and acid-swollen cellulose can be explained in terms of the accessibility of beta-1,4-glucan chains to enzyme attack. Endoglucanases and cellobiohydrolases were much more easily distinguished when the acid-swollen substrate was used.

The cellulolytic system of Clostridium cellulolyticum

Recent findings on the cellulolytic system of the mesophilic Clostridium cellulolyticum are reviewed. Six cellulases and the scaffolding protein, which are, at the present time, the known components of the cellulosome have been cloned. The catalytic and structural properties of the cloned enzymes CelA, CelC, CelD and CelF are described. It was shown that the grafting of the cellulases onto the scaffolding protein was performed using the dockerin-cohesin attachment device and was strictly dependent on the integrity of both components of the complex. The amino-acid sequences of dockerin and cohesin domains of C. cellulolyticum were compared to that of C. cellulovorans and C. thermocellum. This sequence analysis shows that domains belonging to the thermophilic or the mesophilic bacteria can be placed into two well defined groups. The genetic organization of the gene cluster of C. cellulolyticum is discussed.

Structure of the Clostridium stercorarium gene celY encoding the exo-1,4-beta-glucanase Avicelase II

The nucleotide sequence of the celY gene coding for the thermostable exo-1,4-beta-glucanase Avicelase II of Clostridium stercorarium was determined. The gene consists of an ORF of 274Z bp which encodes a preprotein of 914 amino acids with a molecular mass of 103 kDa. The signal-peptide cleavage site was identified by comparison with the N-terminal amino acid sequence of Avicelase II purified from C stercorarium. The celY gene is located in close vicinity to the celZ gene coding for the endo-1,4-beta-glucanase Avicelase I. The CelY-encoding sequence was isolated from genomic DNA of C. stercorarium with the PCR technique. The recombinant enzyme produced in Escherichia coli as a LacZ'-CelY fusion protein could be purified using a simple two-step procedure. The properties of CelY proved to be consistent with those of Avicelase II purified from C. stercorarium. Sequence comparison revealed that CelY consists of an N-terminal catalytic domain flanked by a domain of 95 amino acids with unknown function joined to a type III cellulose-binding domain. The catalytic domain belongs to the recently proposed family L of cellulases (family 48 of glycosyl hydrolases).

The processive endocellulase CelF, a major component of the Clostridium cellulolyticum cellulosome: purification and characterization of the recombinant form

The recombinant form of the cellulase CelF of Clostridium cellulolyticum, tagged by a C-terminal histine tail, was overproduced in Escherichia coli. The fusion protein was purified by affinity chromatography on a Ni-nitrilotriacetic acid column. The intact form of CelF (Mr, 79,000) was rapidly degraded at the C terminus, giving a shorter stable form, called truncated CelF (Mr, 71,000). Both the entire and the truncated purified forms degraded amorphous cellulose (kcat = 42 and 30 min(-1), respectively) and microcrystalline cellulose (kcat = 13 and 10 min(-1), respectively). The high ratio of soluble reducing ends to insoluble reducing ends released by truncated CelF from amorphous cellulose showed that CelF is a processive enzyme. Nevertheless, the diversity of the cellodextrins released by truncated CelF from phosphoric acid-swollen cellulose at the beginning of the reaction indicated that the enzyme might randomly hydrolyze beta-1,4 bonds. This hypothesis was supported by viscosimetric measurements and by the finding that CelF and the endoglucanase CelA are able to degrade some of the same cellulose sites. CelF was therefore called a processive endocellulase. The results of immunoblotting analysis showed that CelF was associated with the cellulosome of C. cellulolyticum. It was identified as one of the three major components of cellulosomes. The ability of the entire form of CelF to interact with CipC, the cellulosome integrating protein, or mini-CipC1, a recombinant truncated form of CipC, was monitored by interaction Western blotting (immunoblotting) and by binding assays using a BIAcore biosensor-based analytical system.

