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Biochem. J. (1994) 298, 705-710 (Printed in Great Britain) 705 Cellulose hydrolysis by the cellulases from Trichoderma reesei: a new model for synergistic interaction Bernd NIDETZKY,*§ Walter STEINER,* Marianne HAYNt and Marc CLAEYSSENSt *Institute of Biotechnology, Technical University of Graz, Petersgasse 12/1, A-8010 Graz, Austria, tinstitute of Biochemistry, University of Graz, Schubertstrasse 1, A-8010 Graz, Austria, and $Laboratory of Biochemistry, Department Biochemistry-Fysiology-Microbiology, University of Gent, K.L.-Ledeganckstraat 25, B-9000 Gent, Belgium The hydrolysis of Whatman no. 1 filter paper by purified cellulolytic components from Trichoderma reesei and the syn- ergistic action of binary combinations of these enzymes on the same substrate were investigated. At 20 g/l filter paper, enzyme concentrations needed to obtain half-maximal hydrolysis rates (KE values) were in the 3-4 1M range for the cellobiohydrolases (CBHs) and 0.05-0.10,1M for the endoglucanases (EGs). Catalytic-core proteins of CBH I and EG III, lacking the cellulose-binding domain, exhibit KE values 2.3 and 5.1 times higher than those of the intact enzymes. In synergistic combi- INTRODUCTION Traditionally, hydrolysis of (semi)crystalline cellulose by fungal cellulase systems is thought to require the cooperative action of so-called endoglucanases (EGs; EC 3.2.1.4) and cellobiohydro- lases or exocellulases (CBHs; EC 3.2.1.91). Since the role of each component in the hydrolysis of the insoluble substrate is not well understood and, also, no clear-cut distinction between the two cellulase types exists (Henrissat et al., 1985; Van Tilbeurgh and Claeyssens, 1985; Vrsanska and Biely, 1992), hypothetical models for synergism have to be re-evaluated. For fungal cellulases, two types of synergism have been put forward. So-called endo-exo-synergism is generally interpreted by a sequential mechanism of enzyme action: endoglucanases in an initial attack on the amorphous regions of the cellulose provide new chain ends for the action of CBHs (Wood and McCrae, 1979; Henrissat et al., 1985; Beldman et al., 1988). Another model assumes competition among individual cellulases for adsorption sites on cellulose (Ryu et al., 1984; Kyriacou et al., 1987), but its relevance for synergism remains unclear. For exo-exo-synergism, described by several authors (Wood and McCrae, 1986; Figerstam and Pettersson, 1980; Henrissat et al., 1985; Kyriacou et al., 1987; Tomme et al., 1988), two mechanistic models have been proposed. One is based on experimental evidence that between the enzymes from Tricho- derma reesei (CBH I and CBH II) a loose complex is formed in solution and that adsorption of the individual components to cellulose is maximal in optimal synergistic admixtures (Tomme et al., 1990). Wood and McCrae (1986), on the other hand, tentatively ascribed synergism between CBH I and CBH II from Penicillium pinophilum to different orientations of the non- reducing end groups in crystalline cellulose, requiring two cello- nations oftwo cellulases, the KE value of at least one enzyme was 3-10-fold reduced. CBH I/CBH II and CBH I/EG III combi- nations showed the most powerful synergism, and optimal ratios were a function of the total protein concentration. Results obtained in activity and adsorption assays using filter paper pretreated with one component, followed by inactivation and subsequent hydrolysis with the same or another cellulase com- ponent, point to a sequential enzymic attack of the cellulose and seems consistent with the mathematical model presented. biohydrolases with different stereochemical specificities. How- ey,r, elucidation of the three-dimensional structure of the core protein of CBH II from T. reesei and modelling of the substrate- binding site (Rouvinen et al., 1990) showed that the cellulose chain may enter the active-site tunnel in two different orientations. Therefore plausible mechanistic concepts for cellulase syn- ergistic action cannot be advanced as results published in the literature are inconclusive and sometimes contradictory. In part, this may be due to the unsatisfactory degree of purity of the individual cellulases (Van Tilbeurgh et al., 1984; Wood and Garcia-Campayo, 1990). Nevertheless some significant findings are: (i) synergism between cellulases depends on the ratio of the individual enzymes (Henrissat et al., 1985; Tomme et al., 1988), (ii) the importance of the degree of