Interaction between glycogen and glycogen phosphorylase of Tetrahymena

The glycogen phosphorylase of Tetrahymena pyriformis complexes with glycogen as judged by its elution pattern from columns of Sepharose 6B. Complex formation does not occur with starch, amylose, or amylopectin, and neither do these polyglucans serve as primers for the enzyme. To study the association between the phosphorylase and glycogen particles in situ, Tetrahymena were grown under differing physiological conditions, phosphorylase was isolated and chromatographed on a Sepharose 6B column. Phosphorylase activity isolated from cells grown in the absence of glucose was only partially associated with glycogen, while in cells exposed to glucose for 30 min or more all the phosphorylase activity was associated with glycogen. The effects of culture age and anaerobiosis on the relative amounts of free and glycogen-bound enzyme in the cells were also studied. It was concluded from the in vivo experiments that there was no simple relation between the fraction of enzyme bound to glycogen and between cell glycogen content.     

Physiological hyperinsulinemia impairs insulin-stimulated glycogen synthase activity and glycogen synthesis

Although chronic hyperinsulinemia has been shown to induce insulin resistance, the basic cellular mechanisms responsible for this phenomenon are unknown. The present study was performed 1) to determine the time-related effect of physiological hyperinsulinemia on glycogen synthase (GS) activity, hexokinase II (HKII) activity and mRNA content, and GLUT-4 protein in muscle from healthy subjects, and 2) to relate hyperinsulinemia-induced alterations in these parameters to changes in glucose metabolism in vivo. Twenty healthy subjects had a 240-min euglycemic insulin clamp study with muscle biopsies and then received a low-dose insulin infusion for 24 (n = 6) or 72 h (n = 14) (plasma insulin concentration = 121 +/- 9 or 143 +/- 25 pmol/l, respectively). During the baseline insulin clamp, GS fractional velocity (0.075 +/- 0.008 to 0.229 +/- 0.02, P < 0.01), HKII mRNA content (0.179 +/- 0.034 to 0.354 +/- 0.087, P < 0.05), and HKII activity (2.41 +/- 0.63 to 3.35 +/- 0.54 pmol x min(-1) x ng(-1), P < 0.05), as well as whole body glucose disposal and nonoxidative glucose disposal, increased. During the insulin clamp performed after 24 and 72 h of sustained physiological hyperinsulinemia, the ability of insulin to increase muscle GS fractional velocity, total body glucose disposal, and nonoxidative glucose disposal was impaired (all P < 0.01), whereas the effect of insulin on muscle HKII mRNA, HKII activity, GLUT-4 protein content, and whole body rates of glucose oxidation and glycolysis remained unchanged. Muscle glycogen concentration did not change [116 +/- 28 vs. 126 +/- 29 micromol/kg muscle, P = nonsignificant (NS)] and was not correlated with the change in nonoxidative glucose disposal (r = 0.074, P = NS). In summary, modest chronic hyperinsulinemia may contribute directly (independent of change in muscle glycogen concentration) to the development of insulin resistance by its impact on the GS pathway.     

Affinity glycogen synthetase to crosslinked glycogen.

The affinity of yeast glycogen synthetase to glycogen modified by crosslinking has been studied under various experimental conditions. It was found that the higher the degree of crosslinking, the lower the affinity of glycogen synthetase to glycogen. The amount of glycogen synthetase adsorbed from the solution depends on the amount of crosslinked glycogen added and is inversely proportional to the concentration of the soluble glycogen. The stability of the complex formed between yeast glycogen synthetase and the crosslinked glycogen was found to be maximal at neutral pH range. The presence of glucose 6-phosphate, uridine 5′-di-phosphate, and uridine 5′-diphosphate glucose enhanced the stability of the complex.     

