Gluconeogenesis and Catabolite Inactivation

Le résumé fourni par la source

The glycolytic chain is a capital pathway for carbohydrate metabolism in yeast ( Figure 19.1 ), but also plays a major role during growth in nonsugar carbon sources, like ethanol, glycerol, pyruvate, etc. Since essential cellular components are derived from sugar phosphates, growth on nonsugar carbon sources requires synthesis of these compounds from the nonsugar precursors. This process is known as gluconeogenesis . Gluconeogenesis implies a reversal of all, or of some, glycolytic reactions, depending on the level at which the nonsugar substrate is incorporated to the glycolytic pathway. Most of the glycolytic reactions are reversible under physiological conditions, and in general, the same type of enzyme is used during growth in fermentable sugars or in nonsugar, nonfermentable carbon sources. However, there are two reactions in the glycolytic chain that cannot be reverted during gluconeogenesis due to the unfavorable concentration of substrates existing in the cell. These are the reactions catalyzed by phosphofructokinase and by pyruvate kinase. To bypass these reactions during gluconeogenesis, different reactions catalyzed by different enzymes have been selected ( Figure 19.1 ). The formation of phosphoenolpyruvate is achieved starting from oxaloacetate and spending energy in the form of ATP (in other organisms GTP is used, Utter and Kolenbrander, 1973), and the formation of fructose-6-P from fructose-1,6-bisphosphate occurs through an hydrolytic reaction. The enzymes catalyzing these steps are phosphoenolpyruvate carboxykinase (PEPCK) and fructose-1,6-bisphos-phatase (FbPase), respectively ( Figure 19.1 ). Depending on the nature of the nonsugar carbon source, additional ancillary enzymes may be needed. In particular, the enzymes of the glyoxylate cycle are required for the metabolism of 2C compounds ( Figure 19.1 ), therefore, we may consider these enzymes as gluconeogenic enzymes. The objective of this chapter is to provide information on the gluconeogenic enzymes and the genes encoding them and also to examine different mechanisms that regulate their level or their activity. Particular attention will be devoted to the process of catabolite inactivation which affects preferentially, although not exclusively, the gluconeogenic enzymes.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.