Control properties of yeast glycogen phosphorylase

Glycogen synthase sensitivity to glucose-6-P is important for controlling glycogen accumulation in Saccharomyces cerevisiae

Glycogen is a storage form of glucose utilized as an energy reserve by many organisms. Glycogen synthase, which is essential for synthesizing this glucose polymer, is regulated by both covalent phosphorylation and the concentration of glucose-6-P. With the yeast glycogen synthase Gsy2p, we recently identified two mutants, R579A/R580A/R582A [corrected] and R586A/R588A/R591A, in which multiple arginine residues were mutated to alanine that were completely insensitive to activation by glucose-6-P in vitro (Pederson, B. A., Cheng, C., Wilson, W. A., and Roach, P. J. (2000) J. Biol. Chem. 275, 27753-27761). We report here the expression of these mutants in Saccharomyces cerevisiae and, as expected from our findings in vitro, they were not activated by glucose-6-P. The R579A/R580A/R582A [corrected] mutant, which is also resistant to inhibition by phosphorylation, caused hyperaccumulation of glycogen. In contrast, the mutant R586A/R588A/R591A, which retains the ability to be inactivated by phosphorylation, resulted in lower glycogen accumulation when compared with wild-type cells. When intracellular glucose-6-P levels were increased by mutating the PFK2 gene, glycogen storage due to the wild-type enzyme was increased, whereas that associated with R579A/R580A/R582A [corrected] was not greatly changed. This is the first direct demonstration that activation of glycogen synthase by glucose-6-P in vivo is necessary for normal glycogen accumulation.

Mechanism of regulation in yeast glycogen phosphorylase

The mechanism of yeast glycogen phosphorylase activation by covalent phosphorylation involves structural elements distinct from the mammalian homologs. To understand the role of the amino-terminal 39-residue extension in the phosphorylation control mechanism, mutants with 22 and 42 amino-terminal residues removed were expressed in Escherichia coli, and their properties were compared with the wild-type (WT) enzyme. The unphosphorylated WT enzyme had a specific activity of 0.1 unit/mg and was not activated significantly by the substrate, glucose 1-phosphate. Phosphorylation by protein kinase resulted in a 1300-fold activation. Glucose 6-phosphate inhibited the unphosphorylated enzyme more effectively than the phosphorylated form, and inhibition of the latter was cooperative. Glucose was a poor inhibitor for both the unphosphorylated and phosphorylated WT enzyme with Ki > 300 mM. The rate of phosphorylation by protein kinase depended on substrates and interactions of the amino terminus. Maltoheptaose increased the rate of phosphorylation of the WT enzyme by yeast phosphorylase kinase 5-fold. The 22-residue deletion mutant (Nd22) had overall kinetic properties similar to the WT enzyme, except that Nd22 was a better substrate for the protein kinase and the rate of phosphorylation was unaffected by maltoheptaose. The 42-residue deletion mutant (Nd42), which lacks the phosphorylation site, was measurably active, although much less active than phosphorylated WT. Sedimentation equilibrium analysis indicated that the WT, Nd22, and Nd42 exist as tetramer, partially dissociated tetramer, and dimer, respectively. Phosphorylation of the WT and Nd22 converted both to dimer. The results indicated that the amino terminus affects quaternary structure and mediates activity regulation through conformational transition.

Glycogen metabolism in a Saccharomyces cerevisiae phosphoglucose isomerase (pgil) disruption mutant

Disruption of the gene pgil of Saccharomyces cerevisiae, which codes for phosphoglucose isomerase, results in a dramatic increase in the amount of intracellular glycogen in early exponential cultures. The level of glucose 6-phosphate was much higher in mutant than in wild-type cells. Phosphorylase a activity and the state of activation of glycogen synthase were also investigated. Phosphorylase a activity was rather low along the culture in wild-type cells, whereas it was consistently higher in mutants. Glycogen synthase was mostly in the active form in early-medium exponential cultures in wild-type cells whereas the activation state of this enzyme in mutant cells, although lower at the earlier steps of the culture, did not differ from wild-type cells at later stages. The fact that the intracellular levels of UDP-glucose are markedly increased in mutant cells suggest that the observed accumulation of glycogen results from a rise in substrate availability rather than from the activation of the enzyme responsible for the synthesis of the polysaccharide.

The family of glycogen phosphorylases: structure and function

Glycogen phosphorylase plays a central role in the mobilization of carbohydrate reserves in a wide variety of organisms and tissues. While rabbit muscle phosphorylase remains the most studied and best characterized of phosphorylases, recombinant DNA techniques have led to the recent appearance of primary sequence data for a wide variety of phosphorylase enzymes. The functional properties of rabbit muscle phosphorylases are reviewed and then compared to properties of phosphorylases from other tissues and organisms. Tissue expression patterns and the chromosomal localization of mammalian phosphorylases are described. Differences in functional properties among phosphorylases are related to new structural information. Evolutionary relationships among phosphorylases as afforded by comparative analysis of proteins and gene sequences are discussed.

