The control of glycogen metabolism in yeast

Abstract

Two interconvertible forms of glycogen synthase and glycogen phosphorylase, one active (a) or the other less active (b), were predominantly present in a thermosensitive adenylate-cyclase-deficient mutant that had been preincubated at the restrictive temperature of 35.degree. C, either in the presence or in the absence of glucose. Glycogen phosphorylase was at least 20-fold less active active after incubation of the cells in the presence of glucose, but this residual activity had kinetic properties identical to those of the active form of enzyme, obtained after incubation in the absence of glucose; this suggests that the b form might be completely inactive and that the low activity measured after glucose treatment must be attributed to a residual amount of phosphorylase a. By contrast, the kinetic properties of the two forms of glycogen synthase were very different. When measured in the absence of glucose 6-phosphate, the two forms of enzyme had a similar affinity for UDP-Glc but different essentially by their Vmax. Glucose 6-phosphate had no effect on synthase a, but increased both Vmax and Km of synthase b; these effects, however, were in great part counteracted by sulfate and by inorganic phosphate, the latter also having the property of increasing the Km of the a form, without affecting Vmax. It was estimated that at physiological concentrations of substrates and effectors, synthase a was about 20-fold more active than synthase b. When an extract of cells that had been preincubated in the absence of glucose was gel-filtered and then incubated at 30.degree. C, phosphorylase was progressively fully inactivated and synthase was partially activated; these reactions were severalfold faster and, in the case of glycogen synthase, more complete in the presence of 10 mM glucose 6-phosphate. When a gel-filtered extract of cells that had been preincubated in the presence of glucose was incubated at 30.degree. C in the presence of ATP-Mg and EGTA, phosphorylase became activated and synthase was inactivated; the first of these two reactions was severalfold stimulated by micromolar concentrations of Ca2+, whereas both reactions were completely inhibited by 10 mM glucose 6-phosphate and only slightly and irregularly stimulated by cyclic AMP.