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Cellulase (CEL) presently constitutes a major group of industrial enzyme based on its diverse ranges of utilization. Apart from such current and well-established applications-as in cotton processing, paper recycling, detergent formulation, juice extraction, and animal feed additives-their uses in agricultural biotechnology and bioenergy have been exploited. Supplementation of CELs to accelerate decomposition of plant residues in soil results in improved soil fertility. So far, applying CELs/antagonistic cellulolytic fungi to crops has shown to promote plant growth performance, including enhanced seed germination and protective effects. Their actions are believed mainly to trigger plant defense mechanisms and/or to act as biocontrol agents that mediate disease suppression. However, the exact interaction between the enzymes/fungi and plants has not been clearly elucidated. Under mild conditions, removal of plant cell wall polysaccharides by CELs for protoplast preparation results in reduced protoplast damage and increased viability and yields. CELs have recently shown great potential in enzyme aid extraction of bioactive compounds from plant materials before selective extraction through enhancing release of target molecules, especially those associated with the wall matrix. To date, attempts have been made to formulate CEL preparation for cellulosic-based bioethanol production. The high cost of CELs has created a bottleneck, resulting in an uneconomic production process. The utilization of low-cost carbohydrates, strain improvement, and gene manipulations has been alternatively aimed at reducing the cost of CEL production. In this review, we focus on and discuss current knowledge of CELs and their applications in agriculture, biotechnology, and bioenergy.

The axy43A gene encoding the intracellular trifunctional xylanolytic enzyme from Paenibacillus curdlanolyticus B-6 was cloned and expressed in Escherichia coli. Recombinant PcAxy43A consisting of a glycoside hydrolase family 43 and a family 6 carbohydrate-binding module exhibited endo-xylanase, ␤-xylosidase, and arabinoxylan arabinofuranohydrolase activities. PcAxy43A hydrolyzed xylohexaose and birch wood xylan to release a series of xylooligosaccharides, indicating that PcAxy43A contained endo-xylanase activity. PcAxy43A exhibited ␤-xylosidase activity toward a chromogenic substrate, p-nitrophenyl-␤-D-xylopyranoside, and xylobiose, while it preferred to hydrolyze long-chain xylooligosaccharides rather than xylobiose. In addition, surprisingly, PcAxy43A showed arabinoxylan arabinofuranohydrolase activity; that is, it released arabinose from both singly and doubly arabinosylated xylose,Moreover, the combination of PcAxy43A and P. curdlanolyticus B-6 endo-xylanase Xyn10C greatly improved the efficiency of xylose and arabinose production from the highly substituted rye arabinoxylan, suggesting that these two enzymes function synergistically to depolymerize arabinoxylan. Therefore, PcAxy43A has the potential for the saccharification of arabinoxylan into simple sugars for many applications. IMPORTANCEIn this study, the glycoside hydrolase 43 (GH43) intracellular multifunctional endo-xylanase, ␤-xylosidase, and arabinoxylan arabinofuranohydrolase (AXH) from P. curdlanolyticus B-6 were characterized. Interestingly, PcAxy43A AXH showed a new property that acted on both the C(O)-2 and C(O)-3 positions of xylose residues doubly substituted with arabinosyl, which usually obstruct the action of xylanolytic enzymes. Furthermore, the studies here show interesting properties for the processing of xylans from cereal grains, particularly rye arabinoxylan, and show a novel relationship between PcAxy43A and endo-xylanase Xyn10C from strain B-6, providing novel metabolic potential for processing arabinoxylans into xylose and arabinose.

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We recently discovered a novel glycoside hydrolase family 6 (GH6) cellobiohydrolase from Paenibacillus curdlanolyticus B-6 (PcCel6A), which is rarely found in bacteria. This enzyme is a true exo-type cellobiohydrolase which exhibits high substrate specificity on amorphous cellulose and low substrate specificity on crystalline cellulose, while this showed no activity on substitution substrates, carboxymethyl cellulose and xylan, distinct from all other known GH6 cellobiohydrolases. Product profiles, HPLC analysis of the hydrolysis products and a schematic drawing of the substrate-binding subsites catalysing cellooligosaccharides can explain the new mode of action of this enzyme which prefers to hydrolyse cellopentaose. PcCel6A was not inhibited by glucose or cellobiose at concentrations up to 300 and 100 mM, respectively. A good synergistic effect for glucose production was found when PcCel6A acted together with processive endoglucanase Cel9R from Clostridium thermocellum and β-glucosidase CglT from Thermoanaerobacter brockii. These properties of PcCel6A make it a suitable candidate for industrial application in the cellulose degradation process.

Rice straw (RS) is an abundant, readily available agricultural waste, which shows promise as a potential feedstock for Asian ethanol production. To enhance release of glucose by enzymatic hydrolysis, RS was pretreated with aqueous ammonia (27% w/w) at two pretreatment temperatures: room temperature and 60 ∘ C. Statistical analysis indicated similarity of enzymatic glucose production at both pretreatment temperatures after 3-day incubation. Chemical composition, FTIR, and EDX analyses confirmed the retention of glucan and xylan in the pretreated solid, but significant reduction of lignin (60.7% removal) and silica. SEM analysis showed the disorganized surfaces and porosity of the pretreated RS fibers, thus improving cellulose accessibility for cellulase. The crystallinity index increased from 40.5 to 52.3%, indicating the higher exposure of cellulose. With 10% (w/v) solid loadings of pretreated RS, simultaneous saccharification and fermentation yielded a final ethanol concentration of 24.6 g/L, corresponding to 98% of maximum theoretical yield. Taken together, aqueous ammonia pretreatment is an effective method to generate highly digestible pretreated RS for bioethanol production and demonstrates potential application in biorefinery industry.