Biotechnological Applications
It is desirable to ferment D-xylose to ethanol however microorganisms that are naturally able to do that have disadvantages. One organism that can naturally ferment D-xylose to ethanol is the yeast Pichia stipitis; however it is not as ethanol and inhibitor tolerant as the traditional ethanol producing yeast Saccharomyces cerevisiae. S. cerevisiae on the other hand can not ferment D-xylose to ethanol. In attempts to generate S.cerevisiae strains that are able to ferment D-xylose the XYL1 and XYL2 genes of P. stipitis coding for the XR and XDH respectively were introduced to S. cerevisiae by means of genetic engineering. In another approach a bacterial xylose isomerase was introduced.
Studies on flux through the pentose phosphate pathway during D-xylose metabolism have revealed that limiting the speed of this step may be beneficial to the efficiency of fermentation to ethanol. Modifications to this flux that may improve ethanol production include a) lowering phosphoglucose isomerase activity, b) deleting the GND1 gene, and c) deleting the ZWF1 gene. Since the pentose phosphate pathway produces additional NADPH during metabolism, limiting this step will help to correct the already evident imbalance between NAD(P)H and NAD+ cofactors and reduce xylitol byproduct.
Another experiment comparing the two D-xylose metabolizing pathways revealed that the XI pathway was best able to metabolize D-xylose to produce the greatest ethanol yield, while the XR-XDH pathway reached a much faster rate of ethanol production.
The aim of this genetic recombination in the laboratory is to develop a yeast strain that efficiently produces ethanol. However, the effectiveness of D-xylose metabolizing laboratory strains do not always reflect their metabolism abilities on raw xylose products in nature. Since D-xylose is mostly isolated from agricultural residues such as wood stocks then the genetically altered strains will need to be effective at metabolizing these less pure natural sources.
Varying expression of the XR and XDH enzyme levels have been tested in the laboratory in the attempt to optimize the efficiency of the D-xylose metabolism pathway.
Read more about this topic: Xylose Metabolism