Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 42

of released sugars need to be well coordinated within the single cell and between
cells and their surroundings at different scales, from molecular levels involving
gene expression and regulation to the intracellular metabolic network, as well as
the kinetics of heterogeneous hydrolysis.
4.2 Strain Development
Unlike conventional sugar- and starch-based feedstocks, hydrolysates of ligno-
cellulosic biomass contain significant amount of pentose sugars such as xylose and
arabinose, in addition to hexose sugars of glucose, mannose and galactose.
Unfortunately, the ethanologenic species, either S. cerevisiae or Z. mobilis, cannot
ferment the pentose sugars into ethanol efficiently. If only hexose sugars from
lignocellulosic biomass are fermented, with pentose sugars left behind, feedstock
consumption for bioethanol production will be significantly high, and in the
meantime the unfermented pentoses will remain with the distillage and increase
the capital investment and energy consumption in the treatment of the distillage.
Two strategies, engineering pentose-utilizing microorganisms with ethanol
production pathways or ethanol producers with pentose-metabolizing pathways, can
be developed for developing recombinants to ferment both pentose and hexose
sugars in the hydrolysate into ethanol [ 62 ]. Although pentose-utilizing bacteria like
Escherichia coli and Klebsiella oxytoca can be engineered for ethanol production
[ 63 ], their poor ethanol tolerance significantly compromises ethanol titers, making
ethanol purification by distillation highly energy-intensive, and in the meantime the
neutral pH values required for their growth and ethanol fermentation increase
the contamination risk of the fermentation system, not to mention the problems
associated with their biomass treatment. Therefore, engineering the ethanologenic
species Z. mobilis and S. cerevisiae with pentose-metabolizing pathways is preferred.
In nature, bacteria employ the isomerase pathway to direct xylose to their
central metabolism, whereas fungi use the reductase and dehydrogenase pathways
to convert xylose to xylulose via the intermediate xylitol. Thus, an overall strategy
for engineering Z. mobilis and S. cerevisiae with xylose-metabolizing pathways is
illustrated in Fig. 9 [ 64 ].
4.2.1 Z. mobilis
Z. mobilis, a facultative anaerobic Gram-negative bacterium, can ferment glucose
into ethanol and CO 2 through the ED pathway, which generates more ethanol
due to less biomass production compared with the Embden-Meyerhof pathway in
S. cerevisiae [ 48 ]. In addition, Z. mobilis can tolerate concentrations as high as
120 g/L ethanol [ 63 ], much higher than that tolerated by other bacteria, and its
biomass is generally recognized as safe (GRAS) for animal feed, making
this species suitable for metabolic engineering with pentose-fermenting ability.