The control of glycogen metabolism in yeast

Abstract

The addition of glucose to a suspension of yeast initiated glycogen synthesis and ethanol formation. Other effects of the glucose addition were a transient rise in the concentration of cyclic AMP and a more prolonged increase in the concentration of hexose 6-monophosphate and of fructose 2,6-bisphosphate. The activity of glycogen synthase increased about 4-fold and that of glycogen phosphorylase decreased 3 -5-fold. These changes could be reversed by the removal of glucose from the medium and induced again by a new addition of the sugar. These effects of glucose were also obtained with glucose derivatives known to form the corresponding 6-phosphoester. Similar changes in glycogen synthase and glycogen phosphorylase activity were induced by glucose in a thermosensitive mutant deficient in adenylate cyclase (cdc35) when incubated at the permissive temperature of 26"C, but were much more pronounced at the nonpermissive temperature of 35°C. Under the latter condition, glycogen synthase was nearly fully activated and glycogen phosphorylase fully inactivated. Such large effects of glucose were, however, not seen in another adenylate-cyclase-deficient mutant (cyrl), able to incorporate exogenous cyclic AMP.When a nitrogen source or uncouplers were added to the incubation medium after glucose, they had effects on glycogen metabolism and on the activity of glycogen synthase and glycogen phosphorylase which were directly opposite to those of glucose. By contrast, like glucose, these agents also caused, under most experimental conditions, a detectable rise in cyclic AMP concentration and a series of cyclic-AMP-dependent effects such as an activation of phosphofructokinase 2 and of trehalase and an increase in the concentration of fructose 2,6-bisphosphate and in the rate of glycolysis. Under all experimental conditions, the rate of glycolysis was proportional to the concentration of fructose 2,6-bisphosphate. Uncouplers, but not a nitrogen source, also induced an activation of glycogen phosphorylase and an inactivation of glycogen synthase when added to the cdc35 mutant incubated at the restrictive temperature of 35 "C without affecting cyclic AMP concentration.Glycogen tends to accumulate in yeast in conditions under which growth is restricted. This is the case during the transition between the exponential and stationary phases of growth in the presence of glucose [l -51 and also during incubation of yeast in a medium containing glucose but devoid of other essential elements such as nitrogen [3, 5-71, phosphate or sulfate [3]. Conversely, the addition of a nitrogen source to a nitrogen-deficient medium causes a sudden arrest of glycogen accumulation and stimulates glycolysis [2, 5 -71. Besides these nutritional factors, several drugs are also known to affect glycogen metabolism. Indeed, uncouplers such as 2,4-dinitrophenol and azide have the property of inducing a rapid degradation of the polysaccharide when added either alone or even in the presence of glucose to a yeast suspension [7-111.

Structural Complexity of the Nitrogen Source and Influence on Yeast Growth and Fermentation

“…Supplementation of the medium with complex constituents always improved cellular metabolism. In addition to the studies already discussed in this paper, the complexity of the medium supplementation was reported to interfere with specific metabolic pathways, such as control of glycogen metabolism in yeast 12 , secretion of enzymatic activities 10,20,27,31,32,43,45 , primary 1 and secondary metabolites 9 . The effect induced by specific kinds of peptides on metabolic routes of nitrogen and carbon metabolism was also considered.…”

“…Earlier studies reported on the effect of media composition complexity, in particular in reference to the nitrogen source on various metabolic processes 10,12,20,27,31,32,43,45 . Supplementation of the medium with complex constituents always improved cellular metabolism.…”

In Saccharomyces cerevisiae an unbalanced level of tyrosine phosphorylation down-regulates the Ras/PKA pathway

“…The fast activation of the Ras-cAMP pathway triggered by glucose addition results in mobilization of storage carbohydrates (Francois, Villanueva, & Hers, 1998;Kraakman et al, 1999;Smith, Ward, & Garrett, 1998;Van der Plaat, 1974), such as glycogen, and increased heat-shock sensitivity, partly mediated by the rapid shutdown of genes containing STRE elements in their promoters (Moskivna, Schuller, Mauer, Mager, & Ruis, 1998;Ruis & Schuller, 1995). Stp1 expressing cells growing on synthetic media supplemented with a non-fermentable carbon source, accumulate higher levels of intracellular glycogen than their wild-type counterparts.…”

The mutant type 1 protein phosphatase encoded by glc7-1 from Saccharomyces cerevisiae fails to interact productively with the GAC1-encoded regulatory subunit.

“…Glycogen synthase is interconverted between a phosphorylated, inactive form and a dephosphorylated, active form (32,33,40,49) in wild-type cells but remains almost entirely in the phosphorylated form in the glc7-1 mutant (34,49). glc7-1 is a mutant in GLC7/DIS2S1 (12,27), which encodes the catalytic subunit of PP1 (48).…”