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영어

Cellulosic biomass is the most attractive renewable source for biorefinery processes producing a wide range of products, such as fuels, commodity chemicals or bioplastics. However, traditional biomass bioconversions are economically inefficient multistep processes due to the recalcitrance of cellulose (1). Thus far, no microorganisms able to perform single-step fermentation into products (consolidated bioprocessing; CBP) have been isolated. Metabolic engineering is currently the most attractive way to develop recombinant microorganisms suitable for CBP. In order to develop a system to be used for consolidated bioprocessing of cellulose, a cellodextrin (including cellobiose) utilization system must be established in recombinant microorganism. In this presentation, we describe efforts in our laboratory toward the cellulosic biorefinery products, 2,3-butanediol(2,3-BDO), lactic acid and ethanol. We have successfully engineered an E. coli to utilize cellodextrins, the hydrolysis intermediates from cellulose depolymerization by periplasmic expression of a cellodextrinase. The resulting strain grew well on cellodextrins with varying degrees of polymerization. A synthetic operon for BDO biosynthesis was engineered into the cellodextrinase-expressing cells, resulting in a biocatalyst capable of converting cellodextrin to 2,3-BDO with 84% conversion yield without exogenous β-glucosidase(2). This periplasmic cellodextrinase also conferred the E. coli cells the ability to direct ferment cellodextrin to lactic acid at about 80% of theoretical yield (3). Furthermore, we successfully engineered E. coli biocatalysts to assimilate cellobiose through a phosphorolytic mechanism. Cytoplasmic expression of the cellobiose phosphorylase allowed E. coli to use cellobiose. Subsequent knockout and complementation studies indicated that the endogenous LacY was responsible for the transport of cellobiose (4). This is an important step toward consolidated bioprocessing for production of biofuel and biorefinery products from lignocelluloses. The ability for the biocatalyst to directly use cellodextrins eliminates the need for exogenous β- glucosidase and removes from hydrolysate cellobiose and cellodextrins, potential inhibitors for the cellulases.