Molecular basis for the regulation of human glycogen synthase by phosphorylation and glucose-6-phosphate.

Glycogen synthase (GYS1) is the central enzyme in muscle glycogen biosynthesis. GYS1 activity is inhibited by phosphorylation of its amino (N) and carboxyl (C) termini, which is relieved by allosteric activation of glucose-6-phosphate (Glc6P). We present cryo-EM structures at 3.0-4.0 Å resolution of phosphorylated human GYS1, in complex with a minimal interacting region of glycogenin, in the inhibited, activated and catalytically competent states. Phosphorylations of specific terminal residues are sensed by different arginine clusters, locking the GYS1 tetramer in an inhibited state via intersubunit interactions. The Glc6P activator promotes conformational change by disrupting these interactions and increases the flexibility of GYS1, such that it is poised to adopt a catalytically competent state when the sugar donor UDP-glucose (UDP-glc) binds. We also identify an inhibited-like conformation that has not transitioned into the activated state, in which the locking interaction of phosphorylation with the arginine cluster impedes subsequent conformational changes due to Glc6P binding. Our results address longstanding questions regarding the mechanism of human GYS1 regulation.

A novel GSK3 inhibitor that promotes self-renewal in mouse embryonic stem cells.

Small molecules that regulate cell stemness have the potential to make a major contribution to regenerative medicine. In the course of screening for small molecules that affect stemness in mouse embryonic stem cells (mESCs), we discovered that NPD13432, an aurone derivative, promoted self-renewal of mESCs. Normally, mESCs start to differentiate upon withdrawal of 2i/LIF. However, cells treated with the compound continued to express endogenous Nanog, a pluripotency marker protein essential for sustaining the undifferentiated state, even in the absence of 2i/LIF. Biochemical characterization revealed that NPD13432 inhibited GSK3α and GSK3ß with IC50 values of 92 nM and 310 nM, respectively, suggesting that the compound promotes self-renewal in mESCs by inhibiting GSK3. The chemical structure of the compound is unique among known molecules with this activity, providing an opportunity to develop new inhibitors of GSK3, as well as chemical tools for investigating cell stemness.

Discovery and Development of Small-Molecule Inhibitors of Glycogen Synthase.

The overaccumulation of glycogen appears as a hallmark in various glycogen storage diseases (GSDs), including Pompe, Cori, Andersen, and Lafora disease. Accumulating evidence suggests that suppression of glycogen accumulation represents a potential therapeutic approach for treating these GSDs. Using a fluorescence polarization assay designed to screen for inhibitors of the key glycogen synthetic enzyme, glycogen synthase (GS), we identified a substituted imidazole, (rac)-2-methoxy-4-(1-(2-(1-methylpyrrolidin-2-yl)ethyl)-4-phenyl-1H-imidazol-5-yl)phenol (H23), as a first-in-class inhibitor for yeast GS 2 (yGsy2p). Data from X-ray crystallography at 2.85 Å, as well as kinetic data, revealed that H23 bound within the uridine diphosphate glucose binding pocket of yGsy2p. The high conservation of residues between human and yeast GS in direct contact with H23 informed the development of around 500 H23 analogs. These analogs produced a structure-activity relationship profile that led to the identification of a substituted pyrazole, 4-(4-(4-hydroxyphenyl)-3-(trifluoromethyl)-1H-pyrazol-5-yl)pyrogallol, with a 300-fold improved potency against human GS. These substituted pyrazoles possess a promising scaffold for drug development efforts targeting GS activity in GSDs associated with excess glycogen accumulation.

Structure and Regulation of Glycogen Synthase in the Brain.

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

Identification of the substrate recruitment mechanism of the muscle glycogen protein phosphatase 1 holoenzyme.

Glycogen is the primary storage form of glucose. Glycogen synthesis and breakdown are tightly controlled by glycogen synthase (GYS) and phosphorylase, respectively. The enzyme responsible for dephosphorylating GYS and phosphorylase, which results in their activation (GYS) or inactivation (phosphorylase) to robustly stimulate glycogen synthesis, is protein phosphatase 1 (PP1). However, our understanding of how PP1 recruits these substrates is limited. Here, we show how PP1, together with its muscle glycogen-targeting (GM) regulatory subunit, recruits and selectively dephosphorylates its substrates. Our molecular data reveal that the GM carbohydrate binding module (GM CBM21), which is amino-terminal to the GM PP1 binding domain, has a dual function in directing PP1 substrate specificity: It either directly recruits substrates (i.e., GYS) or recruits them indirectly by localization (via glycogen for phosphorylase). Our data provide the molecular basis for PP1 regulation by GM and reveal how PP1-mediated dephosphorylation is driven by scaffolding-based substrate recruitment.

