Studies of the cellulolytic system of Trichoderma reesei QM 9414. Analysis of domain function in two cellobiohydrolases by limited proteolysis

Monoclonal antibodies against core and cellulose-binding domains of Trichoderma reesei cellobiohydrolases I and II and endoglucanase I

Cellulases from Trichoderma reesei form an enzyme group with a common structural organization. Each cellulase enzyme is composed of two functional domains, the core region containing the active site and the cellulose-binding domain (CBD). To facilitate the specific detection of each domain, monoclonal antibodies (mAb) against cellobiohydrolase I (CBHI), cellobiohydrolase II (CBHII) and endoglucanase I (EGI) were produced. Five mAb were obtained against CBHI, ten against CBHII and eight against EGI. The location of the antigenic epitope for each antibody was mapped by allowing the antibodies to react with truncated cellulases, synthesized from deleted cDNA in Saccharomyces cerevisiae. Proteolytic fragments of Trichoderma cellulases, obtained by papain digestion, were used to confirm the results. Specific antibodies were detected against the core and the CBD epitopes for all three cellulases. Using the truncated enzymes, it was possible to locate the epitopes to a reasonably short region within the protein. To obtain a quantitative assay for each enzyme, a specific mAb against each antigen was chosen, based on the affinity to the corresponding antigen on Western-blot staining and on filter blots of the cellulolytic yeasts. The mAb were used to quantitative the corresponding enzymes in T. reesei culture medium. Specific quantitation of each cellulase enzyme has not been possible by biochemical assays or using polyclonal antibodies, due to their cross-reactions. Now, these mAb can be specifically used to recognize and quantitate different domains of these three important cellulolytic enzymes.

Purification and properties of a novel type of exo-1,4-beta-glucanase (avicelase II) from the cellulolytic thermophile Clostridium stercorarium

Avicelase II was purified to homogeneity from culture supernatants of Clostridium stercorarium. A complete separation from the major cellulolytic enzyme activity (avicelase I) was achieved by FPLC gel filtration on Superose 12 due to selective retardation of avicelase II. The enzyme has an apparent molecular mass of 87 kDa and a pI of 3.9. Determination of the N-terminal amino acid indicates that avicelase II is not a proteolytically processed product of avicelase I. Maximal activity of avicelase II is observed between pH 5 and 6. In the presence of Ca2+, the enzyme is highly thermostable, exhibiting a temperature optimum around 75 degrees C. Hydrolysis of avicel occurs at a linear rate for three days at 70 degrees C. Avicelase II is active towards unsubstituted celluloses, cellotetraose and larger cellodextrins. It lacks activity towards carboxymethylcellulose and barley beta-glucan. Unlike other bacterial exoglucanases, avicelase II does not hydrolyze aryl-beta-D-cellobiosides. Avicel is degraded to cellobiose and cellotriose at a molar ratio of approximately 4:1. With acid-swollen avicel as substrate, cellotetraose is also formed as an intermediary product, which is further cleaved to cellobiose. The degradation patterns of reduced cellodextrins differ from that expected for a cellobiohydrolase attacking the non-reducing ends of chains; cellopentaitol is degraded to cellobiitol and cellotriose, while cellohexaitol is initially cleaved into cellobiitol and cellotetraose. These findings, taken together, indicate that avicelase II represents a novel type of exoglucanase (cellodextrinohydrolase), which, depending on the accessibility of the substrate, releases cellotetraose, cellotriose, or cellobiose from the non-reducing end of the cellulose chains.

Domains in microbial beta-1, 4-glycanases: sequence conservation, function, and enzyme families

Several types of domain occur in beta-1, 4-glycanases. The best characterized of these are the catalytic domains and the cellulose-binding domains. The domains may be joined by linker sequences rich in proline or hydroxyamino acids or both. Some of the enzymes contain repeated sequences up to 150 amino acids in length. The enzymes can be grouped into families on the basis of sequence similarities between the catalytic domains. There are sequence similarities between the cellulose-binding domains, of which two types have been identified, and also between some domains of unknown function. The beta-1, 4-glycanases appear to have arisen by the shuffling of a relatively small number of progenitor sequences.

