Biocatalysis and Biotransformation of Lignocellulose into Bioethanol: Process Intensification and Mechanism Research

摘要

As one kind of non-renewable resources, fossil fuels will be exhausted in the near future.At the constant production and consumption, the presently known reserves of oil will lastaround 45.7 years, natural gas 62.8 years, and coal 119 years(1). In addition, globalwarming caused by greenhouse gas emission attribute mostly to the combustion of fossilfuels. Bioethanol is one of the most potential renewable energies. For large-scale usage,lignocellulosic material is a promising choice to produce a second generation bioethanolfuel(2). As the renewable source, lignocellulose not only has extensive sources, such assugarcane bagasse, corn stover, corncob, wheat straw, sawdust, etc, but also significantlydecrease net the carbon dioxide emission. However, there are a number of obstacles suchas slow reaction rate, high production costs, non-cellulosic material block, and low ethanolconcentration, which diminishing the enzyme performance and hinder the progress ofbiomass industrialization(3,4). To decrease reducing product inhibition effect oncellobiohydrolases and β-glucosidase, simultaneous saccharification and fermentation(SSF) is often used in bio-ethanol production by fermentable sugars to ethanol timely(5). Atthe same time, various strategies were performed for the purpose of cost-effectiveenzymatic conversion such as reducing enzymes production cost, producing cellulase withhigher specific activity(6) and enzyme recycling including cellulase adsorption onto freshsubstrate(7) and cellulase immobilization(8,9).This paper summarizes our work on biocatalysis and biotransformation of lignocelluloseinto bioethanol aiming to enhance enzyamtic hydrolysis efficiency and increase final ethanolconcentration, and the mechanism of cellulase on cellulose was also investigated.(1) Enzyme complex. Hemicellulose and possibly pectin are thought to restrict theaccess of cellulases to their substrate in pretreated lignocellulose. Addition of xylanasesand pectinases can degrade these non-cellulosic saccharides and thus increase celluloseconversion(10). The digestion of cellulose was increased by enzyme complexes of cellulaseand accessory enzymes including β-glucosidase, xylanase and pectinase to hydrolyzecellobiose, hemicellulose and pectin, respectively (Fig. 1).To enhance the conversion of the cellulose and hemicellulose, the corncob pretreated byaqueous ammonia soaking was hydrolyzed by enzyme complexes. Firstly, corncob werepretreated by aqueous ammonia soaking and the saturation limits for cellulase and β-glucosidase were determined, which is 15mg protein/g glucan (50FPU/g glucan) forcellulase (Spezyme CP). Second, the accessory enzymes (β-glucosidase, xylanase andpectinase) were supplemented to hydrolyze cellobiose (cellulase-inhibiting product),hemicellulose and pectin (the component covering the fiber surfaces), respectively. It wasfound that β-glucosidase (Novozyme 188) loading of 1.45mg protein/g glucan (30CBU/g