Construction and characterization of the chimeric enzymes between the Bacillus subtilis cellulase and an alkalophilic Bacillus cellulase

Improving the activity of endoglucanase I (EGI) from Saccharomyces cerevisiae by DNA shuffling

To enhance the endo-β-1,4-glucanase activity of three mixedTrichodermasp. (Trichoderma reesei, Trichoderma longibrachiatum, andTrichoderma pseudokoningii), we optimized the efficiency of the encoding gene using DNA shuffling andSaccharomyces cerevisiaeINVSc1 as a host.

Directed evolution for engineering pH profile of endoglucanase III from Trichoderma reesei

The potential of cellulase has been revealed not only in biomass conversion but also in various industrial processes, including food, textiles, laundry, pulp, and paper. Due to the need for alkali-tolerant cellulase with high specific activity at alkaline pH, for example, for application in detergent industry an error-prone PCR approach was employed for enhancing the alkali-tolerant ability of endoglucanase III (EG III) from Trichoderma reesei by error-prone PCR. One mutant (N321T) which exhibited an optimal activity at pH 5.4, corresponded to a basic shift of 0.6 pH unit compared to the wild-type enzyme, was selected and characterized. In addition, two site-directed mutations, N321D and N321H, were designed to study the role of residue at position 321. As expected, the N321D mutation changed enzyme's optimal activity to pH 4.0, resulting in a large decrease in the specific activity. However, the N321H mutated enzyme was active over a broader pH range compared to the wild type, with no much change in the specific activity. These properties suggest that the residue at position 321 is important amino acid residue in determining the pH activity profile of the EG III from T. reesei.

Developments in the use of Bacillus species for industrial production

Bacillus species continue to be dominant bacterial workhorses in microbial fermentations. Bacillus subtilis (natto) is the key microbial participant in the ongoing production of the soya-based traditional natto fermentation, and some Bacillus species are on the Food and Drug Administration's GRAS (generally regarded as safe) list. The capacity of selected Bacillus strains to produce and secrete large quantities (20-25 g/L) of extracellular enzymes has placed them among the most important industrial enzyme producers. The ability of different species to ferment in the acid, neutral, and alkaline pH ranges, combined with the presence of thermophiles in the genus, has lead to the development of a variety of new commercial enzyme products with the desired temperature, pH activity, and stability properties to address a variety of specific applications. Classical mutation and (or) selection techniques, together with advanced cloning and protein engineering strategies, have been exploited to develop these products. Efforts to produce and secrete high yields of foreign recombinant proteins in Bacillus hosts initially appeared to be hampered by the degradation of the products by the host proteases. Recent studies have revealed that the slow folding of heterologous proteins at the membrane-cell wall interface of Gram-positive bacteria renders them vulnerable to attack by wall-associated proteases. In addition, the presence of thiol-disulphide oxidoreductases in B. subtilis may be beneficial in the secretion of disulphide-bond-containing proteins. Such developments from our understanding of the complex protein translocation machinery of Gram-positive bacteria should allow the resolution of current secretion challenges and make Bacillus species preeminent hosts for heterologous protein production. Bacillus strains have also been developed and engineered as industrial producers of nucleotides, the vitamin riboflavin, the flavor agent ribose, and the supplement poly-gamma-glutamic acid. With the recent characterization of the genome of B. subtilis 168 and of some related strains, Bacillus species are poised to become the preferred hosts for the production of many new and improved products as we move through the genomic and proteomic era.

Protein engineering of cellulases

Cellulases are enzymes which hydrolyse the beta-1,4-glucosidic linkages of cellulose. They fall into 13 of the 82 glycoside hydrolase families identified by sequence analysis, but they are traditionally divided into two classes termed 'endoglucanases' (EC 3.2.1.4) and 'cellobiohydrolases' (3.2.1.91). Both types of cellulases degrade soluble cellodextrins and amorphous cellulose but, with a few notable exceptions, it is only the cellobiohydrolases which degrade crystalline cellulose efficiently. Site-directed mutagenesis has been central to the characterisation of cellulases, ranging from the identification and characterisation of putative catalytic and binding residues, the trapping of enzyme-substrate complexes by crystallography through to the construction of new and improved biocatalysts including 'glycosynthases'. Whilst studies on soluble substrates and substrate analogues have provided a wealth of information, understanding the mechanism of degradation of the natural substrate, crystalline cellulose, remains a great challenge.

Alkaliphiles: some applications of their products for biotechnology

The term "alkaliphile" is used for microorganisms that grow optimally or very well at pH values above 9 but cannot grow or grow only slowly at the near-neutral pH value of 6.5. Alkaliphiles include prokaryotes, eukaryotes, and archaea. Many different taxa are represented among the alkaliphiles, and some of these have been proposed as new taxa. Alkaliphiles can be isolated from normal environments such as garden soil, although viable counts of alkaliphiles are higher in samples from alkaline environments. The cell surface may play a key role in keeping the intracellular pH value in the range between 7 and 8.5, allowing alkaliphiles to thrive in alkaline environments, although adaptation mechanisms have not yet been clarified. Alkaliphiles have made a great impact in industrial applications. Biological detergents contain alkaline enzymes, such as alkaline cellulases and/or alkaline proteases, that have been produced from alkaliphiles. The current proportion of total world enzyme production destined for the laundry detergent market exceeds 60%. Another important application is the industrial production of cyclodextrin by alkaline cyclomaltodextrin glucanotransferase. This enzyme has reduced the production cost and paved the way for cyclodextrin use in large quantities in foodstuffs, chemicals, and pharmaceuticals. It has also been reported that alkali-treated wood pulp could be biologically bleached by xylanases produced by alkaliphiles. Other applications of various aspects of alkaliphiles are also discussed.