Identification of a novel starch synthase III from the picoalgae Ostreococcus tauri.

Hydrosoluble glycogen is the major energy storage compound in bacteria, archaea, fungi, and animal cells. In contrast, photosynthetic eukaryotes have evolved to build a highly organized semicrystalline granule of starch. Several enzymes are involved in polysaccharide synthesis, among which glycogen or starch synthase catalyze the elongation of the α-1,4-glucan chain. Ostreococcus tauri, accumulates a single starch granule and contains three starch synthase III (SSIII) isoforms, known as OsttaSSIII-A, OsttaSSIII-B and OsttaSSIII-C. After amino acids sequence analysis we found that OsttaSSIII-C lacks starch-binding domains, being 49% identical to the catalytic region of the SSIII from Arabidopsis thaliana and 32% identical to the entire Escherichia coli glycogen synthase. The recombinant, highly purified OsttaSSIII-C exhibited preference to use as a primer branched glycans (such as rabbit muscle glycogen and amylopectin), rather than amylose. Also, the enzyme displayed a high affinity toward ADP-glucose. We found a marked conservation of the amino acids located in the catalytic site, and specifically determined the role of residues R270, K275 and E352 by site-directed mutagenesis. Results show that these residues are important for OsttaSSIII-C activity, suggesting a strong similarity between the active site of the O. tauri SSIII-C isoform and other bacterial glycogen synthases.

A highly prevalent equine glycogen storage disease is explained by constitutive activation of a mutant glycogen synthase.

BACKGROUND: Equine type 1 polysaccharide storage myopathy (PSSM1) is associated with a missense mutation (R309H) in the glycogen synthase (GYS1) gene, enhanced glycogen synthase (GS) activity and excessive glycogen and amylopectate inclusions in muscle. METHODS: Equine muscle biochemical and recombinant enzyme kinetic assays in vitro and homology modelling in silico, were used to investigate the hypothesis that higher GS activity in affected horse muscle is caused by higher GS expression, dysregulation, or constitutive activation via a conformational change. RESULTS: PSSM1-affected horse muscle had significantly higher glycogen content than control horse muscle despite no difference in GS expression. GS activity was significantly higher in muscle from homozygous mutants than from heterozygote and control horses, in the absence and presence of the allosteric regulator, glucose 6 phosphate (G6P). Muscle from homozygous mutant horses also had significantly increased GS phosphorylation at sites 2+2a and significantly higher AMPKα1 (an upstream kinase) expression than controls, likely reflecting a physiological attempt to reduce GS enzyme activity. Recombinant mutant GS was highly active with a considerably lower Km for UDP-glucose, in the presence and absence of G6P, when compared to wild type GS, and despite its phosphorylation. CONCLUSIONS: Elevated activity of the mutant enzyme is associated with ineffective regulation via phosphorylation rendering it constitutively active. Modelling suggested that the mutation disrupts a salt bridge that normally stabilises the basal state, shifting the equilibrium to the enzyme's active state. GENERAL SIGNIFICANCE: This study explains the gain of function pathogenesis in this highly prevalent polyglucosan myopathy.

Redox Switch for the Inhibited State of Yeast Glycogen Synthase Mimics Regulation by Phosphorylation.

Glycogen synthase (GS) is the rate limiting enzyme in the synthesis of glycogen. Eukaryotic GS is negatively regulated by covalent phosphorylation and allosterically activated by glucose-6-phosphate (G-6-P). To gain structural insights into the inhibited state of the enzyme, we solved the crystal structure of yGsy2-R589A/R592A to a resolution of 3.3 Å. The double mutant has an activity ratio similar to the phosphorylated enzyme and also retains the ability to be activated by G-6-P. When compared to the 2.88 Å structure of the wild-type G-6-P activated enzyme, the crystal structure of the low-activity mutant showed that the N-terminal domain of the inhibited state is tightly held against the dimer-related interface thereby hindering acceptor access to the catalytic cleft. On the basis of these two structural observations, we developed a reversible redox regulatory feature in yeast GS by substituting cysteine residues for two highly conserved arginine residues. When oxidized, the cysteine mutant enzyme exhibits activity levels similar to the phosphorylated enzyme but cannot be activated by G-6-P. Upon reduction, the cysteine mutant enzyme regains normal activity levels and regulatory response to G-6-P activation.