Nucleotide sequence of the endoglucanase C gene (cenC) of Cellulomonas fimi, its high-level expression in Escherichia coli, and characterization of its products

The cenC gene of Cellulomonas fimi, encoding endoglucanase CenC, has an open reading frame of 1101 codons closely followed by a 9 bp inverted repeat. The predicted amino acid sequence of mature CenC, which is 1069 amino acids long, is very unusual in that it has a 150-amino-acid tandem repeat at the N-terminus and an unrelated 100-amino-acid tandem repeat at the C-terminus. CenC belongs to subfamily E1 of the beta-1,4-glycanases. High-level expression in Escherichia coli of cenC from a 3.6 kbp fragment of C. fimi DNA leads to levels of CenC which exceed 10% of total cell protein. Most of the CenC is in the cytoplasm in an inactive form. About 60% of the active fraction of CenC is in the periplasm. The catalytic properties of the active CenC are indistinguishable from those of native CenC from C. fimi. The Mr of CenC from E. coli and C. fimi is approximately 130 kDa. E. coli and C. fimi also produce an endoglucanase, CenC', of approximate Mr 120kDa and with the same N-terminal amino acid sequence and catalytic properties as CenC. CenC' appears to be a proteolytic product of CenC. CenC and CenC' can bind to cellulose and to Sephadex. CenC is the most active component of the C. fimi cellulase system isolated to date.

A re-appraisal of multiplicity of endoglucanase I from Trichoderma reesei using monoclonal antibodies and plasma desorption mass spectrometry

An endo beta-1,4-glucanase (EC 3.2.1.4, 1.4-(1,3;1,4)-beta-D-glucan 4 glucanhydrolase) was purified to apparent homogeneity from culture filtrates of Trichoderma reesei QM 9414. Identity of the protein with endoglucanase I (EG I) was examined by subjecting CNBr fragments of the protein to analysis by plasma desorption mass spectrometry. Seven non-glycosylated fragments, mapped on the eg1 gene sequence, could be identified, hence proving at least 39.4% identity of the amino acid sequence. No sign for microheterogeneity was observed. Purified EG I was used to prepare monoclonal antibodies. 17 stable clones were obtained, of which one--Mab EG 3--was used to analyze several commercial T. reesei cellulase preparations as well as culture filtrates from T. pseudokoningii and T. longibrachiatum for the presence of EG I. Most of them contained immunoreactive material migrating as a prominent 50-55 kDa band on SDS-PAGE, resembling EG I, but in some instances additional lower molecular weight bands were also observed. Cultivation of T. reesei at low pH led to an increase of these lower molecular weight bands. EG I was rather stable against proteolysis by papain in vitro, but after prolonged treatment, immunopositive products of 50 and 45 kDa were produced at the expense of the 55 kDa band. Our monoclonal antibodies failed to react with a low-molecular-weight endoglucanase, which was previously shown to be detectable with polyclonal antiserum against EG I. However, all monoclonals reacted with a 118 kDa protein which is most probably a dimer of EG I. These results are discussed with respect to the occurrence of multiple forms of EG I in T. reesei cellulase preparations.

The non-catalytic C-terminal region of endoglucanase E from Clostridium thermocellum contains a cellulose-binding domain

Mature endoglucanase E (EGE) from Clostridium thermocellum consists of 780 amino acid residues and has an Mr of 84,016. The N-terminal 334 amino acids comprise a functional catalytic domain. Full-length EGE bound to crystalline cellulose (Avicel) but not to xylan. Bound enzyme could be eluted with distilled water. The capacity of truncated derivatives of the enzyme to bind cellulose was investigated. EGE lacking 109 C-terminal residues (EGEd) or a derivative in which residues 367-432 of the mature form of the enzyme had been deleted (EGEb), bound to Avicel, whereas EGEa and EGEc, which lack 416 and 246 C-terminal residues respectively, did not. The specific activity of EGEa, consisting of the N-terminal 364 amino acids, was 4-fold higher than that of the full-length enzyme. The truncated derivative also exhibited lower affinity for the substrate beta-glucan than the full-length enzyme. It is concluded that EGE contains a cellulose-binding domain, located between residues 432 and 671, that is distinct from the active site. The role of this substrate-binding domain is discussed.

Modification of the properties of a Ruminococcus albus endo-1,4-beta-glucanase by gene truncation

An endo-1,4-beta-glucanase (EgI) gene isolated from Ruminococcus albus was deleted at the 5'-flanking region by gene truncation or at the 3'-flanking region by insertion of an omega (omega) fragment with a universal stop codon at the EcoRI or BamHI site. These modified genes were integrated into pUC vectors to construct chimera plasmids for Escherichia coli. The truncated EgIs were produced from transformants (E. coli) harboring the chimera plasmids. An EgI with a 15-amino-acid N-terminal deletion exibited higher activity at lower pH and temperature compared with the activity of the original EgI. The EgIs with 59- and 75-amino-acid deletions from the N and C terminals, respectively, had no activity, indicating that both terminal moieties are essential for enzyme activity.

