Ethanol Production by Recombinant CBP Yeasts

Toward the realization of a low-carbon society, there is a need to establish “biorefineries” to produce biofuels and chemicals from lignocellulosic biomass resources that are renewable and do not compete with food. Taking the production of bioethanol using a carbohydrate platform as an example, a biorefinery basically consists of a pretreatment process to swell crystallized biomass, an enzymatic treatment process to hydrolyze the biomass, a fermentation process by microorganisms, and a separation and recovery process of the products. The key to practical application is the success or failure of developing energy-efficient, low-cost processes. Consequently, consolidated bioprocessing (CBP), which integrates enzyme production, saccharification, and fermentation in a single step, is garnering attention as a promising system for bioethanol production. CBP requires highly engineered yeast with multiple process-specific properties. Utilizing the yeast Saccharomyces cerevisiae, which is resistant to many inhibitory chemical compounds released from biomass, as a host and expressing several components of an exogenous cellulolytic system derived from fungi and bacteria are the primary strategies for designing biocatalysts for CBP. The protein display strategy permits the accumulation of hydrolytic enzymes such as cellulases and xylanases on the cell surface of fermenting yeast, which is of critical importance to the realization of CBP. This chapter describes candidate anchor proteins for expressing functional enzymes on the surface of CBP yeast, highly efficient methods for surface expression of anchor proteins on the cell wall, and the application of CBP yeast to bioethanol production using lignocellulose biomass as the carbon source.