Bioethanol Production from Woody Biomass by Recombinant Saccharomyces cerevisiae Expressing Thermostable Enzyme

Abstract

Biomass is one of the major natural resources and biomass has always been a major source of energy for mankind and it is presently estimated to contribute of the order 10–14% of the world energy supply. The more efficient use of biomass is demanded to solve the global crisis such as depletion of fossil fuel and global warming. Woody biomass, including agriculture residues, wood chips, municipal solid wastes, paper wastes, etc., has already been transferred to bio-ethanol and bio-diesel in some cases and used as energy related products, although many issues such as efficiency and productivity still exist to be overcome. Xylose is one of the major fermentable sugars present in woody biomass, the second most abundant carbohydrate polymer in nature to glucose. The efficient fermentation of glucose as well as xylose are required to develop economically viable processes for producing biofuels such as ethanol from woody biomass. Saccharomyces cerevisieae is the favored ethanol-producing microorganism, but it can not metabolize xylose which is an unfermentable sugar of the pentose category. Xylose consuming yeasts such as Pichia stipitis can metabolize xylose through expression of xylose reductase (XR) and xylitol dehydrogenase (XDH). However, Pichia stipitis is susceptible to ethanol and obliges stumpy and carefully controlled oxygenation, these disadvantages avert its usage for industrial ethanol production. Saccharomyces strains producing XR and XDH from Pichia stipitis has the ability to produce ethanol from xylose. During this course, the problems are limited production of ethanol and high xylitol excretion. The making of thermostable XDH carried out in this study is expected to resolve these problems. The significant enhancement was found in Y-C4 strain that contained thermostable enzyme XDH, 51% decrease of unfavorable xylitol excretion with 19% increased ethanol production, when compared with the reference strain expressing the wild-type XDH.