Plant cell wall degrading enzymes and their potential application to biofuel production

Plant cell wall;biofuel;cellulose;hemicellulose;xylanases;beta-glucosidase;thermophilic;anaerobe

Asangba, Abigail E. ; Cann ; Isaac K.

关键词: Plant cell wall;    biofuel;    cellulose;    hemicellulose;    xylanases;    beta-glucosidase;    thermophilic;    anaerobe;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/34518/Asangba_Abigail.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The soaring high price of petroleum price as well as the impact of the fossil fuel combustion process on atmospheric carbon dioxide (CO2) has necessitated the development of alternative fuel energy. Even with the most recent technology, oil, natural gas, and coal will emit not only climate-threatening greenhouse gases and other pollutants, but also the quantity of undiscovered stocks will always be a matter of great concernPlants have evolved photosynthetic mechanisms in which solar energy is used to fix CO2 into carbohydrates. Thus, combustion of biofuels, derived from plant biomass, can be considered a potentially carbon neutral process. Plants cell walls which are composed mostly of lignocellulosic materials (lignin, cellulose, and hemicellulose) are recalcitrant and are difficult to breakdown. This therefore represents a major limitation in the conversion of plant biomass to biofuels. Microbes have evolved a plethora of enzymatic strategies for hydrolyzing plant cell wall into its constituent sugars for subsequent fermentation to biofuels. Therefore, microorganisms are considered an important source of biocatalysts in the emerging biofuel industry. Caldicellulosiruptor bescii and Themoanaerobacterium bryantii are both thermophilic anaerobes capable of secreting enzymes for degrading plant cell walls. To gain insight into the cellular machinery that these organisms elaborate to degrade cellulose and the hemicellulosic polymer xylan, genes with various putative functions from both organisms were identified, cloned and expressed the recombinant proteins in Escherichia coli.A hemicellulase gene cluster from T. bryantii encoding genes predicted to have endoxylanase activity, -xylosidase activity, -glucoronidase activity, acetyl xylan esterase activity as well as oxido-reductase activity was isolated for further studies. We hypothesized that the endoxylanase should work synergistically with the other accessory enzymes in the cluster to break down xylan releasing mainly xylose for the utilization of the organism. As expected the endoxylanase was able to hydrolyze a number of xylan containing polysaccharides releasing undecorated xylo-oligosaccharides with the -xylosidases converting the xylobiose released to xylose. A putative β-glucosidase Cb486 from C. bescii was screened for enzymatic properties, and it was shown to exhibit activity against pNP-α-L-arabinopyranoside, pNP-β-D-fucopyranoside, pNP-β-D-galactopyranoside, pNP-β-D-glucopyranoside, pNP-β-D-xylopyranoside and pNP-β-D-cellobioside suggesting that Cb486 is a multi-functional enzyme. We therefore hypothesized that Cb486 incubated with endoglucanases or endoxylanases should be able to enhance the release of glucose or xylose from cellulose and xylans respectively. As expected, Cb486 was also able to work synergistically with a number of endo-glucanases from the genome of C. bescii to release glucose from cellulosic substrates as well as xylose from xylan containing substrates.