"De novo" synthesis of cellulase' induced by sophorose in Trichoderma viride cells

Comparison of catalytic properties of multiple β-glucosidases of Trichoderma reesei

Ten putative Trichoderma reesei β-glucosidase (BGL) isozymes were heterologously expressed in Escherichia coli and Aspergillus oryzae and purified to homogeneity. Catalytic properties of nine enzymes which showed hydrolytic activity on cellobiose and p-nitrophenyl-β-D-glucopyranoside (pNPG) were investigated. Three BGLs, encoded by the genes cel3A, cel3B, and cel3E, contained a predicted signal peptide, showed higher hydrolytic activity on cello-oligosaccharides than on pNPG, and preferred longer oligosaccharides. Another three putative extracellular BGLs, Cel3B, Cel3F, and Cel3G, and two intracellular enzymes, Cel3C and Cel3D, exhibited preference for pNPG. Intracellular Cel1A showed the highest affinity for cellobiose as a typical cellobiase. Four BGLs, Cel3A, Cel3B, Cel3E, Cel1A, that showed high activity against cello-oligosaccharides were capable of catalyzing transglycosylation reactions from cellobiose, leading to formation of cellotriose and isomeric glucobioses. While Cel3A, Cel3B, and Cel3E synthesized mainly gentiobiose, glycosyl transfer reactions of Cel1A led mainly to sophorose and laminaribiose. Conversion of cellobiose to sophorose by Cel1A reached about 3.6 and 10 % at 1 and 10 % cellobiose concentration, respectively. The formation and persistence of individual cellobiose isomers in incubation mixtures of four BGLs (Cel3A, Cel3B, Cel3E, and Cel1A) with cellobiose correlated well with the k cat values for isomeric glucobioses. Cel1A also showed the lowest sensitivity to inhibition by glucose. Based on all studied catalytic properties, Cel1A appears to be unambiguously the best candidate for site-directed mutations or directed evolution toward improvement of activity, thermostability, and, eventually, efficiency of sophorose synthesis.

High Production of beta-Glucosidase in Schizophyllum commune: Isolation of the Enzyme and Effect of the Culture Filtrate on Cellulose Hydrolysis

Optimization experiments with response surface statistical analysis were performed with Schizophyllum commune to obtain high beta-glucosidase yields. The factors in the optimization experiment were the concentrations of cellulose, peptone, and KH(2)PO(4). Their optimal values were 3.2, 3.0, and 0.2 g/100 ml, respectively. Enzyme assays revealed very high beta-glucosidase (22.2 U/ml) and cellobiase (68.9 U/ml) yields. The avicelase yield was low as compared with that from Trichoderma reesei. Mixtures of S. commune and T. reesei culture filtrates caused faster and more extensive saccharification of Avicel than could be achieved by either filtrate alone. A beta-glucosidase was isolated and purified from the optimized culture filtrate of S. commune. The electrophoretic mobility of the purified beta-glucosidase indicated a molecular weight of 97,000. The amino acid composition was similar to that of beta-glucosidase from T. reesei. The acidic (aspartate and glutamate) residues or their amides or both made up approximately 20% of the protein. The NH(2)-terminal amino acid of the enzyme was histidine.

Synthesis of isopropyl-1-thio-beta-D-glucopyranoside (IPTGlc), an inducer of Aspergillus niger B1 beta-glucosidase production

Production of beta-glucosidase in Aspergillus niger B1 is subjected to catabolic repression by glucose. Aspergillus niger B1 grown on bran as a carbon source secreted beta-glucosidase. The maximum level of the enzyme was reached after 7 d of fermentation. Addition of 1% glucose to the medium suppressed beta-glucosidase production to undetectable levels. In this study, the organic synthesis of a potential inducer of beta-glucosidase production by A. niger B1's reported. Isopropyl-1-thio-beta-D-glucopyranoside (IPTGlc) was synthesized using a two-step organic synthesis protocol. The H-NMR data agreed with those reported previously for the galactoside analog. When IPTGlc was added 24 h after inoculation at a final concentration of 0.4 mM, similar levels of beta-glucosidase were reached 3 to 4 d earlier as compared to fermentation without IPTGlc induction. In practice, this may translate to a more efficient method of producing beta-glucosidase from this fungus.

Chemical modification of a beta-glucosidase from Schizophyllum commune: evidence for essential carboxyl groups