Incorporation of phosphate into glycogen by glycogen synthase.

The storage polymer glycogen normally contains small amounts of covalently attached phosphate as phosphomonoesters at C2, C3 and C6 atoms of glucose residues. In the absence of the laforin phosphatase, as in the rare childhood epilepsy Lafora disease, the phosphorylation level is elevated and is associated with abnormal glycogen structure that contributes to the pathology. Laforin therefore likely functions in vivo as a glycogen phosphatase. The mechanism of glycogen phosphorylation is less well-understood. We have reported that glycogen synthase incorporates phosphate into glycogen via a rare side reaction in which glucose-phosphate rather than glucose is transferred to a growing polyglucose chain (Tagliabracci et al. (2011) Cell Metab13, 274-282). We proposed a mechanism to account for phosphorylation at C2 and possibly at C3. Our results have since been challenged (Nitschke et al. (2013) Cell Metab17, 756-767). Here we extend the evidence supporting our conclusion, validating the assay used for the detection of glycogen phosphorylation, measurement of the transfer of (32)P from [ß-(32)P]UDP-glucose to glycogen by glycogen synthase. The (32)P associated with the glycogen fraction was stable to ethanol precipitation, SDS-PAGE and gel filtration on Sephadex G50. The (32)P-signal was not affected by inclusion of excess unlabeled UDP before analysis or by treatment with a UDPase, arguing against the signal being due to contaminating [ß-(32)P]UDP generated in the reaction. Furthermore, [(32)P]UDP did not bind non-covalently to glycogen. The (32)P associated with glycogen was released by laforin treatment, suggesting that it was present as a phosphomonoester. The conclusion is that glycogen synthase can mediate the introduction of phosphate into glycogen, thereby providing a possible mechanism for C2, and perhaps C3, phosphorylation.

Upregulation of miR-497 induces hepatic insulin resistance in E3 rats with HFD-MetS by targeting insulin receptor.

OBJECTIVE: The study aims to find regulatory microRNA(s) responsible for down-regulated insulin receptor (InsR) in the liver of HFD-MetS E3 rats with insulin resistance. METHODS: Firstly, hepatic insulin resistance in HFD-MetS E3 rats was evaluated by RT-qPCR, western blotting, immunohistochemistry and PAS staining. Secondly, the candidate miRNAs targeting rat InsR were predicted through online softwares and detected in the liver of HFD-MetS E3 rats with insulin resistance. Then, the expression of InsR, phosphorylated IRS-1 (pIRS-1) at Tyr632, phosphorylated AKTs (pAKTs) at Ser473 and Thr308, phosphorylated GSK-3ß (p GSK-3ß) at Ser9, phosphorylated GS (pGS) at Ser641 and the glycogen content were detected in CBRH-7919 cells treated with 100 nM insulin for different time periods by western blotting or PAS staining respectively, after transient transfection with miR-497 mimics or inhibitors for 24 h. Lastly, the relation between miR-497 and InsR was further determined using dual luciferase reporter assay. RESULTS: Elevated miR-497 was negatively related with down-regulated InsR in the liver of HFD-MetS E3 rats with insulin resistance. Comparing with the mNC group, glycogen content and the expression of InsR, pIRS-1 (Tyr632), pAKTs (Ser473 and Thr308) and pGSK-3ß (Ser9) decreased significantly in CBRH-7919 cells, while pGS (Ser641) increased significantly, after transient transfection with miR-497 mimics for 24 h and treatment with 100 nM insulin for corresponding time periods, counter to those results in CBRH-7919 cells after similar procedures with miR-497 inhibitors and insulin. In addition, dual luciferase reporter assay further confirmed that miR-497 can bind to the 3'UTR of rat InsR. CONCLUSION: Insulin receptor is the target gene of miR-497, and elevated miR-497 might induce hepatic insulin resistance in HFD-MetS E3 Rats through inhibiting the expression of insulin receptor and confining the activation of IRS-1/PI3K/Akt/GSK-3ß/GS pathway to insulin.

Selective photoregulation of the activity of glycogen synthase and glycogen phosphorylase, two key enzymes in glycogen metabolism.