substrate saturation (Woodward et al., 1988) and (iii) the influence of the physico- chemical properties of the substrate itself (Henrissat et al., 1985). The present paper deals with the synergistic action of the four major cellulases from T. reesei, namely CBH I, CBH II, EG I and EG III on Whatman no. 1 filter paper with a crystallinity index of about 45 % (Henrissat et al., 1985). Furthermore, we present a mathematical and mechanistic model for the co-operative action of the cellulases. EXPERIMENTAL Materials Filter paper no. 1 was from Whatman (Maidstone, Kent, U.K.). D-Glucose was determined either by the hexokinase assay (Chemie Linz, Linz, Austria) or the D-glucOse oxidase (GOD)/ peroxidase (POD) method (Boehringer, Mannheim, Germany). 2'-Chloro-4'-nitrophcnyl (CNP) fl-glycosides were prepared as Abbreviations used: CBD, cellulose-binding domain; CBH, cellobiohydrolase, 1,4-,8-D-glucan cellobiohydrolase; CNP, 2'-chloro-4'-nitrophenyl; CNPLac, 2'-chloro-4'-nitrophenyl lactoside; EG, endoglucanase, 1,4-f-D-glucan glucanohydrolase; i.e.f., isoelectric focusing; GOD, o-glucose oxidase; POD, peroxidase. § Present address and address for correspondence: Institut fOr Lebensmitteltechnologie, UnIversitAt fur Bodenkultur, Peter-Jordan-Strasse 82, A-1190 Wien, Austria. 705Biochem. J. (1 994) 298, 705-71 0 (Printed in Great Britain) 706 B. Nidetzky and others described by Van Tilbeurgh et al. (1988). fl-Glucosidase from Aspergillus niger (Novozym 188; Novo, Bagsvaerd, Denmark) was purified by Bio-Gel P6 gel filtration (Bio-Rad, Richmond, VA, U.S.A.). Its activity was determined with 4-nitrophenyl fl-D- glucoside (Ghose, 1987). Enzyme purification CBH I, CBH II and EG I were prepared from the culture filtrate of T. reesei MCG 77 using consecutive ion-exchange chromat- ography on DEAE- and CM-Sepharose. CBH I, CBH II and EG I were further purified by affinity chromatography (Van Tilbeurgh et al., 1984). The core protein and the cellulose-binding domain (CBD) ofCBH I were prepared by limited proteolysis as described by van Tilbeurgh et al. (1986). EG III from T. reesei QM 9414 and its core, purified by reported methods (Macarron et al., 1993; Stahlberg et al., 1988), were gifts from respectively Dr. R. Macarron and Dr. G. Pettersson. The purity of the individual enzymes was checked and verified by SDS/PAGE, isoelectric focusing (i.e.f.)/PAGE and by the very sensitive measurement of the specific activity (50 °C) against small chromogenic substrates (Van Tilbeurgh et al., 1988). So, under these conditions, no appreciable activity of CBHII or EG III against CNP lactoside (CNPLac) could be detected (less than 0.01 % EG I and 0.1 % CBH I activity). No contamination (less than 0.1 %) ofCBH I by EG I could be detected using activity measurements on 1 mM CNPLac in the presence of 20 mM cellobiose, which completely inhibits CBH I, but not EG I, activity. No release of the chromophore by CBHI, CBHII, and EG I was measured when using CNP cellotrioside as a substrate, thus ruling out con- tamination by EG III (less than 0.05 %). The concentration of the individual cellulases was determined spectrophotometrically (280 nm) using the following molar ab- sorption coefficients (M-1 cm-1): CBH I (67200), CBH I core (62500), CBH II (79100), EG I (54800), EG III (77000) (Saloheimo et al., 1988), and EG III core (63600) (Stahlberg et al., 1988). Enzymic hydrolysis To prevent strong product inhibition by cellobiose during cellulose hydrolysis, individual enzymes or combinations were dissolved in 0.05 M sodium acetate buffer (pH 4.8) supplemented with fl-glucosidase (0.2 unit/ml). No cellulolytic activity of the f- glucosidase against filter paper was detected. The reaction was started by adding a piece of filter paper (10±0.5 mg) to 0.5 ml of preincubated (50 °C, 10 min) solution containing either indi- vidual enzyme or binary enzyme combinations in the following concentration ranges: CBH I (0.2-8.0 1sM); CBH I core (0.4- 15.0,M); CBH 11 (0.1-7.0 1tM); EG I (0.01-0.50 ,M); EG III (0.02-0.90 1tM); EG III core (0.02-0.90 ,uM). Further incubation was at 50 °C and agitation at 200 rev./min (Aquatron; Infors, Bottmingen, Switzerland). At stated times (0.5-5 h), samples were taken (15,ul), centrifuged [10000 rev./min (ray 7.5 cm), 5 min] and further incubated at 50 °C for 30 min to complete the conversion of soluble hydrolysis prodducts into D-glucose. The latter was determined by the hexokinase assay measurin, absorb- ances at 340 nm. The rates of