Regulation and Function of Pyruvate Kinase and Malate Enzyme in Yeast*

Abstract

The concentrations of pyruvate kinase and malate enzyme have been determined in the yeast Rhodotorula glutinis and shown to vary considerably according to the carbon compounds consumed by the cells. The results so obtained, together with the kinetic studies of the synthesis of the enzymes, seem to indicate that pyruvate kinase is nutritionally induced by glucose whereas malate enzyme is repressed by glucose and by acetate.In R. glutinis, the function of pyruvate kinase is to form pyruvate from phosphoenolpyruvate during glycolysis and that of malate enzyme to provide pyruvate from malate. The reverse reactions probably do not play any physiological role. R. glutinis cells growing in a medium containing glucose could at the same time utilize aspartate as source of carbon. Under such conditions, asparate could not supply phosphoenolpyruvate nor pyruvate for biosynthetic and energetic purposes, because both phosphoenolpyruvate carboxykinase and malate enzyme are repressed by glucose, but could, however, provide oxaloacetate for biosynthesis and at the same time spare the pyruvate glycolytically formed by inhibiting its carboxylation. Once glucose was used up by the cells and after a clear diauxia, the yeast continued growing solely on the remaining aspartate. The disappearance of glucose from the medium determined a change in enzyme levels, pyruvate kinase being no longer induced and phosphoenolpyruvate carboxykinase and malate enzyme being no longer repressed. This enzymatic change allowed aspartate to fulfill by itself the functions of supplying energy and providing precursors for the synthesis of all cell constituents.