Unusual sequence organization in CenB, an inverting endoglucanase from Cellulomonas fimi

The nucleotide sequence of the cenB gene was determined and used to deduce the amino acid sequence of endoglucanase B (CenB) of Cellulomonas fimi. CenB comprises 1,012 amino acids and has a molecular weight of 105,905. The polypeptide is divided by so-called linker sequences rich in proline and hydroxyamino acids into five domains: a catalytic domain of 607 amino acids at the N terminus, followed by three repeats of 98 amino acids each which are greater than 60% identical, and a C-terminal domain of 101 amino acids which is 50% identical to the cellulose-binding domains of C. fimi cellulases Cex and CenA. A deletion mutant of the cenB gene encodes a polypeptide lacking the C-terminal 333 amino acids of CenB. The truncated polypeptide is catalytically active and, like intact CenB, binds to cellulose, suggesting that CenB has a second cellulose-binding site. The sequence of amino acids 1 to 461 of CenB is 35% identical, with a further 15% similarity, to that of a cellulase from avocado, which places CenB in cellulase family E. CenB releases mostly cellobiose and cellotetraose from cellohexaose. Like CenA, CenB hydrolyzes the beta-1,4-glucosidic bond with inversion of the anomeric configuration. The pH optimum for CenB is 8.5, and that for CenA is 7.5.

The conserved terminal region of Trichoderma reesei cellulases forms a strong antigenic epitope for polyclonal antibodies

The specificity of polyclonal antibodies (Pab) raised against Trichoderma reesei cellulases has been studied. cDNAs lacking regions coding for certain functional domains were produced by preparing series of 3'-end deletions from the cDNAs for two cellobiohydrolases, CBH I and CBH II, and an endoglucanase, EG I. The proteins coded by the full length cDNAs and the truncated proteins coded by the deleted cDNAs were expressed in yeast Saccharomyces cerevisiae, under the control of the ADC1 promoter. Each polyclonal antiserum showed cross-reactivity with other cellulases. Pabs for CBH I and CBH II both recognized EG I. Pab for EG I strongly recognized both CBH I and CBH II. By analyzing the truncated proteins, we found that these antibodies were almost entirely directed against the conserved tail of the cellulase enzymes.

The tertiary structure of a bacterial cellulase determined by small-angle X-ray-scattering analysis

CenA from Cellulomonas fimi is a beta-1,4-endoglucanase that binds tightly to cellulose. X-ray-scattering analyses show that the enzyme is tadpole-shaped: the previously identified catalytic and cellulose-binding domains comprise the head and tail respectively. It appears that this structural and functional organization is common to several cellulases from bacteria and fungi.

The competitive inhibition of Trichoderma reesei C30 cellobiohydrolase I by guanidine hydrochloride

The p-nitrophenylcellobiosidase (PNPCase) activity of Trichoderma reesei cellobiohydrolase I (CBH I) was competitively inhibited by concentrations of guanidine hydrochloride (Gdn HCl) that did not affect the tryptophan fluorescence of this enzyme. The Km of CBH I, 3.6 mM, was increased to 45.4 mM in the presence of 0.14 M Gdn HCl, the concentration that was required to inhibit the enzyme by 50%. A similar concentration of lithium chloride and urea had little effect on the PNPCase activity of CBH I. Maximal inhibition was pH dependent, occurring in the range of pH 4.0 to 5.0, which is in the range for maximal activity. Analysis of the inhibition data indicated that 1.2 molecules of Gdn HCl combined reversibly with 1 molecule of CBH I. Other hydrolases and proteases were also inhibited by Gdn HCl. It is suggested that the inhibition of CBH I by Gdn HCl occurs as a result of the interaction between the positively charged guanidinium group of Gdn HCl and the carboxylate group of glutamic acid 126, postulated to be in the catalytic center of this enzyme.