hydrolysis were calculated from the linear range of D-glucose produced versus reaction time or, in case of significant deviation from linearity, by taking the amount of D-glucose released after 0.5 h. Shaking intensities higher than 200 rev./min did not increase the reaction rates, thus indicating no mass-transfer limitation of hydrolysis under the conditions used. Suitable test experiments proved that the addition of ,-glucosidase did not alter the amount of D-glucose released by individual cellulases or combin- ations of the enzymes after 1 h (no synergism between the cellulases and the p-glucosidase). Enzymic pretreatment of filter paper The effect of substrate pretreatment was examined in a two-step experiment. Step 1 Filter-paper discs (5.05 mm diameter; 2.0+0.5 mg) in 100 ,l 0.05 M sodium acetate buffer, pH 5.6, containing 0.2 unit of /8- glucosidase/ml and different cellulases (either 2.6 ,uM CBH I, 2.6 ,uM CBH I core, 2.6 ,uM CBH 11, 0.4 ,uM EG I or 0.7 ,uM EG III) were shaken (Thermomixer 5436; Eppendorf, Munich, Germany; 50 °C and 1300 rev./min). After 1 h, 70 1el of the reaction mixture were withdrawn and the same amount of buffer was added. The tubes were heated (90 min at 95 °C) to inactivate the enzyme adsorbed to the cellulose and, after cooling to room temperature, the enzymically pretreated filter paper was washed several times with 70 ,ul fresh buffer. Controls containing buffer and filter paper only were treated similarly. Step 2 The enzyme used in the second step was added to the preincubated (50 °C, 10 min) solutions containing the pretreated filter paper (final volume 100 ,ul). Conditions and final concentrations were as in step 1. After 15, 30, 45 and 60 min, samples were taken (7 ,e1), further incubated for 30 min at 50 °C and analysed for D- glucose by the GOD/POD assay (100l,u reagent), measuring absorbances at 405 nm in microtitre plates (Easy Reader; SLT Lab Instruments, Graz, Austria). The complete inactivation of the enzyme used in step 1 was checked by addition of buffer and fl-glucosidase to the pretreated filter paper and measuring formation of D-glucose as described above. Adsorpfton Adsorption of individual cellulases on to pretreated filter paper was determined by measuring the specific activity of non- adsorbed enzyme (1 h incubation) in the supernatants (Tomme et al., 1990). CBH I (core), EG I, and EG III activities were determined at 50 °C by on-line monitoring of the release of 2- chloro-4-nitrophenol (et 1.8 x 104 M-1 cm-' at 405 nm and pH 5.6) from the CNP ,-glycosides of lactose (1 mM), cellobiose (1 mM), and cellotriose (0.25 mM) respectively (Van Tilbeurgh et al., 1988). Prior to these activity measurements any cellobiose present in the samples was hydrolysed by adding fl-glucogidase as described above. In the subsequent activity measurement, the ,8-glucosidase (and contaminating ,3-galactosidase) activities in the samples had to be totally inhibited by addition of 10 mM glucono-6-lactone and galactono-&-lactone. Residuai C1H II after adsorption was determined by measuring activities of supernatant samnples against untreated filter paper (20 g/l). Computational methods Parameters in the mathematical models (see the Appendix) were estimated by non-linear least-squares regression using theBMDP statistical software package (BMDP statistical software, Los Angeles, CA, U.S.A.). Synergistic action of Trichoderma reesei cellulases 707 Table 1 Kinetic constants calculated by non-linear regression according to the models in the Appendix for individual cellulases and their binary, synergistic combinations acting on flIter paper (results are means + S.D.) (a) Range Vm= KE Enzyme (#M) (g/h per litre) (#M) Vma/K Ratio CBH 0.20-8.00 0.448 + 0.072 4.243 + 1.285 0.106 + 0.036 l CBH core 0.40-15.00 0.471 +0.101 9.663 + 2.548 0.049 + 0.016 2.17 CBH II 0.10-7.00 0.518 + 0.052 3.301 + 0.644 0.157 + 0.034 EG 0.01-0.50 0.172+ 0.021 0.095+ 0.034 1.810+ 0.5031 EG III 0.02-0.90 0.299 + 0.010 0.047+ 0.007 6.361 + 0.971 6 EG IlIl core 0.02-0.90 0.271 + 0.012 0.241 + 0.031 1.125+ 0.153 665 (b) (c) Calculated optimal ratios (total enzyme concentration) Vs.,rtx. V(E)+ V(E2) K1 K Enzyme 1/enzyme 2 (g/h per litre) (g/h per litre) (#M) (ciM) (1 ,uM) (10 #uM) CBH l/CBH 11 CBH l/EG CBH l/EG Ill CBH Il/EG IlIl 2.183 + 0.141 0.494 + 0.053 1.277 + 0.080 0.635+ 0.069 0.966 + 0.089 0.620 + 0.075 0.747 + 0.073 0.817+ 0.053 4.243* 4.243* 1.488 + 0.193 0.803 + 0.216 0.297 + 0.063 0.022 + 0.010 0.008 + 0.002 0.042 + 0.009 