The concurrent regulation of metabolically related enzymes. The Krebs cycle and glyoxylate shunt enzymes in Neurospora.

Abstract

The kinetics of derepression of isocitrate lyase, malate synthase, phosphoenolpyruvate carboxykinase, citrate synthase, fumarate hydratase, and malate dehydrogenase were studied following transfer of mycelium grown on a medium containing sucrose to one containing acetate as sole source of carbon. The maximum rate of isocitrate lyase synthesis was proportional to the number of isocitrate lyase structural genes. Comparisons of the repressed levels of isocitrate lyase and malate synthase on sucrose medium and the derepressed levels in acetate medium showed an over‐all coordination between the levels of these two enzymes. A similar relationship existed between the levels of fumarate hydratase, citrate synthase and malate dehydrogenase. The Krebs cycle enzymes also showed coordinate repression on sucrose medium under conditions of enhanced catabolite repression. Other results, however, showed that neither for the glyoxylate shunt enzymes nor for the Krebs cycle enzymes was this coordination invariable. Under conditions in which some of the Krebs cycle enzymes assumed different roles no coordination was observed. It is suggested that this type of coordination, which is frequently observed in the pathways of central carbon metabolism in eucaryotes, results from the metabolic repressors of each enzyme being in equilibrium or in a constant ratio to one another. The coordination does not arise because the structural genes of each enzyme belong to the same operon.