Crystallographic studies on acyl ureas, a new class of glycogen phosphorylase inhibitors, as potential antidiabetic drugs

Acyl ureas were discovered as a novel class of inhibitors for glycogen phosphorylase, a molecular target to control hyperglycemia in type 2 diabetics. This series is exemplified by 6-{2,6-Dichloro- 4-[3-(2-chloro-benzoyl)-ureido]-phenoxy}-hexanoic acid, which inhibits human liver glycogen phosphorylase a with an IC(50) of 2.0 microM. Here we analyze four crystal structures of acyl urea derivatives in complex with rabbit muscle glycogen phosphorylase b to elucidate the mechanism of inhibition of these inhibitors. The structures were determined and refined to 2.26 Angstroms resolution and demonstrate that the inhibitors bind at the allosteric activator site, where the physiological activator AMP binds. Acyl ureas induce conformational changes in the vicinity of the allosteric site. Our findings suggest that acyl ureas inhibit glycogen phosphorylase by direct inhibition of AMP binding and by indirect inhibition of substrate binding through stabilization of the T' state.     

Comparison of AMP and NADH binding to glycogen phosphorylase b

The binding sites for the allosteric activator, AMP, to glycogen phosphorylase b are described in detail utilizing the more precise knowledge of the native structure obtained from crystallographic restrained least-squares refinement than has hitherto been available. Localized conformational changes are seen at the allosteric effector site that include shifts of between 1 and 2 A for residues Tyr75 and Arg309 and very small shifts for the region of residues 42 to 44 from the symmetry-related subunit. Kinetic studies demonstrate that NADH inhibits the AMP activation of glycogen phosphorylase b. Crystallographic binding studies at 3.5 A resolution show that NADH binds to the same sites on the enzyme as AMP, i.e. the allosteric effector site N, which is close to the subunit-subunit interface, and the nucleoside inhibitor site I, which is some 12 A from the catalytic site. The conformations of NADH at the two sites are different but both conformations are "folded" so that the nicotinamide ring is close (approx. 6 A) to the adenine ring. These conformations are compared with those suggested from solution studies and with the extended conformations observed in the single crystal structure of NAD+ and for NAD bound to dehydrogenases. Possible mechanisms for NADH inhibition of phosphorylase activation are discussed.     

Inhibition of the interaction between protein phosphatase 1 glycogen-targeting subunit and glycogen phosphorylase increases glycogen synthesis in primary rat hepatocytes

In Type 2 diabetes, increased glycogenolysis contributes to the hyperglycaemic state, therefore the inhibition of GP (glycogen phosphorylase), a key glycogenolytic enzyme, is one of the possibilities to lower plasma glucose levels. Following this strategy, a number of GPis (GP inhibitors) have been described. However, certain critical issues are associated with their mode of action, e.g. an impairment of muscle function. The interaction between GP and the liver glycogen targeting subunit (termed G(L)) of PP1 (protein phosphatase 1) has emerged as a new potential anti-diabetic target, as the disruption of this interaction should increase glycogen synthesis, potentially providing an alternative approach to counteract the enhanced glycogenolysis without inhibiting GP activity. We identified an inhibitor of the G(L)-GP interaction (termed G(L)-GPi) and characterized its mechanism of action in comparison with direct GPis. In primary rat hepatocytes, at elevated glucose levels, the G(L)-GPi increased glycogen synthesis similarly to direct GPis. Direct GPis significantly reduced the cellular GP activity, caused a dephosphorylation of the enzyme and decreased the amounts of GP in the glycogen-enriched fraction; the G(L)-GPi did not influence any of these parameters. Both mechanisms increased glycogen accumulation at elevated glucose levels. However, at low glucose levels, only direct GPis led to increased glycogen amounts, whereas the G(L)-GPi allowed the mobilization of glycogen because it did not block the activity of GP. Due to this characteristic, G(L)-GPi in comparison with GPis could offer an advantageous risk/benefit profile circumventing the potential downsides of a complete prevention of glycogen breakdown while retaining glucose-lowering efficacy, suggesting that inhibition of the G(L)-GP interaction may provide an attractive novel approach for rebalancing the disturbed glycogen metabolism in diabetic patients.     

Activation of protein kinase and glycogen phosphorylase in isolated rat liver cells by glucagon and catecholamines.