62/38 70/30 74/26 90/10 82/18 92/8 74/26 86/14 * Set constant during regression analysis because of highly linear correlation between Vsy and K1. RESULTS AND DISCUSSION Hydrolysis of cellulose by individual cellulases and their core proteins Under the conditions used and for each of the six enzymes studied (always in the presence of fl-glucosidase), initial hy- drolysis rates of Whatman No. 1 filter paper proved to be linear (first 30 min) when D-glucose was measured by the coupled assay (see the Experimental section). At longer reaction times a steady decrease in reaction rate was observed for EG I and EG III (core), probably indicating a rapid depletion of available sub- strate sites (results not shown). Adsorption of cellulases on to the surface of their insoluble substrate is generally assumed to be a prerequisite step for hydrolysis. Therefore it seemed appropriate to analyse initial- rate data ofcellulose hydrolysis at various enzyme concentrations and constant substrate concentration and to use equations deviating from classical Michaelis-Menten kinetics (see the Appendix). For the individual, intact cellulases and for the respective core proteins of CBH I and EG III, kinetic constants VMax. and KE were estimated by non-linear regression analysis (Table la). The resulting 'catalytic efficiencies' point to 10-50- fold higher values for the endocellulases compared with those for the exo-type enzymes and 2-6-fold reduced values for the core proteins compared with those for intact CBH I or EG III. The maximal hydrolysis rate values agree with those found for the same enzymes with other types of cellulosic substrates (Henrissat et al., 1985; Tomme et al., 1988). Within statistical significance, maximal hydrolysis rates (Vm.. in Table la) are not influenced by proteolytic modification. Thus one function of the CBD can simply be to enhance the enzymes' efficiency by increasing the adsorption partition coefficient on crystalline cellulose (Tomme et al., 1988; Teeri et al., 1992). It seems thus improbable that the fungal CBD induces any ad- ditional 'active' property to the enzyme, as suggested by some authors (Knowles et al., 1987), at least not in the case of filter- paper hydrolysis. Furthermore, no synergistic effect on filter- paper hydrolysis was observed when CBH I core and the isolated CBD of CBH I were admixed either in equimolar amounts or with a 5-fold molar excess of CBD. Hydrolysis of cellulose by binary combinations of Intact cellulases Four combinations, CBH I, CBH II, EG I and EG III, showed positive co-operativity (Table lb). This was investigated at various total enzyme concentrations and different ratios of the individual components. In all cases the synergistic factors, which were defined as the ratio of the activity of a given combination to the sum of the individual activities, were found to be constant during hydrolysis of filter paper up to 5 h reaction time. From a large amount of hydrolysis-rate data, parameters could be extracted by using the mathematical model outlined in the Appendix (eqns. A2a and A2b). Values for V'yn_max.. and the respective constants, K1 and K2, were determined by non-linear regression for different combin- ations of the cellulases (Table lb). Experimental hydrolysis rates were fitted to the model (eqns. A2a and A2b) using the parameter values from Table la. As shown in Figs l(a)-1(d), no serious trend deviation can be detected, indicating that the model is valid to describe the experimental data. The highest synergistic rates were observed with the combin- ations CBH I/CBH II and CBH I/EG III (Table lb). The half- saturation constant of at least one enzyme in a synergistic combination (K1 or K2 in Table lb) is reduced drastically when compared with the results in Table l(a). This may indicate an increased affinity of the individual enzymes for the substrate when acting in a synergistic combination. From eqns. (A2a) and (A2b) the maximal hydrolysis rates for different ratios of the components at a given total enzyme concentration can be calculated (Table ic). It is evident that the B. Nidetzky and others 1.0 A 0.8 - 0.6 - 0.4 - 0.2 - 0 2.64(a) 0.5 - A -I / A- - 1.84 0.4 / 6 1.20 _ / I-- -0.1 I.~~~~~D// - I/ * ---0.' g. sL- -0° 0 0.40 0.80 1.20 [CBH II] (/zM) 80 40 1.60 2.00 0.3 0.2 0.1 I EG _. 0.29 ,- * _ 0.12 A ,- _ S0.06 ,7-* 0.03 i' '- -*-_ 0.02 AA _--A Ag, A, _/- AAAA'. o 1.0 - EG III 0.8 - 20-(C) _- 0.25 20 0.10 A *~~~~~0.05 A*8-3 ' - -A - - 0.03 1.8 ~ A 1.4 -/,A ' 0