Sequence analysis of the Clostridium stercorarium celZ gene encoding a thermoactive cellulase (Avicelase I): identification of catalytic and cellulose-binding domains

The nucleotide sequence of the celZ gene coding for a thermostable endo-beta-1,4-glucanase (Avicelase I) of Clostridium stercorarium was determined. The structural gene consists of an open reading frame of 2958 bp which encodes a preprotein of 986 amino acids with an Mr of 109,000. The signal peptide cleavage site was identified by comparison with the N-terminal amino acid sequence of Avicelase I purified from C. stercorarium culture supernatants. The recombinant protein expressed in Escherichia coli is proteolytically cleaved into catalytic and cellulose-binding fragments of about 50 kDa each. Sequence comparison revealed that the N-terminal half of Avicelase I is closely related to avocado (Persea americana) cellulase. Homology is also observed with Clostridium thermocellum endoglucanase D and Pseudomonas fluorescens cellulase. The cellulose-binding region was located in the C-terminal half of Avicelase I. It consists of a reiterated domain of 88 amino acids flanked by a repeated sequence about 140 amino acids in length. The C-terminal flanking sequence is highly homologous to the non-catalytic domain of Bacillus subtilis endoglucanase and Caldocellum saccharolyticum endoglucanase B. It is proposed that the enhanced cellulolytic activity of Avicelase I is due to the presence of multiple cellulose-binding sites.

Three-dimensional structure of cellobiohydrolase II from Trichoderma reesei

The enzymatic degradation of cellulose is an important process, both ecologically and commercially. The three-dimensional structure of a cellulase, the enzymatic core of CBHII from the fungus Trichoderma reesei reveals an alpha-beta protein with a fold similar to but different from the widely occurring barrel topology first observed in triose phosphate isomerase. The active site of CBHII is located at the carboxyl-terminal end of a parallel beta barrel, in an enclosed tunnel through which the cellulose threads. Two aspartic acid residues, located in the center of the tunnel are the probable catalytic residues.

Spatial separation of protein domains is not necessary for catalytic activity or substrate binding in a xylanase

Xylanase A (XYLA) from Pseudomonas fluorescens subspecies cellulosa shows sequence conservation with two endoglucanases from the same organism. The conserved sequence in XYLA, consisting of the N-terminal 234 residues, is not essential for catalytic activity. Full-length XYLA and a fusion enzyme, consisting of the N-terminal 100 residues of XYLA linked to mature alkaline phosphatase, bound tightly to crystalline cellulose (Avicel), but not to xylan. The capacity of truncated derivatives of the xylanase to bind polysaccharides was investigated. XYLA lacking the first 13 N-terminal amino acids did not bind to cellulose. However, a catalytically active XYLA derivative (XYLA'), in which residues 100-234 were deleted, bound tightly to Avicel. Substrate specificity, cellulose-binding capacity, specific activity and Km for xylan hydrolysis were evaluated for each of the xylanases. No differences in any of these parameters were detected for the two enzymes. It is concluded that XYLA contains a cellulose-binding domain consisting of the N-terminal 100 residues which is distinct from the active site. Spatial separation of the catalytic and cellulose-binding domains is not essential for the enzyme to function normally.

The N-terminal region of an endoglucanase from Pseudomonas fluorescens subspecies cellulosa constitutes a cellulose-binding domain that is distinct from the catalytic centre

The substrate specificity of an endoglucanase (EGB) from Pseudomonas fluorescens subspecies cellulosa was determined. The enzyme was most active against barley beta-glucan, but showed significant activity against amorphous and crystalline cellulose. EGB was purified to homogeneity by affinity chromatography with crystalline cellulose (Avicel). The Mr of the purified enzyme was 50,000, which is in good agreement with the size of EGB deduced from the nucleotide sequence of the celB gene, coding for EGB. The N-terminal region of the mature form of EGB showed strong homology to another endoglucanase and to a xylanase expressed by the same organism; homologous sequences included highly conserved serine-rich regions. Truncated forms of celB, in which the gene sequence encoding the conserved domain had been deleted, directed the synthesis of a functional endoglucanase that did not bind to crystalline cellulose. This indicates that the conserved region of endoglucanases and xylanases expressed by P. fluorescens subsp. cellulosa constitutes a cellulose-binding domain, which is distinct from the active centre. The possible role of this substrate-binding region is discussed.

Nucleotide sequence of the engXCA gene encoding the major endoglucanase of Xanthomonas campestris pv. campestris

The nucleotide sequence of the gene (engXCA) encoding the major extracellular endoglucanase (ENGXCA) of the phytopathogenic bacterium Xanthomonas campestris pv. campestris (X. c. campestris) was determined and compared with the N-terminal amino acid (aa) sequence of the purified enzyme. An open reading frame of 1479 bp encoding 493 aa was identified, of which the N-terminal 25 aa represent a potential signal peptide. Determination of the exact position of a Tn5 insertion within engXCA, which did not reduce the encoded enzyme activity, indicated that the C-terminal region of the protein is not crucial for ENGXCA activity. Comparison of the complete deduced aa sequence with those deduced from other endoglucanase- and exoglucanase-encoding genes revealed a region with a high degree of homology, located towards the C terminus of the protein. These data indicate that the X. c. campestris ENGXCA may have a domain structure similar to that of many other bacterial and fungal cellulolytic enzymes. Hydrophobic cluster analysis was performed on the deduced aa sequence. Comparison of this analysis with those of 30 other cellulase sequences belonging to six different families indicated that the X. c. campestris enzyme can be classified in family A. The two aa residues which had previously been identified as 'potentially catalytic' within this family of cellulases, are conserved in the X. c. campestris ENGXCA.