In liver cells isolated from fed female rats, glucagon (290nM) increased adenosine 3':5'-monophosphate (cyclic AMP) content and decreased cyclic AMP binding 30 s after addition of hormones. Both returned to control values after 10 min. Glucagon also stimulated cyclic AMP-independent protein kinase activity at 30 s and decreased protein kinase activity assayed in the presence of 2 muM cyclic AMP at 1 min. Glucagon increased the levels of glycogen phosphorylase a, but there was no change in total glycogen phosphorylase activity. Glucagon increased glycogen phosphorylase a at concentrations considerably less than those required to affect cyclic AMP and protein kinase. The phosphodiesterase inhibitor, 1-methyl-3-isobutyl xanthine, potentiated the action of glucagon on all variables, but did not increase the maximuM activation of glycogen phosphorylase. Epinephrine (1muM) decreased cyclic AMP binding and increased glycogen phosphorylase a after a 1-min incubation with cells. Although 0.1 muM epinephrine stimulated phosphorylase a, a concentration of 10 muM was required to increase protein kinase activity. 1-Methyl-3-isobutyl xanthine (0.1 mM) potentiated the action of epinephrine on cyclic AMP and protein kinase. (-)-Propranolol (10muM) completely abolished the changes in cyclic AMP binding and protein kinase due to epinephrine (1muM) in the presence of 0.1mM 1-methyl-3-isobutyl xanthine, yet inhibited the increase in phosphorylase a by only 14 per cent. Phenylephrine (0.1muM) increased glycogen phosphorylase a, although concentrations as great as 10 muM failed to affect cyclic AMP binding or protein kinase in the absence of phosphodiesterase inhibitor. Isoproterenol (0.1muM) stimulated phosphorylase and decreased cyclic AMP binding, but only a concentration of 10muM increased protein kinase. 1-Methyl-3-isobutyl xanthine potentiated the action of isoproterenol on cyclic AMP binding and protein kinase, and propranolol reduced the augmentation of glucose release and glycogen phosphorylase activity due to isoproterenol. These data indicate that both alpha- and beta-adrenergic agents are capable of stimulating glycogenolysis and glycogen phosphorylase a in isolated rat liver cells. Low concentrations of glucagon and beta-adrenergic agonists stimulate glycogen phosphorylase without any detectable increase in cyclic AMP or protein kinase activity. The effects of alpha-adrenergic agents appear to be completely independent of changes in cyclic AMP protein kinase activity.     

Inhibition of rabbit muscle glycogen phosphorylase by D-gluconohydroximo-1,5-lactone-N-phenylurethane

The effect of the beta-glycosidase inhibitor D-gluconohydroximo-1,5-lactone-N-phenylurethane (PUG) on the kinetic and ultracentrifugation properties of glycogen phosphorylase has been studied. Recent crystallographic work at 2.4 A resolution [D. Barford et al. (1988) Biochemistry 27, 6733-6741] has shown that PUG binds in the catalytic site of phosphorylase b crystals with its gluconohydroximolactone moiety occupying a position similar to that observed for other glucosyl compounds and the N-phenylurethane side chain fitting into an adjacent cavity with little conformational change in the enzyme. In solution, PUG was shown to be a potent inhibitor of phosphorylase b, directly competitive with alpha-D-glucopyranose 1-phosphate (glucose-1-P) (Ki = 0.40 mM) and noncompetitive with respect to glycogen and AMP. When PUG was tested for synergistic inhibition in the presence of caffeine, the Dixon plots of reciprocal velocity versus PUG concentration at different fixed caffeine concentrations provided intersecting lines with interaction constant (alpha) values of 0.95-1.38, indicating that the binding of one inhibitor is not significantly affected by the binding of the other. For glycogen phosphorolysis, PUG was noncompetitive with respect to phosphate, suggesting that it can bind to the central enzyme-AMP-glycogen-phosphate complex. PUG was shown to inhibit phosphorylase alpha (without AMP) activity (Ki = 0.43 mM) in a manner similar to that of the b form. However, in the presence of AMP, PUG exhibited complex kinetics, acting as a noncompetitive inhibitor with respect to glucose-1-P, while a twofold decrease of PUG binding to the enzyme-AMP-glycogen complex was observed. Ultracentrifugation experiments demonstrated that PUG does not cause any significant dissociation of phosphorylase alpha tetramer. Furthermore the dimerization of phosphorylase alpha by glucose is completely prevented in the presence of PUG. These observations are consistent with PUG binding to both the R and the T conformations of phosphorylase.     