Mechanistic consequences of mutation of active site carboxylates in a retaining beta-1,4-glycanase from Cellulomonas fimi

The exoglucanase/xylanase Cex from Cellulomonas fimi is a retaining glycosidase which functions via a two-step mechanism involving the formation and hydrolysis of a covalent glycosyl-enzyme intermediate. The roles of three conserved active site carboxylic acids in this enzyme have been probed by detailed kinetic analysis of mutants modified at these three positions. Elimination of the catalytic nucleophile (E233A) results in an essentially inactive enzyme, consistent with the important role of this residue. However addition of small anions such as azide or formate restores activity, but as an inverting enzyme since the product formed under these conditions is the alpha-glycosyl azide. Shortening of the catalytic nucleophile (E233D) reduces the rates of both formation and hydrolysis of the glycosyl-enzyme intermediate some 3000-4000-fold. Elimination of the acid/base catalyst (E127A) yields a mutant for which the deglycosylation step is slowed some 200-300-fold as a consequence of removal of general base catalysis, but with little effect on the transition state structure at the anomeric center. Effects on the glycosylation step due to removal of the acid catalyst depend on the aglycon leaving group ability, with minimal effects on substrates requiring no general acid catalysis but large (> 10(5)-fold) effects on substrates with poor leaving groups. The Brønsted beta 1g value for hydrolysis of aryl cellobiosides was much larger (beta 1g approximately -1) for the mutant than for the wild-type enzyme (beta 1g = -0.3), consistent with removal of protonic assistance. The pH-dependence was also significantly perturbed. Mutation of a third conserved active site carboxylic acid (E123A) resulted in rate reductions of up to 1500-fold on poorer substrates, which could be largely restored by addition of azide, but without the formation of glycosyl azide products. These results suggest a simple strategy for the identification of the key active site nucleophile and acid/base catalyst residues in glycosidases without resort to active site labeling.

Microbial hydrolysis of polysaccharides

Microorganisms are efficient degraders of starch, chitin, and the polysaccharides in plant cell walls. Attempts to purify hydrolases led to the realization that a microorganism may produce a multiplicity of enzymes, referred to as a system, for the efficient utilization of a polysaccharide. In order to fully characterize a particular enzyme, it must be obtained free of the other components of a system. Quite often, this proves to be very difficult because of the complexity of a system. This realization led to the cloning of the genes encoding them as an approach to eliminating other components. More than 400 such genes have been cloned and sequenced, and the enzymes they encode have been grouped into more than 50 families of related amino acid sequences. The enzyme systems revealed in this manner are complex on two quite different levels. First, many of the individual enzymes are complex, as they are modular proteins comprising one or more catalytic domains linked to ancillary domains that often include one or more substrate-binding domains. Second, the systems are complex, comprising from a few to 20 or more enzymes, all of which hydrolyze a particular substrate. Systems for the hydrolysis of plant cell walls usually contain more components than systems for the hydrolysis of starch and chitin because the cell walls contain several polysaccharides. In general, the systems produced by different microorganisms for the hydrolysis of a particular polysaccharide comprise similar enzymes from the same families.

Characterization of CenC, an enzyme from Cellulomonas fimi with both endo- and exoglucanase activities