The control of glycogen metabolism in yeast. 2. A kinetic study of the two forms of glycogen synthase and of glycogen phosphorylase and an investigation of their interconversion in a cell-free extract

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 degrees C, either in the presence or in the absence of glucose. Glycogen phosphorylase was at least 20-fold less 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 differed 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 degrees 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 degrees 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.

Convergent and divergent evolution of regulatory sites in eukaryotic phosphorylases

The activity of many proteins in eukaryotic cells is regulated by reversible covalent phosphorylation. This regulatory modification is often linked to other allosteric controls within the same protein, and such overlapping regulatory mechanisms are best characterized for glycogen phosphorylase (EC 2.4.1.1). Phosphorylases from different organisms or cell types exhibit markedly contrasting regulatory features; this makes the enzyme attractive for studying the evolution of interacting molecular regulatory mechanisms. Extensive biochemical and crystallographic studies of rabbit muscle phosphorylase have led to a characterization of five regulatory regions (phosphorylation, glycogen storage, AMP, glucose and purine sites). Here we report the complete primary structure of the yeast Saccharomyces cerevisiae glycogen phosphorylase, deduced from the sequence of the cloned gene. Regions that are highly conserved between muscle and yeast enzymes include the active site, the glycogen storage site and possibly the glucose and purine inhibition sites. Partial conservation of the residues involved in AMP-binding suggests a binding site for the yeast enzyme inhibitor, glucose 6-phosphate. Other parts of the AMP site and the intersubunit contacts involved in AMP allostery are disrupted in the yeast enzyme by extreme sequence divergence. The poor alignment of amino termini and lack of homology at phosphorylation sites indicate that regulation by reversible phosphorylation evolved independently in yeast and vertebrate phosphorylases.

Effect of glucose on alpha-glucan degradation in Polyporus circinatus

The effect of glucose on the enzymes involved in the degradation of a reserve alpha-glucan in Polyporus circinatus was studied. The levels of phosphorylase activity, endoamylase, amylo-1,6-glucosidase were regulated by glucose concentration.

Glycogen phosphorylase from Neurospora crassa: purification of a high-specific-activity, non-phosphorylated form

A highly active glycogen phosphorylase was purified from Neurospora crassa by polyethylene glycol fractionation at pH 6.16 combined with standard techniques (chromatography and salt fractionation). The final preparation had a specific activity of 65 +/- 5 U/mg of protein (synthetic direction, pH 6.1, 30 degrees C) and was homogeneous by the criteria of gel electrophoresis, amino-terminal analysis, gel filtration, and double immunodiffusion in two dimensions. The enzyme had a native molecular weight of 180,000 +/- 10,000 (by calibrated gel filtration and gel electrophoresis) and a subunit molecular weight of 90,000 +/- 5,000 (by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Each subunit contained one molecule of pyridoxal phosphate. No phosphoserine or phosphothreonine was detected by amino acid analysis optimized for phosphoamino acid detection. The enzyme isolated from cells grown on high-specific-activity 32Pi (as sole source of phosphorus) contained one atom of 32P per subunit. All the radioactivity was removed by procedures that removed pyridoxal phosphate. Thus, the enzyme could not be classified as an a type (phosphorylated, active in the absence of a cofactor) or as a b type (non-phosphorylated, inactive in the absence of a cofactor). The level of phosphorylase was markedly increased in mycelium taken from older cultures in which the carbon source (glucose or sucrose) had been depleted. The polyethylene glycol fractionation scheme applied at pH 7.5 to mycelial extracts of younger cultures (taken before depletion of the sugar) resulted in co-purification of glycogen phosphorylase and glycogen synthetase.

Regulation of locust fat-body phosphorylase

1. Glycogen phosphorylase of locust fat-body was partially purified by differential centrifugation and dissociation from glycogen particles at two pH values. 2. Optimum activity was obtained at pH6.6-6.7. 3. The calculated apparent K(m) values for glycogen and glucose 1-phosphate were 0.08% and 10-13mm respectively. 4. 5'-AMP activated in the range 5mum-1mm. 5. Glucose 6-phosphate is a competitive inhibitor for the substrate glucose 1-phosphate (K(i)=1.7mm). 5'-AMP abolishes this inhibition. Glucose weakly inhibits (K(i)=25-30mm), but trehalose does not inhibit even at 100mm. 6. It is suggested that glucose 6-phosphate is a major regulator of glycogen phosphorylase activity in locust fat-body.