Glycogen is a polymer of α-1,4- and α-1,6-linked glucose units that provides a readily available source of energy in living organisms. Glycogen synthase (GS) and glycogen phosphorylase (GP) are the two enzymes that control, respectively, the synthesis and degradation of this polysaccharide and constitute adequate pharmacological targets to modulate cellular glycogen levels, by means of inhibition of their catalytic activity. Here we report on the synthesis and biological evaluation of a selective inhibitor that consists of an azobenzene moiety glycosidically linked to the anomeric carbon of a glucose molecule. In the ground state, the more stable (E)-isomer of the azobenzene glucoside had a slight inhibitory effect on rat muscle GP (RMGP, IC50 = 4.9 mM) and Escherichia coli GS (EcGS, IC50 = 1.6 mM). After irradiation and subsequent conversion to the (Z)-form, the inhibitory potency of the azobenzene glucoside did not significantly change for RMGP (IC50 = 2.4 mM), while its effect on EcGS increased 50-fold (IC50 = 32 µM). Sucrose synthase 4 from potatoes, a glycosyltransferase that does not operate on glycogen, was only slightly inhibited by the (E)-isomer (IC50 = 0.73 mM). These findings could be rationalized on the basis of kinetic and computer-aided docking analysis, which indicated that both isomers of the azobenzene glucoside mimic the EcGS acceptor substrate and exert their inhibitory effect by binding to the glycogen subsite in the active center of the enzyme. The ability to selectively photoregulate the catalytic activity of key enzymes of glycogen metabolism may represent a new approach for the treatment of glycogen metabolism disorders.

Advances in understanding glycosyltransferases from a structural perspective.

Glycosyltransferases (GTs), the enzymes that catalyse glycosidic bond formation, create a diverse range of saccharides and glycoconjugates in nature. Understanding GTs at the molecular level, through structural and kinetic studies, is important for gaining insights into their function. In addition, this understanding can help identify those enzymes which are involved in diseases, or that could be engineered to synthesize biologically or medically relevant molecules. This review describes how structural data, obtained in the last 3-4 years, have contributed to our understanding of the mechanisms of action and specificity of GTs. Particular highlights include the structure of a bacterial oligosaccharyltransferase, which provides insights into N-linked glycosylation, the structure of the human O-GlcNAc transferase, and the structure of a bacterial integral membrane protein complex that catalyses the synthesis of cellulose, the most abundant organic molecule in the biosphere.

Structural basis for the recruitment of glycogen synthase by glycogenin.

Glycogen is a primary form of energy storage in eukaryotes that is essential for glucose homeostasis. The glycogen polymer is synthesized from glucose through the cooperative action of glycogen synthase (GS), glycogenin (GN), and glycogen branching enzyme and forms particles that range in size from 10 to 290 nm. GS is regulated by allosteric activation upon glucose-6-phosphate binding and inactivation by phosphorylation on its N- and C-terminal regulatory tails. GS alone is incapable of starting synthesis of a glycogen particle de novo, but instead it extends preexisting chains initiated by glycogenin. The molecular determinants by which GS recognizes self-glucosylated GN, the first step in glycogenesis, are unknown. We describe the crystal structure of Caenorhabditis elegans GS in complex with a minimal GS targeting sequence in GN and show that a 34-residue region of GN binds to a conserved surface on GS that is distinct from previously characterized allosteric and binding surfaces on the enzyme. The interaction identified in the GS-GN costructure is required for GS-GN interaction and for glycogen synthesis in a cell-free system and in intact cells. The interaction of full-length GS-GN proteins is enhanced by an avidity effect imparted by a dimeric state of GN and a tetrameric state of GS. Finally, the structure of the N- and C-terminal regulatory tails of GS provide a basis for understanding phosphoregulation of glycogen synthesis. These results uncover a central molecular mechanism that governs glycogen metabolism.

Major change in regiospecificity for the exo-1,3-ß-glucanase from Candida albicans following its conversion to a glycosynthase.