Regulation of hepatic glycogen phosphorylase and glycogen synthase by calcium and diacylglycerol

Incubation of rat hepatocytes with angiotensin II (1 nM) produced a time-dependent accumulation of 1, 2-diacylglycerol and inactivation of glycogen synthase with maximum effects at 10 min. The level of diacylglycerol then gradually declined and the activity of glycogen synthase I returned to control values at 30 min. In contrast, angiotensin II caused an increase in cytosolic Ca2+ and an activation of glycogen phosphorylase which were rapid and transient, reaching maximum values in less than 2 min and then returning to control levels at 15 min. There were excellent correlations between the changes in glycogen synthase I and diacylglycerol levels and between the changes in phosphorylase alpha and cytosolic Ca2+ in these time-course studies. However, there was no correlation between the changes in diacylglycerol and phosphorylase alpha or between the changes in cytosolic Ca2+ and glycogen synthase I. Norepinephrine also caused a slow increase in diacylglycerol and inactivation of glycogen synthase, and a rapid increase in cytosolic free Ca2+ and activation of glycogen phosphorylase. Addition of an alpha1-adrenergic blocker (prazosin or phentolamine) caused rapid decreases in cytosolic free Ca2+ and phosphorylase alpha, but only slowly reversed the inactivation of synthase and accumulation of diacylglycerol. The dose-response curves for norepinephrine and prazosin on glycogen synthase were well correlated with those on diacylglycerol. It is proposed that in liver cells, Ca2+-mobilizing hormones regulate phosphorylase a through a Ca2+-dependent mechanism and inactivate glycogen synthase through the generation of diacylglycerol, at least in part. The data provide additional support for the view that protein kinase C may be important in the regulation of glycogen synthase in liver.     

Kinetics of the reaction between muscle glycogen phosphorylase B and glycogen

Kinetics of glycogen binding by glycogen phosphorylase b has been studied by stopped flow and temperature jump methods. This reaction is followed by increase in light scattering whose amplitude depends upon the enzyme binding sites concentration of glycogen particles occupied by the enzyme. It has been shown that the complex formation has the first order with respect to enzyme and glycogen concentrations. Relaxation kinetics is compatible with proposed bimolecular reaction scheme. Microscopic rate constants of the forward and reverse reactions of glycogen binding by glycogen phosphorylase b are determined in temperature range from 12,7 to 30 degrees C. The possibility of diffusional control of the binding rate is discussed.     

Schistosoma haematobium: histochemistry of glycogen, glycogen phosphorylase a and glycogen branching enzyme in niridazole-treated females.

The body posterior to the ovary of Schistosoma haematobium females was investigated. Glycogen, glycogen phosphorylase a (EC 2.4.1.1) and glycogen branching enzyme (EC 2.4.1.18) activities were detected in the subtegumental muscle system, parenchyma and mature vitelline cells, whereas no activities were detected in the tegument and immature vitelline cells of the parasite. Administration of a single niridazole dose of 250 mg kg-1 to the pouched mouse (Saccostomus camestris) produced the following changes in S. haematobium females: a relatively rapid depletion of glycogen stores due to disruption of the absorptive surface of the parasite, and to an increase in the activity of glycogen phosphorylase a; a reduction in the phosphorylase a to phosphorylase b-conversion capacity of glycogen phosphorylase phosphatase (EC 3.1.3.17); a decrease in glycogen branching enzyme activity; and a relatively rapid degeneration of parasite cells possibly due to their loss of endogenous energy reserves.     

Determination of glycogen and enzymes of glycogen metabolism in human hair follicles

The skin epithelium and its organelles use glycogen as well as glucose as source of energy. Therefore the characterisation of glycogen metabolism and the enzymes involved is important in the study of mechanisms regulating the normal or abnormal differentiation of skin organelles such as sebaceous glands and hair follicles.The present paper describes fluorimetric methods for the determination of glycogen and for the measurements of phosphorylase and phosphorylase kinase activity in one and the same lysate of minute tissue samples. The methods were tested for their suitability on freshly isolated human hair follicles and cultured hair follicle cells. The possible use of these techniques for studies on the pathophysiology of acne and hirsutism is discussed.     

Binding of glycogen, oligosaccharides, and glucose to glycogen debranching enzyme

The binding of glucose and a series of oligosaccharides to glycogen debranching enzyme was determined by the ability of the saccharides to decrease the rate of reaction of sulfhydryl groups with 5,5'-dithiobis(2-nitrobenzoate) (DTNB). At pH 7.2, the strength of binding increases with chain length from glucose to maltotriose to maltopentaose but not to maltohexaose, and the free energies for binding of the oligosaccharides suggest subsites of equivalent affinities for the four glucose units following the initial reducing moiety. The rate of reaction of DTNB with enzyme saturated with saccharide is the same for all compounds, suggesting that all the saccharides, including glucose, induce the same conformational state. The site of binding may be that which binds the alpha-1,6-linked side chain of the natural limit dextrin substrate. At pH 8.0, this site exhibits similar characteristics, but an additional site, which may bind the four terminal glucose units of the main chain of the natural substrate, is manifested and exhibits different characteristics, including a very low affinity for glucose itself. The binding of glycogen to the debranching enzyme was monitored by centrifugal separation from the protein and exhibits a much lower dissociation constant than that for the oligomers, suggesting that branched polymers have more than one set of subsites.     