Construction of chimeric beta-glucosidases with improved enzymatic properties

The amino acid sequences of beta-glucosidases from Cellvibrio gilvus and Agrobacterium tumefaciens show about 40% similarity. The pH/temperature optima and stabilities and substrate specificities of the two enzymes are quite different. C. gilvus beta-glucosidase exhibits an optimum pH of 6.2-6.4 and temperature of 35 degrees C, whereas the corresponding values for A. tumefaciens are 7.2-7.4 and 60 degrees C, respectively. The substrate specificity of A. tumefaciens enzyme toward different aryl glycosides is broader than C. gilvus enzyme. To analyze these properties further, three chimeric beta-glucosidases were constructed by substituting segments from the C-terminal homologous region of C. gilvus beta-glucosidase gene with that of A. tumefaciens. The chimeric enzymes were characterized with respect to pH/temperature activity and stability and substrate specificity. Chimeric enzymes exhibited chromatographic behavior similar to that of C. gilvus enzyme. However, enzymatic properties of chimeras were admixtures of those of the two parents. The chimeric enzymes were optimally active at 45-50 degrees C and pH 6.6-7.0. Km values of chimeric enzymes for the various saccharides were admixtures of both parental enzymes. These results suggest that the two domains of C. gilvus and A. tumefaciens enzymes probably can fold independently. The homologous C-terminal region in beta-glucosidase appears to play an important role in determining enzyme characteristics. Changes in the properties on substitution of segments in this region might be related to the enzyme specificity, and beta-glucosidases with improved properties can be prepared by manipulating this region.

Analysis of a cis-active sequence mediating catabolite repression in gram-positive bacteria

One form of catabolite repression (CR) in the Gram-positive genus, Bacillus, is mediated by a cis-acting element (CRE). We use here a consensus sequence to identify such elements in sequenced genes of Gram-positive bacteria. These are analysed with respect to position and type of gene in which they occur. CRE sequences near the promoter region are mainly identified in genes encoding carbon catabolic enzymes, which are thus likely to be subject to CR by a global mechanism. Functional aspects of CREs are evaluated.

Determinants for the enhanced thermostability of hybrid (1-3,1-4)-beta-glucanases

Hybrid (1-3,1-4)-beta-glucanases which contain an N-terminal region derived from the Bacillus amyloliquefaciens enzyme and a C-terminal region of the closely related B. macerans enzyme may exhibit a thermostability superior to both parental enzymes. A systematic series of hybrid enzymes were constructed in order to delineate the amino acid residues that affect protein stability. Hybrid enzymes with between one and four of the N-terminal residues for the mature B. amyloliquefaciens (1-3,1-4)-beta-glucanase exhibit no significant changes in biochemical characteristics as compared with the parental B. macerans enzyme. However, significantly enhanced thermostability was observed in the hybrid enzyme containing an N-terminal segment of eight amino acid residues derived from the B. amyloliquefaciens enzyme. Site-directed mutagenesis revealed that the combined effect of Gln1, Thr2, Ser5 and Phe7 confer enhanced stability on hybrid enzymes, probably by improving the hydrogen bonding that stabilizes the interactions between the N-terminal and the centre of the folded molecule, as well as between the two termini of the polypeptide chain. Furthermore, deletion of Tyr13 in the hybrid enzyme containing the 12 N-terminal amino acids from the B. amyloliquefaciens (1-3,1-4)-beta-glucanase results in a dramatic increase in stability at 70 degrees C with the half-life of 6 min increased to around 4 h. This is twofold higher than the hitherto most stable hybrid enzyme in which the N-terminal domain consisted of 16 residues of the B. amyloliquefaciens enzyme.

Analysis of functional domains of endoglucanases from Clostridium cellulovorans by gene cloning, nucleotide sequencing and chimeric protein construction

The nucleotide sequence of engD, an endo-beta-1,4-glucanase gene from Clostridium cellulovorans was determined (Genbank Accession No. M37434). The COOH-terminal part of the gene product, EngD, contained a Thr-Thr-Pro repeated sequence followed by a region that has homology to the exoglucanase of Cellulomonas fimi. EngD and EngB, another C. cellulovorans endoglucanase, show 75% amino acid sequence homology at their NH2-termini, in contrast to their carboxyterminal domains which show no homology. EngD had endoglucanase activity on carboxymethylcellulose (CMC), cellobiosidase activity on p-nitrophenyl-cellobioside (p-NPC), and partial hydrolytic activity on crystalline cellulose (Avicel), while EngB showed hydrolytic activity against only CMC. Chimeric proteins between EngB and EngD were constructed by exchanging the non-homologous COOH-terminal regions. Chimeric proteins that contained the NH2-terminus of EngD retained cellobiosidase activity but chimeras with the EngB NH2-terminus showed no cellobiosidase activity. Hydrolysis of crystalline cellulose (Avicelase activity) was observed only with the enzyme containing the EngD NH2-terminus and EngD COOH-terminus.