Comparison of the hydrolytic activity and fluorescence of native, guanidine hydrochloride-treated and renatured cellobiohydrolase I from Trichoderma reesei

Guanidine hydrochloride (GdnHCl) is an effective agent for the elution of cellulase protein from unhydrolyzed cellulosic residues, but once eluted the enzyme is inactive. The studies described in this paper examine the effect of GdnHCl on the hydrolytic activity and tryptophan fluorescence of cellobiohydrolase I (CBH I) from Trichoderma reesei. CBH I was found to be completely inactivated by 0.25 M GdnHCl, but higher concentrations of GdnHCl were required to partially unfold this enzyme, as determined from the measurement of a decrease in its tryptophan fluorescence. Binding of CBH I to microcrystalline cellulose was prevented by 4 M GdnHCl, suggesting that a conformational change of CBH I resulted in the loss of substrate binding. Removal of the denaturant from CBH I by dialysis or gel filtration allowed the kinetics of the reactivation of CBH I, after 4 M GdnHCl treatment, to be studied. The fluorescence and specific hydrolytic activity of native and renatured CBH I were comparable. It is concluded, therefore, that GdnHCl may be used to elute cellulase components, such as CBH I, adsorbed on undigested cellulosic substrates since this component can easily be renatured and subsequently reused.

Structural changes in cellobiohydrolase I upon binding of a macromolecular ligand as evident by SAXS investigations

Xylan from Rhodymenia palmata binds to the cellobiohydrolase I from Trichoderma reesei (CBH I) or its core protein, inhibiting their activity. Adsorption onto microcrystalline cellulose (Avicel) is reduced approximately 30% for intact CBH I and nearly 50% for the core, whereas the effects with cellobiose are negligible. Structural changes concomitant with this binding are studied in solution by small angle X-ray scattering. In the "tadpole" structure typical for the CBH I [Abuja et al., 1988] the lengthening of the tail part is the most salient observation when xylan is present which accounts for an increase in Dmax (18.0 to 22.0 nm) and radius of gyration (4.74 to 5.18 nm). When xylan binds to the core the radius of gyration remains nearly unchanged. Here a model can be constructed showing a xylan molecule on the surface of the core protein near the tail part.

Studies of the cellulolytic system of Trichoderma reesei QM 9414. Binding of small ligands to the 1,4-beta-glucan cellobiohydrolase II and influence of glucose on their affinity

Binding onto cellobiohydrolase II from Trichoderma reesei of glucose, cellobiose, cellotriose, derivatized and analogous compounds, is monitored by protein-difference-absorption spectroscopy and by titration of ligand fluorescence, either at equilibrium or by the stopped-flow technique. The data complete earlier results [van Tilbeurgh, H., Pettersson, L. G., Bhikhabhai, R., De Boeck, H. and Claeyssens, M. (1985) Eur. J. Biochem. 148, 329-334] indicating an extended active center, with putative subsites ABCD. Subsite A specifically complexes with beta-D-glucosides and D-glucose; at 25 degrees C the latter influences the concomitant binding of other ligands at neighbouring sites. For several ligands this cooperative effect for binding (at 0.33 M glucose and temperature range 4-37 degrees C) was characterized by a substantial increase of the enthalpic term (delta delta H = -35 kJ mol-1). Glucose (0.33 M) decreases the association and dissociation rate parameters of 4-methylumbelliferyl beta-D-cellobioside by one order of magnitude: k+ = (3.6 +/- 0.5) x 10(-5) M-1 s-1 versus (5.1 +/- 0.1) x 10(-6) M-1 s-1 (in the absence of glucose) and k- = (1.3 +/- 0.1) s-1 versus (14.0 +/- 0.3) s-1. As deduced from substrate-specificity studies and inhibition experiments, subsite B interacts with terminal non-reducing glucopyranosyl residues of oligomeric ligands and substrates, whereas catalytic (hydrolytic) cleavage occurs between C and D. Association constants 10-100 times higher than those for cellobiose or its glycosides were observed for D-glucopyranosyl-(1----4)-beta-D-xylopyranose and cellobionolactone derivatives, suggesting 'transition-state'-type binding for these ligands at subsite C. Although subsite D can accomodate a bulky chromophoric group (MeUmb) its preference for a glucosyl residue is reflected in the lower binding enthalpy of cellotriose (-34 kJ mol-1) as compared to cellobiose (-28.3 kJ mol-1) and MeUmb(Glc)2 (-11.6 kJ mol-1). This model indicates that oligomeric ligands (substrates) interact through cooperativity of their subunits at the extended binding site of cellobiohydrolase II.

