Metabolism of the obligatory aerobic yeast Rhodotorula gracilis

Summary

The transients in metabolite concentrations of the yeast Rhodotorula gracilis have been analyzed following the addition of D-glucose and D-xylose. In addition, the effect of varied metabolite concentrations on pyruvate kinase (important for the switch: glycolysis vs. gluconeogenesis) were tested. These results when seen together with the already known enzyme equipment of the yeast, lead to the following conclusions:

• 1. The pentose phosphate cycle is the main pathway of D-glucose metabolism; glucose molecules are broken down not only oxidatively, but also through the reversed transaldolase and transketolase reactions, eventually forming xylulose-5-P.

• 2. Xylulose-5-P is split to a C₂-unit and glyceraldehyde-3-P. The former compound (presumably glycollate) is oxidized to glyoxylate and further metabolized in the glyoxylate shunt of the tricarboxylic acid cycle, the latter compound is transformed to acetyl-CoA through the glycolytic reactions.

• 3. Addition of D-xylose stimulates endogenous metabolism. It is postulated that under these conditions fragments of an endogenous substrate (predominantly cell lipids) are transformed to glucose-6-P in reactions combined from the lower part of the glycolytic pathway and the non-oxidative part of the pentose phosphate shunt.

• 4. Crossover plots indicate the loci of glycolytic regulation between triosephosphates and phosphoglyceric acids, between phosphoglyceric acids and pyruvate (or PEP) as well as between PEP (or pyruvate) and dicarboxylic acids of the TCA cycle (malate).

• 5. A scheme of metabolic pathways in the yeast is postulated and discussed. For comparison, the metabolite concentrations in Saccharomyces carlsbergensis, a yeast with complete glycolysis, were also analyzed.