The interaction of muscle glycogen phosphorylase b with glycogen

Interaction of muscle glycogen phosphorylase b (EC 2.4.1.1) with glycogen was studied by sedimentation, stopped-flow and temperature-jump methods. The equilibrium enzyme concentration was determined by sedimentation in an analytical ultracentrifuge equipped with absorption optics and a photoelectric scanning system. The maximum adsorption capacity of pig liver glycogen is 3.64 mumol dimeric glycogen phosphorylase b per g glycogen, which corresponds to 20 dimeric enzyme molecules per average glycogen molecule of Mr 5.5 X 10(6). Microscopic dissociation constants were determined for the enzyme-glycogen complex within the temperature range from 12.7 to 30.0 degrees C. Enzyme-glycogen complexing is accompanied by increasing light scattering and its increment depends linearly on the concentration of the binding sites on a glycogen particle that are occupied by the enzyme. Complex formation and relaxation kinetics are in accordance with the proposed bimolecular reaction scheme. The monomolecular dissociation rate constant of the complex increases as the temperature increases from 12.7 to 30.0 degrees C, whereas the bimolecular rate constant changes slightly and is about 10(8) M-1 X S-1. These data point to the possibility of diffusional control of the complex formation.     

Glucose transport rate and glycogen synthase activity both limit skeletal muscle glycogen accumulation

We varied rates of glucose transport and glycogen synthase I (GS-I) activity (%GS-I) in isolated rat epitrochlearis muscle to examine the role of each process in determining the rate of glycogen accumulation. %GS-I was maintained at or above the fasting basal range during 3 h of incubation with 36 mM glucose and 60 microU/ml insulin. Lithium (2 mM LiCl) added to insulin increased glucose transport rate and muscle glycogen content compared with insulin alone. The glycogen synthase kinase-3beta inhibitor GF-109203 x (GF; 10 microM) maintained %GS-I about twofold higher than insulin with or without lithium but did not increase glycogen accumulation. When %GS-I was lowered below the fasting range by prolonged incubation with 36 mM glucose and 2 mU/ml insulin, raising rates of glucose transport with bpV(phen) or of %GS-I with GF produced additive increases in glycogen concentration. Phosphorylase activity was unaffected by GF or bpV(phen). In muscles of fed animals, %GS-I was approximately 30% lower than in those of fasted rats, and insulin-stimulated glycogen accumulation did not occur unless %GS-I was raised with GF. We conclude that the rate of glucose transport is rate limiting for glycogen accumulation unless %GS-I is below the fasting range, in which case both glucose transport rate and GS activity can limit glycogen accumulation.     

Crystal structure of glycogen synthase: homologous enzymes catalyze glycogen synthesis and degradation

Glycogen and starch are the major readily accessible energy storage compounds in nearly all living organisms. Glycogen is a very large branched glucose homopolymer containing about 90% alpha-1,4-glucosidic linkages and 10% alpha-1,6 linkages. Its synthesis and degradation constitute central pathways in the metabolism of living cells regulating a global carbon/energy buffer compartment. Glycogen biosynthesis involves the action of several enzymes among which glycogen synthase catalyzes the synthesis of the alpha-1,4-glucose backbone. We now report the first crystal structure of glycogen synthase in the presence and absence of adenosine diphosphate. The overall fold and the active site architecture of the protein are remarkably similar to those of glycogen phosphorylase, indicating a common catalytic mechanism and comparable substrate-binding properties. In contrast to glycogen phosphorylase, glycogen synthase has a much wider catalytic cleft, which is predicted to undergo an important interdomain 'closure' movement during the catalytic cycle. The structures also provide useful hints to shed light on the allosteric regulation mechanisms of yeast/mammalian glycogen synthases.