Purification and characterization of endoglucanase C of Cellulomonas fimi, cloning of the gene, and analysis of in vivo transcripts of the gene

Two nonglycosylated endoglucanases which bind to Sephadex were purified from culture supernatants of Cellulomonas fimi grown on microcrystalline cellulose. Their Mrs were 120,000 and 130,000. The N-terminal amino acid sequences of the enzymes were identical, suggesting that the enzymes were related. A DNA fragment encoding this N-terminal sequence was cloned in Escherichia coli. The nucleotide sequence corresponding to the N-terminal amino acid sequence was preceded by a sequence encoding a typical leader peptide. Transcripts hybridizing to the cloned fragment were detected in total RNA isolated from C. fimi cells grown on carboxymethyl cellulose but not from cells grown on glycerol or glucose. Transcription started at a cluster of sites 53 to 59 nucleotides upstream of a GUG translation initiation codon and terminated at either of two closely spaced C residues immediately downstream of a region of potential secondary structure. The size of the transcript was approximately 3.5 kilobases, sufficient to encode a polypeptide of 130 kilodaltons. The 130-kilodalton polypeptide is designated endoglucanase C (CenC), and the gene encoding it is designated cenC.

Crystallization of the core protein of cellobiohydrolase II from Trichoderma reesei

Single crystals of the core protein of the cellulase cellobiohydrolase II have been grown in polyethylene glycol 6000 with the hanging drop method. Successful crystallization occurred only when 82 amino acids were removed from the N terminus by papain cleavage. Crystals belong to the space group P2(1) and have cell constants a = 49.1 A, b = 75.8 A, c = 92.9 A, beta = 103.2. The diffraction pattern extends to better than 2.0 A.

Cellulase families revealed by hydrophobic cluster analysis

The amino acid sequences of 21 beta-glycanases have been compared by hydrophobic cluster analysis. Six families of cellulases have been identified on the basis of primary structure homology: (A) endoglucanases B, C and E of Clostridium thermocellum; endoglucanases of Erwinia chrysanthemi and Bacillus sp.; endoglucanase III of Trichoderma reesei; endoglucanase I of Schizophyllum commune; (B) cellobiohydrolase II of T. reesei; endoglucanases of Cellulomonas fimi and Streptomyces sp; (C) cellobiohydrolases I of T. reesei and of Phanerochaete chrysosporium; endoglucanase I of T. reesei; (D) endoglucanase A of C. thermocellum and an endoglucanase from Ce. uda; (E) endoglucanase D of C. thermocellum and an endoglucanase from Pseudomonas fluorescens; (F) xylanases of C. thermocellum and of Cryptococcus albidus and the cellobio-hydrolase of Ce. fimi. For each family, conserved potentially catalytic residues have have been listed and previous allocations of the active-site residues are evaluated in the light of the alignment of the amino acid sequences. A strong homology is also reported for the putative cellulose-binding domains of cellulases of Ce. fimi and of P. fluorescens.

Fungal cellulase systems. Comparison of the specificities of the cellobiohydrolases isolated from Penicillium pinophilum and Trichoderma reesei

Reaction patterns for the hydrolysis of chromophoric glycosides from cello-oligosaccharides and lactose by the cellobiohydrolases (CBH I and CBH II) purified from Trichoderma reesei and Penicillium pinophilum were determined. They coincide with those found for the parent unsubstituted sugars. CBH I enzyme from both organisms attacks these substrates in a random manner. Turnover numbers are, however, low and do not increase appreciably as a function of the degree of polymerization of the substrates. The active-site topology of the CBH I from T. reesei was further probed by equilibrium binding experiments with cellobiose, cellotriose, lactose and some of their derivatives. These point to a single interaction site (ABC), spatially restricted as deduced from the apparent independency of the thermodynamic parameters. It appears that the putative subsite A can accommodate a galactopyranosyl or glucopyranosyl group, and subsite B a glucopyranosyl group, whereas in subsite C either a glucopyranosyl or a chromophoric group can be bound, scission occurring between subsites B and C. The apparent kinetic parameters (turnover numbers) for the hydrolysis of cello-oligosaccharides (and their derivatives) by the CBH II type enzyme increase as a function of chain length, indicative of an extended binding site (A-F). Its architecture allows for specific binding of beta-(1----4)-glucopyranosyl groups in subsites A, B and C. Binding of a chromophore in subsite C produces a non-hydrolysable complex. The thermodynamic interaction parameters of some ligands common to both type of enzyme were compared: these substantiate the conclusions reached above.

