Page 100 - Lignocellulosic Biomass to Liquid Biofuels

76 Lignocellulosic Biomass to Liquid Biofuels

Through metabolic engineering, bacterial and yeast strains have been
constructed which feature traits that are advantageous for ethanol produc-
tion using lignocellulose sugars, in order to minimize by-product forma-
tion and to increase the ability to utilize all sugars present in lignocellulose
substrate, ethanol yield and productivity, tolerance to ethanol and inhibi-
tors, and tolerance to process hardness [41]. Metabolic engineering is a
powerful method to improve, redirect, or generate new metabolic reac-
tions or complete pathways in microorganisms. A number of different
strategies have been applied to engineer yeasts capable of efficiently
producing ethanol from xylose, including the introduction of initial xylose
metabolism and xylose transport, changing the intracellular redox balance,
and overexpression of xylulokinase and pentose phosphate (PP) pathways
[42,43].
Since many years, Escherichia coli, Klebsiella oxytoca, and Z. mobilis have
been genetically engineered to produce ethanol efficiently from all hexose
and pentose sugars present in the polymers of hemicellulose [44 49].
Moreover, a recombinant E. coli strain from wheat straw was investigated
at high-solid loading by both separate hydrolysis and fermentation and
fed-batch simultaneous saccharification and fermentation (SSF) [50].
In the last decades, numerous microorganisms used in industry, includ-
ing E. coli, Bacillus sp., lactic acid bacteria, Corynebacterium glutamicum [51],
and S. cerevisiae, have been engineered to tolerate toxic compounds and
metabolize a range of carbon sources present in hemicellulose [27,29].
The genetic improvement of S. cerevisiae strains was widely reviewed
[40,52,53]. Genetic and metabolic engineering have been used to insert
heterologous genes encoding D-xylose reductase and xylitol dehydroge-
nase in S. cerevisiae, resulting in yeast strains able to utilize the pentose
D-xylose and to ferment it to ethanol [54 68].
Moreover, genetic engineering have allowed to select an efficient L-
arabinose-fermenting S. cerevisiae strain. In particular, S. cerevisiae strain was
engineered by expression of a bacterial pathway for catabolism of L-arabi-
nose, comprising L-arabinose isomerase, L-ribulokinase, and L-ribulose-5-P
4-epimerase. This has been achieved in order to be able to utilize the
pentose sugar L-arabinose for growth and to ferment it to ethanol [69].In
addition, S. cerevisiae was engineered to construct an acetate-tolerant strain,
because acetate shows a negative effect on the growth of contaminated
bacteria and is a safe and inexpensive reagent for inhibiting bacterial
growth [70]. S. cerevisiae was engineered also to increase its tolerance to
high temperatures [71,72], to ethanol [73], to acetic acid and formic acid