The cenC gene, encoding beta-1,4-glucanase C (CenC) from Cellulomonas fimi, was overexpressed in Escherichia coli with a tac-based expression vector. The resulting polypeptide, with an apparent molecular mass of 130 kDa, was purified from the cell extracts by affinity chromatography on cellulose followed by anion-exchange chromatography. N-terminal sequence analysis showed the enzyme to be properly processed. Mature CenC was optimally active at pH 5.0 and 45 degrees C. The enzyme was extremely active on soluble, fluorophoric, and chromophoric glycosides (4-methylumbelliferyl beta-glycosides, 2'-chloro-4'-nitrophenyl-beta-D-cellobioside, and 2'-chloro-4'-nitrophenyl-lactoside) and efficiently hydrolyzed carboxymethyl cellulose, barley beta-glucan, lichenan, and, to a lesser extent, glucomannan. CenC also hydrolyzed acid-swollen cellulose, Avicel, and bacterial microcrystalline cellulose. However, degradation of the latter was slow compared with its degradation by CenB, another C. fimi cellulose belonging to the same enzyme family. CenC acted with inversion of configuration at the anomeric carbon, in accordance with its classification as a family 9 member. The enzyme released mainly cellobiose from soluble cellodextrins and insoluble cellulose. Attack appeared to be from the reducing chain ends. Analysis of carboxymethyl cellulose hydrolysis suggests that CenC is semiprocessive enzyme with both endo- and exoglucanase activities.

Substrate specificity of endoglucanase A from Cellulomonas fimi: fundamental differences between endoglucanases and exoglucanases from family 6

Values of kcat. and Km for the hydrolysis of cellotetraose, cellotriose, beta-cellobiosyl fluoride and various beta-aryl cellobiosides by endoglucanase A (CenA) from Cellulomonas fimi indicate that specific binding interactions between the reducing-end glucose residues of cellotetraose and cellotriose and the enzyme at the transition state provide enormous stabilization, endowing glucose with the "effective leaving group ability' of 2,4-dinitrophenol. As has been seen with several other inverting glycosidases, CenA hydrolyses the "wrong' anomer of its glycosyl fluoride substrate, alpha-cellobiosyl fluoride, according to non-Michaelian kinetics. This indicates that CenA carries out this hydrolysis by a mechanism involving binding of two substrate molecules in the active site (Hehre, Brewer and Genghof (1979) J. Biol. Chem. 254, 5942-5950] in contrast with that reported for cellobiohydrolase II, another family-6 enzyme [Konstantinidis, Marsden and Sinnott (1993) Biochem. J. 291, 833-838]. The pH profiles for wild-type CenA indicate that kcat. for CenA depends on the presence of both a protonated group and a deprotonated group for full activity, consistent with the presence of an acid and a base catalyst at the active site. By contrast, the profile for the Asp252Ala mutant of CenA shows a dependence only on a base-catalytic group, thereby confirming the role of Asp-252 as an acid catalyst. These results show that hydrolysis by CenA occurs by a typical inverting mechanism involving both acid and base catalysis, as first proposed by Koshland. It also suggests that endoglucanases from family 6 may function by fundamentally different mechanisms for exoglucanases in this family.

Molecular study and overexpression of the Clostridium cellulolyticum celF cellulase gene in Escherichia coli

The CelF-encoding sequence was isolated from Clostridium cellulolyticum genomic DNA using the inverse PCR technique. The gene lies between cipC (the gene encoding the cellulosome scaffolding protein) and celC (coding for the endoglucanase C) in the large cel cluster of this mesophilic cellulolytic Clostridium species. Comparisons between the deduced amino acid sequence of the mature CelF (693 amino acids, molecular mass 77626) and those of other beta-glycanases showed that this enzyme belongs to the recently proposed family L of cellulases (family 48 of glycosyl hydrolases). The protein was overproduced in Escherichia coli using the T7 expression system. It formed both cytoplasmic and periplasmic inclusion bodies when induction was performed at 37 degrees C. Surprisingly, the protein synthesized from the cytoplasmic production vector was degraded in the Ion protease-deficient strain BL21(DE3). The induction conditions were optimized with regard to the concentration of inductor, cell density, and temperature and time of induction in order to overproduce an active periplasmic protein (CelFp) which was both soluble and stable. It was collected using the osmotic shock method. The enzymic degradation of various cellulosic substrates by CelFp was studied. CelFp degraded swollen Avicel more efficiently than substituted soluble CM-cellulose or crystalline Avicel and was not active on xylan. Its activity is therefore quite different from that of endoglucanases, which are most active on CM-cellulose.