Catalytic and substrate-binding domains of endoglucanase 2 from Bacteroides succinogenes

Endoglucanase 2 (EG2) of the cellulolytic ruminal anaerobe Bacteroides succinogenes is a 118-kilodalton (kDa) enzyme which binds to cellulose and produces cellotetraose as the end product of hydrolysis. The purified enzyme was treated with the protease trypsin in an attempt to isolate peptides which retained the ability to either hydrolyze soluble carboxymethyl cellulose or bind to insoluble cellulose. There was no loss in endoglucanase activity (carboxymethylcellulase) over a period of 2 h following the addition of trypsin. In comparison, there was a greater than eightfold reduction in the binding of carboxymethylcellulase activity to crystalline cellulose. A Lineweaver-Burk plot with amorphous cellulose as the substrate revealed that the trypsin-digested enzyme had an identical Vmax but a 1.9-fold-lower Km in comparison with the intact enzyme. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the trypsin-digested enzyme revealed two major peptides of 43 and 51 kDa (p43 and p51). The 43-kDa peptide was able to bind to both amorphous and crystalline cellulose, whereas p51 did not. Purified p51 had a molar activity toward carboxymethyl cellulose which was identical to that of the intact enzyme, but activity toward both amorphous and crystalline cellulose was reduced approximately twofold. Two high-titer monoclonal antibodies from mice immunized with the intact protein recognized p43 but not p51. The results are consistent with a bifunctional organization of EG2, in which the 118-kDa enzyme is composed of a 51-kDa catalytic domain and a highly antigenic 43-kDa substrate-binding domain. In terms of its domain structure and activity toward cellulose, EG2 is very similar to cellobiohydrolase II of Trichoderma reesei.

Monoclonal antibodies against different domains of cellobiohydrolase I and II from Trichoderma reesei

Monoclonal antibodies have been produced against two functionally different domains present in two cellobiohydrolases from Trichoderma reesei (CBH I and CBH II). Four groups of antibodies were obtained, which specifically recognized (Western blotting, ELISA) (a) the core protein within CBH I, (b) the core protein within CBH II, (c) the BA region of CBH I, and (d) the ABB' region of CBH II. No cross-reactivities within these four groups were observed. The antibodies reacted also specifically with proteins of similar size to CBH I and CBH II (SDS-PAGE) from other Trichoderma strains (Western blotting), whereas no reaction was observed with cellulases from other fungal sources. Analysis of culture filtrates of T. reesei QM 9414 harvested at various times of growth on cellulose under buffered conditions (pH 5-6) indicated the presence of only single bands of CBH I and CBH II, even after prolonged cultivation (160 h). Cultivation on cellulose in unbuffered media, however, showed the appearance (Western blotting) of additional lower molecular weight proteins, which reacted with the monoclonal antibodies directed against the cores of CBH I and II, but not with those recognizing the respective BA and ABB' regions. The appearance of these lower molecular weight bands was most pronounced in unbuffered media, supplemented with a 3-fold (w/w) amount of organic nitrogen (peptone). Analysis of some commercial cellulase preparations from T. harzianum revealed the same pattern of lower molecular weight proteins, in contrast to samples from other fungal cellulases. Those samples or preparations, showing a multiple pattern of CBH I and CBH II, exhibited higher activities of an acid proteinase. These results imply that the use of unbuffered, high nitrogen-supplemented culture conditions for production of cellulases may lead to considerable proteolytic modification of the secreted cellobiohydrolases.

Regulatory aspects of cellulase biosynthesis and secretion

The cellulase enzyme system consists of cellobiohydrolase, endoglucanase, and beta-glucosidase and has been extensively studied with respect to its biosynthesis, properties, mode of action, application, and, most recently, secretion mechanisms. A knowledge of the factors governing the biosynthesis and secretion of these enzymes at the molecular level will be useful in maximizing enzyme productivity in extracellular fluid. Among other topics, the regulatory effects of sorbose (a noninducing sugar which is not a product of cellulose hydrolysis) on cellulase synthesis and release are described. Cellulase genes have recently been cloned into a number of microorganisms with a view to understanding the gene structure and expression and to obtaining the enzyme components in pure form. The factors governing biosynthesis and secretion of cellulases in recombinant cells are also discussed. Cellulases are known to be glycoproteins, therefore, the role of O- and N-linked glycosylation on enzyme stability and secretion is also detailed.

