Conversion of plant cell walls to ethanol constitutes second generation bioethanol production. The process consists of several steps: biomass selection/genetic modification, physiochemical pretreatment, enzymatic saccharification, fermentation and separation. Ultimately, it is desirable to combine as many of the biochemical steps as possible in a single organism to achieve CBP (consolidated bioprocessing). A commercially ready CBP organism is currently unreported. Production of second generation bioethanol is hindered by economics, particularly in the cost of pretreatment (including waste management and solvent recovery), the cost of saccharification enzymes (particularly exocellulases and endocellulases displaying kcat ~1 s−1 on crystalline cellulose), and the inefficiency of co-fermentation of 5- and 6-carbon monosaccharides (owing in part to redox cofactor imbalances in Saccharomyces cerevisiae).

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