Biomedical Engineering Reference
In-Depth Information
Besides the strains mentioned from nature, now many recombinant strains have
been obtained by genetic engineering techniques. The two main ideas are as below:
One is to introduce pentose metabolic pathways to efficient hexose-utilizing strains,
and the other is to introduce key genes of high ethanol yield to strains that can
utilize mixed sugar but the yield is low. The commonly used host strains contain
Saccharomyces cerevisiae
,
Zymomonas mobilis,
and
E. coli
. Sun et al. amplified
genes
pdc
and
adhB
encoding essential ethanologenic enzymes in
Z. mobilis
using
the polymerase chain reaction (PCR) technique. Recombinant plasmids pKK-PA
and pEtac-PA were constructed in which genes
pdc
and
adhB
were placed under the
control of the
lac
promoter, respectively. The results indicated that by introducing
both
pdc
and
adhB
the ethanologenic pathway was successfully constructed in
E. coli
[
40
,
41
]. Bao et al. cloned
xylA
gene from
Thermus thermophilus
and
transformed it into
S. cerevisiae
after treatment. The recombinant xylose isomerase
showed activity. Recombinant
S. cerevisiae
strain that can simultaneous express
xylA
and overexpress
TK11
and
TA11
can grow in medium in which xylose as the
sole carbon source. By flask fermentation, the strain fermented xylose to ethanol,
and the yield was 1.3 g
L
1
[
41
,
42
].
(2) Mechanism of pentose fermentation
Currently, the study of the mechanism of pentose fermentation has mainly concen-
trated on the microbial metabolism of xylose. Xylose isomerization is the initial
biochemical reaction. First, xylose is converted to xylulose catalyzed by xylose
isomerase in microbial cells and to phosphate xylulose catalyzed by xylulose kinase.
Then, it enters the pentose phosphate cycle. After a series of biochemical reactions,
it is finally changed into ethanol and other metabolites.
In xylose isomerization, metabolic pathways of bacteria, filamentous fungi, and
yeasts are different. To most bacteria and actinomycetes, it is only a one-step
reaction catalyzed by xylose isomerase. To yeast and filamentous fungi, first xylose
is reduced to xylitol by a NADPH-dependent xylose reductase (XR), and then xylitol
is oxidized to xylulose by NAD-dependent xylitol dehydrogenase (XDH). Take
yeast as an example: The yeast xylose fermentation is conducted under facultative
anaerobic conditions, and the overall reaction formula is as follows:
6
C
5
H
10
O
5
! 9
C
2
H
5
OH
C 12
CO
2
Therefore, the theoretical yield of xylose fermentation to ethanol is 0.46 g
ethanol
g
1
(in terms of glucose mass), which is lower than the yield of glucose
fermentation to ethanol, 0.51 g ethanol
g
1
(in terms of glucose mass) [
41
].
6.2.3.2
Immobilized Cell Fermentation
There are several advantages of immobilized cell fermentation. For example,
cell concentration is increased, and the cell can be used continuously. Thus, the
final alcohol concentration in the fermentation broth can be improved. Currently,
