Design of superior cell factories for a sustainable biorefinery by synthetic bioengineering

Abstract

To build an energy and material secure future, we must pioneer the next generation of renewable fuels and chemicals using environmentally benign production processes. Since biomass represents an abundant carbon-neutral renewable resource for the production of biofuels, numerous environmental and social benefits could result from the replacement of petroleum-based transport fuels with bioethanol converted from biomass. One of the key technologies for the development of biorefineries is cell surface engineering, which is a powerful tool for engineering and functionalizing many organisms. Using the technology, various kinds of functional proteins, such as enzymes, can be expressed on the cell surface without loss of cell activity. The display of amylolytic and cellulolytic enzymes on the surface of Saccharomyces cerevisiae has accomplished direct ethanol production from starchy and cellulosic biomass. Moreover, the display of hemicellulase on the surface of S. cerevisiae that has a xylose-assimilating metabolic pathway has enabled production of ethanol from hemicellulosic materials. Furthermore, reutilization of the cell-surface engineered yeast has an advantage in the reduction of enzyme cost, which enables reuse of enzymes on the cell surface by collecting the cells. Thus, cell surface engineering is a promising technology for the development of a consolidated bioprocess by integrating enzyme production, saccharification and fermentation. Regardless of the biomass hydrolysis, metabolic engineering of microorganisms is emphasized for the efficient production of ethanol from biomass. Specifically, lignocellulosic hydrolysates contain high concentrations of inhibitors that negatively affect metabolism and ethanol yields. To circumvent these difficulties, robust S. cerevisiae strains that efficiently ferment mixtures of hexose and pentose sugars in the presence of various chemical contexts for industrial ethanol production should be constructed through metabolic engineering approaches. A combination of a cell-surface displayed enzyme system and an intracellular metabolic engineering system is a very effective approach for developing cells with improved fermentation ability for industrial applications. The technology (synthetic bioengineering) will open up new applications of cell factories to industrially important processes.