Domain structure of cellobiohydrolase II as studied by small angle X-ray scattering: close resemblance to cellobiohydrolase I

Evidence for a domain structure of cellobiohydrolase II (CBH II, 58 kDa) from Trichoderma reesei (Teeri et al., 1987; Tomme et al., 1988) is corroborated by results from SAXS experiments. They indicate a 'tadpole' structure for the intact CBH II in solution (Dmax = 21.5 +/- 0.5 nm; Rg = 5.4 +/- 0.1 nm) and a more isotropic, ellipsoid shape for the core protein (Dmax = 6.0 +/- 0.3 nm; Rg = 2.1 +/- 0.1 nm). The latter was obtained by partial proteolysis with papain which cleaves the native CBH II to give two fragments (Tomme et al., 1988): the core (45 kDa) with the active (hydrolytic) domain and a smaller fragment (11 kDa) coinciding with the tail part of the model and containing the binding domain for unsoluble cellulose. This peptide fragment is conserved in most cellulolytic enzymes from Trichoderma reesei (Teeri et al., 1987). It contains a conserved region (block A) and glycosylated parts (blocks B and B' duplicated and located N-terminally in CBH II). In spite of different domain arrangements in CBH I (blocks B-A at C-terminals) SAXS measurements (Abuja et al., 1988) indicate similar tertiary structures for both cellobiohydrolases although discrete differences in the tail parts exist.

Cloning of the Thermomonospora fusca Endoglucanase E2 Gene in Streptomyces lividans: Affinity Purification and Functional Domains of the Cloned Gene Product

Thermomonospora fusca YX grown in the presence of cellulose produces a number of β-1-4-endoglucanases, some of which bind to microcrystalline cellulose. By using a multicopy plasmid, pIJ702, a gene coding for one of these enzymes (E2) was cloned into Streptomyces lividans and then mobilized into both Escherichia coli and Streptomyces albus. The gene was localized to a 1.6-kilobase Pvu II- Cla I segment of the originally cloned 3.0-kilobase Sst I fragment of Thermomonospora DNA. The culture supernatants of Streptomyces transformants contain a major endoglucanase that cross-reacts with antibody against Thermomonospora cellulase E2 and has the same molecular weight (43,000) as T. fusca E2. This protein binds quickly and tightly to Avicel, from which it can be eluted with guanidine hydrochloride but not with water. It also binds to filter paper but at a slower rate than to Avicel. Several large proteolytic degradation products of this enzyme generated in vivo lose the ability to bind to Avicel and have higher activity on carboxymethyl cellulose than the native enzyme. Other smaller products bind to Avicel but lack activity. A weak cellobiose-binding site not observed in the native enzyme was present in one of the degradation products. In E. coli , the cloned gene produced a cellulase that also binds tightly to Avicel but appeared to be slightly larger than T. fusca E2. The activity of intact E2 from all organisms can be inactivated by Hg 2+ ions. Dithiothreitol protected against Hg 2+ inactivation and reactivated both unbound and Avicel-bound Hg 2+ -inhibited E2, but at different rates.

A binding-site-deficient, catalytically active, core protein of endoglucanase III from the culture filtrate of Trichoderma reesei

From the culture filtrate of Trichoderma reesei we have isolated a novel endoglucanase (38 kDa) which was shown to be identical to endoglucanase III (E III, 50 kDa), but lacking the first 61 N-terminal amino acids. This core protein, designated E III core, is fully active against soluble substrates, such as carboxymethylcellulose, whereas both activity against and adsorption to microcrystalline cellulose (Avicel) is markedly decreased. Sedimentation velocity experiments revealed that the intact E III enzyme has much higher asymmetry than the E III core protein, suggesting that the N-terminal region split off constitutes a protruding part of the native enzyme. These results lead to the proposal that native E III consists of two functionally separated domains: a catalytically active core and a protruding N-terminal domain which acts in the binding to insoluble cellulose. The N-terminal peptide missing in E III core corresponds to the heavily glycosylated common structural element found also in the N-terminus of cellobiohydrolase II and in the C-termini of cellobiohydrolase I and endoglucanase I. A similar bifunctional organization could thus be the rule for Trichoderma cellulases, endoglucanases as well as cellobiohydrolases.