Glycolysis is the lysis, or splitting, of one molecule of glucose into two molecules of pyruvate, producing a net gain of two ATP molecules. Pyruvate can then be used in anaerobic (fermentation) or aerobic (respiration) metabolism. The glycolysis pathway and the genes involved are illustrated here.

PGI1 encodes phosphoglucose isomerase, a tetrameric enzyme that catalyzes the conversion of glucose-6-phosphate to fructose-6-phosphate during glycolysis and the reverse reaction during gluconeogenesis (4, 6, 5)

Gluconeogenesis is the process whereby glucose is synthesized from non-carbohydrate precursors, which enables yeast cells to grow on non-sugar carbon sources like ethanol, glycerol, or peptone. The reactions of gluconeogenesis, shown here, mediate conversion of pyruvate to glucose, which is the opposite of glycolysis, the formation of pyruvate from glucose. While these two pathways have several reactions in common, they are not the exact reverse of each other. As the glycolytic enzymes phosphofructokinase (Pfk1p, Pfk2p) and pyruvate kinase (Cdc19p) only function in the forward direction, the gluconeogenesis pathway replaces those steps with the enzymes pyruvate carboxylase (Pyc1p, Pyc2p) and phosphoenolpyruvate carboxykinase (Pck1p)-generating oxaloacetate as an intermediate from pyruvate to phosphoenolpyruvate-and also the enzyme fructose-1,6-bisphosphatase (Fbp1p) (reviewed in 7). Overall, the gluconeogenic reactions convert two molecules of pyruvate to a molecule of glucose, with the expenditure of six high-energy phosphate bonds, four from ATP and two from GTP.

Cells lacking Pgi1p are unable to grow on fructose or glucose as a sole carbon source, and both growth and sporulation are glucose dependent (1). Because fructose 6-phosphate (F6-P) is also a precursor of the cell wall components chitin and mannoprotein, temperature-sensitive (cdc30) mutations in phosphoglucose isomerase cause cell-cycle arrest probably due to an inability to synthesize components needed for cytokinesis and cell separation (8). Transcription of PGI1 does not appear to be regulated by glycolytic or gluconeogenic growth, or between exponential and stationary phases (9).

Last updated: 2005-07-22