Relative contributions of alternate metabolic pathways for glucose in lipid production by rhodotorula gracilis

Abstract

The oleaginous yeast, Rh. gracilis is a strictly aerobic microorganism. This organism can accumulate over 50\% of cell dry weight as lipid. In understanding the metabolic mechanism related to lipid accumulation in Rh. gracilis, it is necessary to examine the relationship between Embden-Meyerhof-Parnas pathway activity, Hexose-Mono-Phosphate pathway activity, and lipogensis in glucose metabolism. Glucose metabolism via the EMP and HMP pathways and the extent of lipid biosynthesis was estabilished by incubating the cells grown in a chemostat under nitrogen-and carbon-limited conditions with D-glucose labeled with $^{14}C$ at different positions and measuring the labeling pattern of various products. Namely, methods based on yields of $^{14}CO_2$ and $^{14}C$ incorporated into lipid were used to access the relationship between these metabolic and biosynthetic parameters. And, NADPH producing enzymes, i.e. the malic enzyme and glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were assayed. In these experiments, $^{14}CO_2$ ratios and contributions of the HMP pathway were found to vary with the specific rate of lipid biosynthesis (SRLB), not with the lipid contents in the nitrogen-limited cultures. However, under carbon-limited conditions, only $^{14}CO_2$ ratios were found to vary with the SRLB, but, the contribution of the HMP pathway had not varied with the SRLB and also the lipid content. Those enzymes examined in this study have not show particular trends. From the results, obtained, it seems that the proportion of the EMP and HMP pathways change with the various nutritional and physiological conditions and most of the NADPH for lipid biosynthesis may be supplied by the HMP pathway. And the availability of NADPH appears be a limiting factor for the operation of the HMP and/or EMP pathway.