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Chapter 2 : Lignocellulosic Biomass Conversion to Ethanol by Saccharomyces
Category: Applied and Industrial Microbiology
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Traditional Saccharomyces cerevisiae ferments glucose to ethanol rapidly and efficiently, but it is limited in its fermentation of pentose sugars (xylose and arabinose) to ethanol. For future sustainable and cost-efficient lignocellulosic biomass conversion to ethanol, there exist two major challenges: heterogeneous sugar utilization and stress tolerance in engineering microbial catalytic fermentors for bioethanol production. This chapter reviews the current knowledge on the composition and structure of lignocellulosic biomass, its pretreatment and enzymatic saccharification to simple sugars. It also discusses strain development of S. cerevisiae for efficient fermentation of the biomass-derived sugars to ethanol. Endoglucanase from Mucor circinelloides strain was found to have a wide pH stability and activity. There is an increasing demand for the development of thermostable, environmentally compatible products and for substrate-tolerant cellulases with increased specificity and activity for the application of converting cellulose to glucose for the fuel ethanol industry. Well-functioning hexose transporter family members maintain an easy flow of glucose uptake for the yeast and produce ethanol through glycolysis. Significant progress has been made recently for inhibitors generated from biomass pretreatment. The chapter focuses on the representative inhibitors furfural and 5-hydroxymethylfurfural (HMF). Development of yeast strains that can efficiently utilize heterogeneous sugars and withstand stress conditions in the bioethanol process is key for sustainable, economic and cost-competitive industry dealing with lignocellulosic biomass conversion to ethanol. A comprehensive genomic engineering approach will allow to meet the challenges for efficient lignocellulosic biomass conversion to ethanol in the next decade and beyond.
Hexose and pentose catabolism pathways of Saccharomyces cerevisiae. Shown are catabolic pathways of yeast in utilization of major hexoses including glucose, galactose, and mannose; and of pentoses including xylose and arabinose for ethanol production. In the diagram, underlined EC numbers represent endogenous enzymes, and those not underlined indicate an exogenous origin (i.e., introduced to the yeast). Enzyme-encoding genes and EC numbers are presented in parentheses as follows: hexokinase (HXK1/HXK2, 22.214.171.124); glucokinase (GLK1, 126.96.36.199); galactokinase (GAL1, 188.8.131.52); galactose-1-phosphate uridylyltransferase (GAL7, 184.108.40.206); UDP-glucose 4-epimerase (GAL10, 220.127.116.11); phosphoglucomutase (GAL5/PGM2, 18.104.22.168); hexokinase 1 (HXK1, 22.214.171.124); mannose-6-phosphate isomerase (PMI40, 126.96.36.199); xylose reductase/aldose (GRE3/xyl1, 188.8.131.52); xylitol dehydrogenase (XYL2/xyl2, 184.108.40.206); xylulokinase (XKS1/xyl3, 220.127.116.11); xylose isomerase (xylA, 18.104.22.168); arabinitol 4-dehydrogenase (lad1, 22.214.171.124); L-xylulose reductase (lxr1, 126.96.36.199); L-arabinose isomerase (araA, 188.8.131.52); L-ribulokinase (araB, 184.108.40.206); and L-ribulose-5-phosphate 4-epimerase (araD, 220.127.116.11) (adapted from Sonderegger et al., 2004b ; Grotkjær et al., 2005 ; Karhumaa et al., 2005 ; Jeffries, 2006 ; and van Maris et al., 2006 ).
Metabolic conversion products of inhibitors. Furfural is converted to 2-furanmethanol (FM, furfuryl alcohol) and 5-hydroxymethylfurfural (HMF) converted to 2,5-furan-dimethanol (FDM; 2,5-bis-hydroxymethylfuran) (Liu et al., submitted).
Cell growth under inhibitor stress. Cell growth of S. cerevisiae NRRL Y-12632 (gray circles) and strain 12HF10 (black circles) in response to furfural and HMF at 12 mM each on a defined medium (Liu et al., submitted).
Metabolic dynamics of S. cerevisiae in presence of inhibitors. Major metabolic conversion dynamics of NRRL Y-12632 (a) and strain 12HF10 (b) including glucose ( ), ethanol ( ), HMF ( ), FDM ( ), furfural ( ), and FM ( ) in the presence of furfural and HMF at 12 mM each on a defined medium as measured by HPLC analysis. Glucose and ethanol were estimated in grams/liter (left axis), and the remaining values are presented in millimolar (right axis) (Liu et al., submitted).
The furfural and HMF conversion pathways. A schematic diagram shows furfural conversion into furan methanol (FM) and HMF into 2,5-furan-dimethanol (FDM) relative to glycolysis and ethanol fermentation for ethanologenic yeast S. cerevisiae (adapted from Liu, 2006 , and Liu et al., submitted).
Enzymes involved in cellulose and hemicellulose degradation
Ethanol fermentation kinetics of selective representative yeast strains
A survey of representative Saccharomyces strains for improved xylose utilization by genetic engineering and directed adaptation selection