The exo-1,3-ß-glucanase (Exg) from Candida albicans is involved in cell wall ß-d-glucan metabolism and morphogenesis through its hydrolase and transglycosidase activities. Previous work has shown that both these activities strongly favor ß-1,3-linkages. The E292S Exg variant displayed modest glycosynthase activity using α-d-glucopyranosyl fluoride (α-GlcF) as the donor and pNP-ß-d-glucopyranoside (pNPGlc) as the acceptor but surprisingly showed a marked preference for synthesizing ß-1,6-linked over ß-1,3- and ß-1,4-linked disaccharide products. With pNPXyl as the acceptor, the preference became ß-1,4 over ß-1,3. The crystal structure of the glycosynthase bound to both of its substrates, α-GlcF and pNPGlc, is the first such ternary complex structure to be determined. The results revealed that the donor bound in the -1 subsite, as expected, while the acceptor was oriented in the +1 subsite to facilitate ß-1,6-linkage, thereby supporting the results from solution studies. A second crystal structure containing the major product of glycosynthesis, pNP-gentiobiose, showed that the -1 subsite allows another docking position for the terminal sugar; i.e., one position is set up for catalysis, whereas the other is an intermediate stage prior to the displacement of water from the active site by the incoming sugar hydroxyls. The +1 subsite, an aromatic "clamp", permits several different sugar positions and orientations, including a 180° flip that explains the observed variable regiospecificity. The p-nitrophenyl group on the acceptor most likely influences the unexpectedly observed ß-1,6-specificity through its interaction with F229. These results demonstrate that tailoring the specificity of a particular glycosynthase depends not only on the chemical structure of the acceptor but also on understanding the structural basis of the promiscuity of the native enzyme.

Screening glycosynthase libraries with a fluoride chemosensor assay independently of enzyme specificity: identification of a transitional hydrolase to synthase mutant.

Glycosynthases have become efficient tools for the enzymatic synthesis of oligosaccharides, glycoconjugates and polysaccharides. Enzyme-directed evolution approaches are applied to improve the performance of current glycosynthases and engineer specificity for non-natural substrates. However, simple and general screening methods are required since most of the reported assays are specific for each particular enzyme. In the present paper, we report a general screening assay that is independent of enzyme specificity, and implemented in an HTS (high-throughput screening) format for the screening of cell extracts in directed evolution experiments. Fluoride ion is a general by-product released in all glycosynthase reactions with glycosyl fluoride donors. The new assay is based on the use of a specific chemical sensor (a silyl ether of a fluorogenic methylumbelliferone) to transduce fluoride concentration into a fluorescence signal. As a proof-of-concept, it has been applied to a nucleophile saturation mutant library of Bacillus licheniformis 1,3-1,4-ß-glucanase. Beyond the expected mutations at the glutamic acid (catalytic) nucleophile, other variants have been shown to acquire glycosynthase activity. Surprisingly, an aspartic acid for glutamic acid replacement renders a highly active glycosynthase, but still retains low hydrolase activity. It appears as an intermediate state between glycosyl hydrolase and glycosynthase.

Structural basis for 2'-phosphate incorporation into glycogen by glycogen synthase.

Glycogen is a glucose polymer that contains minor amounts of covalently attached phosphate. Hyperphosphorylation is deleterious to glycogen structure and can lead to Lafora disease. Recently, it was demonstrated that glycogen synthase catalyzes glucose-phosphate transfer in addition to its characteristic glucose transfer reaction. Glucose-1,2-cyclic-phosphate (GCP) was proposed to be formed from UDP-Glc breakdown and subsequently transferred, thus providing a source of phosphate found in glycogen. To gain further insight into the molecular basis for glucose-phosphate transfer, two structures of yeast glycogen synthase were determined; a 3.0-Å resolution structure of the complex with UMP/GCP and a 2.8-Å resolution structure of the complex with UDP/glucose. Structural superposition of the complexes revealed that the bound ligands and most active site residues are positioned similarly, consistent with the use of a common transfer mechanism for both reactions. The N-terminal domain of the UDP-glucose complex was found to be 13.3° more closed compared with a UDP complex. However, the UMP · GCP complex was 4.8° less closed than the glucose complex, which may explain the low efficiency of GCP transfer. Modeling of either α- or ß-glucose or a mixture of both anomers can account for the observed electron density of the UDP-glucose complex. NMR studies of UDP-Glc hydrolysis by yeast glycogen synthase were used to verify the stereochemistry of the product, and they also showed synchronous GCP accumulation. The similarities in the active sites of glycogen synthase and glycogen phosphorylase support the idea of a common catalytic mechanism in GT-B enzymes independent of the specific reaction catalyzed.

Role of autophagy in glycogen breakdown and its relevance to chloroquine myopathy.

