BIOFUEL PRODUCTION FROM CELLULOSIC BIOMASS BY CELL SURFACE ENGINEERING – DEVELOPMENT OF ARMING TECHNOLOGY TO DESIGN BIOCATALYST FOR CONSOLIDATED BIOPROCESSING (CBP) SYSTEM
Authors: Kouichi Kuroda and Mitsuyoshi Ueda
Abstract: Lignocellulosic biomass, the most abundant renewable material on earth, consists mainly of three polymeric constituents, cellulose, hemicellulose, and lignin. Cellulose and hemicellulose can be converted into bioethanol through their conversion into sugar by cellulolytic enzymes and fermentation. Recently, biofuel produced from lignocellulosic biomass has attracted widespread attention as a promising alternative to petroleum because it does not disturb food supply and carbon dioxide released by biofuel consumption is recycled by photosynthesis. Cellulose is a linear and highly ordered polymer of cellobiose, and is degraded synergistically by cellulolytic enzymes. Endoglucanases produce reducing and nonreducing ends from the internal region of amorphous, soluble, and substituted celluloses. Cellobiohydrolases release cellobiose from crystalline cellulose and produce cellobiose and other short cellooligosaccharides from the reducing or nonreducing ends. Then, β-glucosidase cleaves cellobiose and cellooligosaccharides to produce glucose. This glucose can then be assimilated into fermentative ethanol in the yeast Saccharomyces cerevisiae. Cell surface engineering of yeasts is an attractive strategy for molecular breeding of novel yeasts and whole-cell biocatalysts that can directly produce ethanol from cellulose. Using this technique, the cellulolytic enzymes described above could be displayed on the yeast cell surface with the retention of their enzymatic activities. In addition, the simultaneous display of various enzymes on a single cell surface is also possible. Thus, yeast cells displaying cellulolytic enzymes can produce fermentative ethanol after the hydrolytic degradation of cellulosic biomass. Consolidated bioprocessing (CBP) of lignocellulose to ethanol is the most ideal system combining multisteps such as production of cellulolytic enzymes, cellulose hydrolysis, and fermentation of sugars in one reactor. CBP is expected as a potential approach to low-cost bioethanol production from cellulosic biomass. Cell surface engineering is the epoch-making technology required for CBP to produce biofuels from cellulosic biomass in the future, which would contribute to the establishment of a sustainable society based on biomass resources.