Submitted to: Biotechnology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 25, 1997
Publication Date: N/A
Interpretive Summary: Biomass represents a renewable resource that, if converted into liquid fuel, i.e., ethanol, would reduce our dependence upon foreign oil. D-xylose is one of the most abundant sugars in nature and, as such, accounts for up to 25% of the dry weight of some plants. Most yeast are unable to produce ethanol from xylose. Previous investigators have used recombinant DNA techniques to clone the yeast genes encoding the enzymes that metabolize xylose into yeast that ferment well. However, the new, recombinant yeasts produce large amounts of byproducts, rather than ethanol, due to inherent problems associated with these enzymes. Bacteria do not have this problem since they metabolize xylose differently. Previous reports suggested that the yeast, Rhodosporidium toruloides, had a bacterial-like xylose metabolism. We examined this yeast in the hopes that it would prove to be a source of a gene that would eliminate the problems that have plagued previous research. We were unable to demonstrate the existence of the bacterial-like enzyme in this yeast; however, we did find the conventional yeast metabolizing enzymes. The information obtained will be helpful to others in the field in that it extends our knowledge of xylose metabolism in yeast and also corrects a long standing error in the literature.
Technical Abstract: Most yeasts metabolize xylose by first reducing xylose to xylitol, with xylose reductase and NADPH, and then oxidizing xylitol to xylulose, with xylitol dehydrogenase and NAD+. Thereafter, xylulose is phosphorylated to xylulose-5-phosphate and metabolized via the pentose-phosphate pathway. The yeast genes for the initial steps in the xylose pathway have been cloned and successfully expressed in yeast that were unable to utilize xylose. The results indicated that a cofactor imbalance between NAD+ and NADPH resulted in the incomplete conversion of xylose to xylulose. Hofer et al. (1971. Biochem. Biophys. Acta 252:1-12) examined the pathway of xylose metabolism in the yeast Rhodosporidium toruloides, syn. Rhodotorula glutinis. They presented strong circumstantial evidence that the primary route of xylose catabolism in this yeast was the direct conversion of xylose to xylulose by xylose isomerase. We examined the pathway that this yeast uses to metabolize xylose to determine if it does indeed possess a xylose isomerase. We were unable to detect xylose isomerase activity in cell free extracts; however, xylose reductase and xylitol dehydrogenase activities were readily detectable. If R. toruloides does possess a xylose isomerase, it does not play a major role in the catabolism of xylose.