Biomedical Engineering Reference
In-Depth Information
Fig. 9
Simplified xylose-metabolizing pathways in bacteria and yeast (Adapted from [
60
])
Zhang et al. transferred four genes responsible for xylose assimilation and
pentose phosphate pathways--xylose isomerase (xylA), xylulose kinase (xylB),
transketolase (tktA) and transaldolase (talB)--into Z. mobilis, enabling the
recombinant to use xylose for growth and fermentation [
65
]. Shortly afterwards,
Deanda et al. engineered this species with arabinose utilization by expressing five
genes from E. coli encoding L-arabinose isomerase (araA), L-ribulokinase (araB),
L-ribulose-5-phosphate-4-epimerase (araD), transaldolase (talB) and transketolase
(tktA)[
66
]. To overcome the disadvantage of genetic instability of the plasmid-
bearing recombinants, genomic integration of these heterologous genes was further
developed [
67
].
Although significant progress has been made in engineering Z. mobilis to
co-ferment pentose and hexose sugars for ethanol production, no commercial
applications have been reported to date, due to the incomplete understanding of the
species as well as the complexity of industrial substrates, particularly the inhibition
of various toxic by-products released during the pretreatment of lignocellulosic
biomass and the molecular mechanisms underlying the responses of the species
to environmental stresses. With the sequencing of the Z. mobilis genome and
elucidation of more functional genes, together with the applications of synthetic
and systems biology methodologies [
68
-
71
], more efficient strains are expected to
be engineered. Under the support of the DOE project, the Integrated Corn-Based
Bio-Refinery (ICBR), DuPont and Broin Companies have established a partnership
to produce cellulosic ethanol from corn stover by genetically modified Z. mobilis,
which might be a milestone for commercial application of this species [
72
].
4.2.2 S. cerevisiae
Currently, ethanol production from starch- and sugar-based feedstocks is solely
using strains from S. cerevisiae, which exhibits significant advantages over other
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