The beta-glucosidase from Schizophyllum commune was purified to homogeneity by a modified procedure that employed Con A-Sepharose. The participation of carboxyl groups in the mechanism of action of the enzyme was delineated through kinetic and chemical modification studies. The rates of beta-glucosidase-catalyzed hydrolysis of p-nitrophenyl-beta-D-glucoside were determined at 27 degrees C and 70 mM ionic strength over the pH range 3.0-8.0. The pH profile gave apparent pK values of 3.3 and 6.9 for the enzyme-substrate complex and 3.3 and 6.6 for the free enzyme. The enzyme is inactivated by Woodward's K reagent and various water-soluble carbodiimides; chemical reagents selective for carboxyl groups. Of these reagents, 1-ethyl-3-(4-azonia-4,4-dimethylpentyl)carbodiimide iodide in the absence of added nucleophile was the most effective and a kinetic analysis of the modification indicated that one molecule of carbodiimide is required to bind to the beta-glucosidase for inactivation. Employing a tritiated derivative of the carbodiimide, 44 carboxyl groups in the enzyme were found to be labelled while the competitive inhibitor deoxynojirimycin protected three residues from modification. Treatment of the enzyme with tetranitromethane resulted in the modification of five tyrosine residues with approx. 28% diminution of enzymic activity. Titration of denatured enzyme with dithiobis(2-nitro-benzoic acid) indicated the absence of free thiol groups. Reaction of the enzyme with diethyl pyrocarbonate resulted in the modification of four histidine residues with the retention of 78% of the original enzymatic activity. The divalent transition metals Cu2+ and Hg2+ were found to be potent inhibitors of the enzyme, binding in an apparent irreversible manner.

Enzymatic degradation of cell wall and related plant polysaccharides

Polysaccharides such as starch, cellulose and other glucans, pectins, xylans, mannans, and fructans are present as major structural and storage materials in plants. These constituents may be degraded and modified by endogenous enzymes during plant growth and development. In plant pathogenesis by microorganisms, extracellular enzymes secreted by infected strains play a major role in plant tissue degradation and invasion of the host. Many of these polysaccharide-degrading enzymes are also produced by microorganisms widely used in industrial enzyme production. Most commerical enzyme preparations contain an array of secondary activities in addition to the one or two principal components which have standardized activities. In the processing of unpurified carbohydrate materials such as cereals, fruits, and tubers, these secondary enzyme activities offer major potential for improving process efficiency. Use of more defined combinations of industrial polysaccharases should allow final control of existing enzyme processes and should also lead to the development of novel enzymatic applications.

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.

Extracellular proteins secreted by the basidiomycete Schizophyllum commune in response to carbon source

The secretion of 1,4-beta-D-glucanases by the basidiomycete Schizophyllum commune in response to cellulose or cellobiose has been studied. The proteins were labeled with 35S, and the secretion of enzymes was measured by beta-glucosidase and carboxymethyl cellulase activities and by immunoprecipitation with specific antibodies. The antigen proteins used were a beta-glucosidase (Mr, 93,000), an avicelase (avicelase II; Mr, 64,000), and a carboxymethyl cellulose (carboxymethyl cellulase I; Mr 41,000). The beta-glucosidase was initially secreted as an Mr 110,000 form, which was followed later by lower-molecular-weight (88,000 to 93,000) forms. The avicelase II, which accounted for about 50% of the secreted labeled protein, had an Mr of 64,000. Secretion of the related avicelase I (Mr 61,000) followed later. The carboxymethyl cellulose I was secreted in two molecular weight forms, Mr 44,000 and 41,000. The evidence is consistent with the idea that three genes account for the secreted glucanase activities. Other species result from different glycosylation or proteolytic cleavage processing, which may occur during or after secretion. The beta-glucosidase secretion appears to be regulated differently than that of avicelase II or carboxymethyl cellulase I; the latter two were regulated coordinately under the conditions used in this work. No common immune determinants between the three antigens were observed.

Induction of cellulolytic enzymes in Trichoderma reesei by sophorose

Sophorose (2-O-beta-glucopyranosyl-D-glucose) induces carboxymethyl cellulase in Trichoderma reesei QM6a mycelium with 1.5 to 2 h. The induction response to sophorose concentration, although complicated by the metabolism of sophorose, shows saturation kinetics. Most of the cellulase appears after most of the sophorose has been taken up, but the presence of an inducer is required to maintain cellulase synthesis because enzyme production ceases after separation of the mycelium from the induction medium. Cellulase appears simultaneously in the medium and in the mycelium, and no appreciable levels accumulate in the mycelium. Response to pH suggest either that synthesis and secretion of the enzyme are closely associated or concurrent events affected by surface interactions with the medium. Effects of temperature and pH on cellulase induction by sophorose are similar to those reported for induction by cellulose. The kinetics of absorption by mycelium differs from that of other beta-linked saccharides and glucose, the uptake of sophorose being much slower. Under our cultural conditions, sophorose appears to induce an incomplete array of cellulase enzymes, as indicated by enzymatic and electrophoretic studies.

Sophorose metabolism and cellulase induction in Trichoderma

The cellulase inducer sophorose was rapidly catabolized to CO2 and H2O by Trichoderma: only small amounts were used to induce the synthesis of cellulase. 3H-sophorose uptake began after a lag of 1 h and its half-life in the medium was less than 5 h. Cellulase activity in the medium did not increase till 6 h after the addition of sophorose and reached a half maximum value at 14 h. The presence of free sophorose in the medium was required for continuous cellulase production. Several small sophorose addition induced much more cellulase than an equivalent single dose. These results are attributed to two pathways of sophorose utilization, a catabolic pathway that has a high capacity but low affinity for sophorose and an inductive pathway having a lower capacity but higher affinity for sophorose.