ENO1 and ENO2 are the two S. cerevisiae genes encoding phosphopyruvate hydratase, which catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate during glycolysis. The enolase enzymes function as dimeric phosphopyruvate hydratase complexes (1). Replacement of His373 with asparagine (H373N enolase) or phenylalanine (H373F enolase) reduces enzymatic activity of Eno1p to ca. 10% and 0.0003% of its native enzyme activity, respectively (4).

Log phase cells grown on glucose contain 20-fold more Eno2p than Eno1p, whereas cells grown on ethanol or glycerol plus lactate contain similar amounts of both proteins (1). Enolase catalyses the first common step of glycolysis and gluconeogenesis. During gluconeogenesis, ENO1 and ENO2 catalyze the reverse reaction --- the synthesis of phosphoenolpyruvate from 2-phosphoglycerate (5, ,6, 7). This reaction is important for growth on non-sugar carbon sources like ethanol, glycerol, or peptone, when the gluconeogenesis pathway is used to synthesize glucose.

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 8). 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.

Last updated: 2005-07-05