Outlook for cellulase improvement: screening and selection strategies

Cellulose is the most abundant renewable natural biological resource, and the production of biobased products and bioenergy from less costly renewable lignocellulosic materials is important for the sustainable development of human beings. A reduction in cellulase production cost, an improvement in cellulase performance, and an increase in sugar yields are all vital to reduce the processing costs of biorefineries. Improvements in specific cellulase activities for non-complexed cellulase mixtures can be implemented through cellulase engineering based on rational design or directed evolution for each cellulase component enzyme, as well as on the reconstitution of cellulase components. Here, we review quantitative cellulase activity assays using soluble and insoluble substrates, and focus on their advantages and limitations. Because there are no clear relationships between cellulase activities on soluble substrates and those on insoluble substrates, soluble substrates should not be used to screen or select improved cellulases for processing relevant solid substrates, such as plant cell walls. Cellulase improvement strategies based on directed evolution using screening on soluble substrates have been only moderately successful, and have primarily targeted improvement in thermal tolerance. Heterogeneity of insoluble cellulose, unclear dynamic interactions between insoluble substrate and cellulase components, and the complex competitive and/or synergic relationship among cellulase components limit rational design and/or strategies, depending on activity screening approaches. Herein, we hypothesize that continuous culture using insoluble cellulosic substrates could be a powerful selection tool for enriching beneficial cellulase mutants from the large library displayed on the cell surface.

Cellodextrin preparation by mixed-acid hydrolysis and chromatographic separation

A procedure for preparation of purified cellodextrins in gram quantities was developed for use in biochemical and microbiological studies. Cellodextrins were prepared by hydrolyzing microcrystalline cellulose (Avicel) over a period of 4 to 5.5h in the presence of a mixture of 80% (v/v) concentrated hydrochloric acid ( approximately 37 wt.%) and 20% (v/v) concentrated sulfuric acid ( approximately 98 wt.%) at room temperature (22 degrees C). Acetone precipitation, washing ion exchange, and neutralization with barium hydroxide were used to generate a solution of mixed cellodextrins substantially free of acids and salts. Yields following hydrolysis and precipitation were approximately 0.05, approximately 0.07, approximately 0.06, and approximately 0.02 g/g cellulose for cellotriose (G(3)), cellotetraose (G(4)), cellopentose (G(5)), and cellohexose (G(6)), respectively. Cellodextrins with degrees of polymerization from 3 to 11 were separated chromatographically using a 29 x 5-cm I.D. Bio-Rad AG50W-X4 column arranged in series with a 91 x 5-cm I.D. Bio-Gel P4 column. This two-column system was used to obtain cellodextrin preparations at 240 mg/day for G(3), 330 mg/day for G(4), 260 mg/day for G(5), and 130 mg/day for G(6), with purity >99% for G(3), G(4), and G(5) and >95% for G(6). The overall procedure achieves yields comparable to the highest previously reported, employs a separation system that can readily be reused for multiple runs, and avoids use of fuming HCl.

Substrate specificity of endoglucanases: what determines xyloglucanase activity?

Endoglucanases from Trichoderma viride differ in their activity and mode of action towards xyloglucans. In order to explain the basis for their different behavior, the number of substrate-binding sites of three endoglucanases (endoI, endoIV, and endoV) were determined using bond cleavage frequencies of both normal and reduced cellodextrins and Ko/K(m). EndoIV differed from other endoglucanases described so far, in having at least nine putative binding sites. The specificities of the three endoglucanases towards various xyloglucans derived from apple fruit and potato were determined. Also, the release of oligosaccharides from these substrates in time was monitored. It was concluded that the endoglucanases prefer to bind unbranched glucosyl residues. Because most xyloglucans are composed of XXXG-type of building units, distant subsites are needed to bind xyloglucan. Having at least nine substrate-binding sites, endoIV seems to be well equipped to degrade xyloglucans which was confirmed by its high xyloglucanase activity.

Preparation of cellodextrins and isolation of oligomeric side components and their characterization

Cellodextrin (beta-1,4-glucose oligomer) mixtures are prepared by precipitation of oligomers with 1-propanol and ethanol after partial hydrolysis of cellulose with hydrochloric acid or by acetolysis of cellulose. Cellooligomers (DP3-DP8) can be isolated by high-resolution size-exclusion chromatography on Bio-Gel P 4 using water as eluent. Recycle operation of the columns allows the separation of oligomers up to a degree of polymerization of 12. However, ion-exchange chromatography of their borate complexes demonstrates the heterogeneity of cellodextrins, homogeneous according to size-exclusion chromatography. At least four secondary oligomeric components are observed in the different samples. By preparative affinity chromatography on phenyl-boronate-agarose two of these components could be purified and subsequently characterized. In one series of oligosaccharides the glucose unit at the reducing end of the beta-1,4-glucose oligomers is derivatized to fructose. This enolization reaction occurs during size-exclusion chromatography. The precipitation step with alkanols during preparation of oligomer mixtures generates oligomeric glycosides. Additionally, the formation of amines from respective beta-1,4-glucose oligomers is observed with the ammonium carbonate eluent used in affinity chromatography. Analysis methods combined to assess for the homogeneity of cellodextrins include enzyme- and acid-catalyzed (partial) hydrolysis of the different oligomers and subsequent analysis of degradation products by sugar borate chromatography; 13C and 1H NMR spectroscopy; and fast atom bombardment mass spectroscopy.