Several myopathies are associated with defects in autophagic and lysosomal degradation of glycogen, but it remains unclear how glycogen is targeted to the lysosome and what significance this process has for muscle cells. We have established a Drosophila melanogaster model to study glycogen autophagy in skeletal muscles, using chloroquine (CQ) to simulate a vacuolar myopathy that is completely dependent on the core autophagy genes. We show that autophagy is required for the most efficient degradation of glycogen in response to starvation. Furthermore, we show that CQ-induced myopathy can be improved by reduction of either autophagy or glycogen synthesis, the latter possibly due to a direct role of Glycogen Synthase in regulating autophagy through its interaction with Atg8.

Improving the glycosyltransferase activity of Agrobacterium tumefaciens glycogen synthase by fusion of N-terminal starch binding domains (SBDs).

Glycogen and starch, the major storage carbohydrate in most living organisms, result mainly from the action of starch or glycogen synthases (SS or GS, respectively, EC 2.4.1.21). SSIII from Arabidopsis thaliana is an SS isoform with a particular modular organization: the C-terminal highly conserved glycosyltransferase domain is preceded by a unique specific region (SSIII-SD) which contains three in tandem starch binding domains (SBDs, named D1, D2 and D3) characteristic of polysaccharide degrading enzymes. N-terminal SBDs have a probed regulatory role in SSIII activity, showing starch binding ability and modulating the catalytic properties of the enzyme. On the other hand, GS from Agrobacterium tumefaciens has a simple primary structure organization, characterized only by the highly conserved glycosyltransferase domain and lacking SBDs. To further investigate the functional role of A. thaliana SSIII-SD, three chimeric proteins were constructed combining the SBDs from A. thaliana with the GS from A. tumefaciens. Recombinant proteins were expressed in and purified to homogeneity from Escherichia coli cells in order to be kinetically characterized. Furthermore, we tested the ability to restore in vivo glycogen biosynthesis in transformed E. coli glgA(-) cells, deficient in GS. Results show that the D3-GS chimeric enzyme showed increased capacity of glycogen synthesis in vivo with minor changes in its kinetics parameters compared to GS.

Regulation of glycogen synthase from mammalian skeletal muscle--a unifying view of allosteric and covalent regulation.

It is widely accepted that insufficient insulin-stimulated activation of muscle glycogen synthesis is one of the major components of non-insulin-dependent (type 2) diabetes mellitus. Glycogen synthase, a key enzyme in muscle glycogen synthesis, is extensively regulated, both allosterically (by glucose-6-phosphate, ATP, and others) and covalently (by phosphorylation). Although glycogen synthase has been a topic of intense study for more than 50 years, its kinetic characterization has been confounded by its large number of phosphorylation states. Questions remain regarding the function of glycogen synthase regulation and the relative importance of allosteric and covalent modification in fulfilling this function. In this review, we consider both earlier kinetic studies and more recent site-directed mutagenesis and crystal structure studies in a detailed qualitative discussion of the effects of regulation on the kinetics of glycogen synthase. We propose that both allosteric and covalent modification of glycogen synthase may be described by a Monod-Wyman-Changeux model in terms of apparent changes to L, the equilibrium constant for transition between the T and R conformers. As, with the exception of L, all parameters of this model are independent of the glycogen synthase phosphorylation state, the need to determine kinetic parameters for all possible states is eliminated; only the relationship between a particular state and L must be established. We conclude by suggesting that renewed efforts to characterize the relationship between phosphorylation and the kinetics of glycogen synthase are essential in order to obtain a better quantitative understanding of the function of glycogen synthesis regulation. The model we propose may prove useful in this regard.

Synthesis of 1,2,3-triazoles from xylosyl and 5-thioxylosyl azides: evaluation of the xylose scaffold for the design of potential glycogen phosphorylase inhibitors.

Various acetylenic derivatives and acetylated ß-D-xylopyranosyl azide or the 5-thio-ß-d-xylopyranosyl analogue were coupled by Cu(I)-catalyzed azide alkyne 1,3-dipolar cycloaddition (CuAAC) to afford a series of 1-xylosyl-4-substituted 1,2,3-triazoles. Controlled oxidation of the endocyclic sulfur atom of the 5-thioxylose moiety led to the corresponding sulfoxides and sulfones. Deacetylation afforded 19 hydroxylated xylose and 5-thioxylose derivatives, found to be only sparingly water-soluble. Compared to glucose-based analogues, they appeared to be much weaker inhibitors of glycogen phosphorylase, as the absence of a hydroxymethyl group weakens their binding at the enzyme active site. However, such new xylose derivatives might be useful glycomimetics.