The hepatic PP1 glycogen-targeting subunit interaction with phosphorylase a can be blocked by C-terminal tyrosine deletion or an indole drug

The inhibition of hepatic glycogen-associated protein phosphatase-1 (PP1-G(L)) by glycogen phosphorylase a prevents the dephosphorylation and activation of glycogen synthase, suppressing glycogen synthesis when glycogenolysis is activated. Here, we show that a peptide ((280)LGPYY(284)) comprising the last five amino acids of G(L) retains high-affinity interaction with phosphorylase a and that the two tyrosines play crucial roles. Tyr284 deletion abolishes binding of phosphorylase a to G(L) and replacement by phenylalanine is insufficient to restore high-affinity binding. We show that a phosphorylase inhibitor blocks the interaction of phosphorylase a with the G(L) C-terminus, suggesting that the latter interaction could be targeted to develop an anti-diabetic drug.     

Combined kinetic mechanism describing activation and inhibition of muscle glycogen phosphorylase b by adenosine 5'-monophosphate

The kinetic analysis of the glycogen chain growth reaction catalyzed by glycogen phosphorylase b from rabbit skeletal muscle has been carried out over a wide range of concentrations of AMP under the saturation of the enzyme by glycogen. The applicability of 23 different variants of the kinetic model involving the interaction of AMP and glucose 1-phosphate binding sites in the dimeric enzyme molecule is considered. A kinetic model has been proposed which assumes: (i) the independent binding of one molecule of glucose 1-phosphate in the catalytic site on the one hand, and AMP in both allosteric effector sites and both nucleoside inhibitor sites of the dimeric enzyme molecule bound by glycogen on the other hand; (ii) the binding of AMP in one of the allosteric effector sites results in an increase in the affinity of other allosteric effector site to AMP; (iii) the independent binding of AMP to the nucleoside inhibitor sites of the dimeric enzyme molecule; (iv) the exclusive binding of the second molecule of glucose 1-phosphate in the catalytic site of glycogen phosphorylase b containing two molecules of AMP occupying both allosteric effector sites; and (v) the catalytic act occurs exclusively in the complex of the enzyme with glycogen, two molecules of AMP occupying both allosteric effector sites, and two molecules of glucose 1-phosphate occupying both catalytic sites.     

An engineered liver glycogen phosphorylase with AMP allosteric activation

Liver and muscle glycogen phosphorylases, which are products of distinct genes, are both activated by covalent phosphorylation, but in the unphosphorylated (b) state, only the muscle isozyme is efficiently activated by the allosteric activator AMP. The different responsiveness of the phosphorylase isozymes to allosteric ligands is important for the maintenance of tissue and whole body glucose homeostasis. In an attempt to understand the structural determinants of differential sensitivity of the muscle and liver isozymes to AMP, we have developed a bacterial expression system for the liver enzyme, allowing native and engineered proteins to be expressed and characterized. Engineering of the single amino acid substitutions Thr48Pro, Met197Thr and the double mutant Thr48Pro, Met197Thr in liver phosphorylase, and Pro48Thr in muscle phosphorylase, did not qualitatively change the response of the two isozymes to AMP. These sites had previously been implicated in the configuration of the AMP binding site. However, when nine amino acids among the first 48 in liver phosphorylase were replaced with the corresponding muscle phosphorylase residues (L1M2-48L49-846), the engineered liver enzyme was activated by AMP to a higher maximal activity than native liver phosphorylase. Interestingly, the homotropic cooperativity of AMP binding was unchanged in the engineered phosphorylase b protein, and heterotropic cooperativity between the glucose-1-phosphate and AMP sites was only slightly enhanced. The native liver, native muscle and L1M2-48L49-846 phosphorylases were converted to the a form by treatment with purified phosphorylase kinase; the maximal activity of the chimeric a enzyme was greater than the native liver a enzyme and approached that of muscle phosphorylase a. From these results we suggest that tissue-specific phosphorylase isozymes have evolved a complex mechanism in which the N-terminal 48 amino acids modulate intrinsic activity (Vmax), probably by affecting subunit interactions, and other, as yet undefined regions specify the allosteric interactions with ligands and substrates.     

Interaction of glycogen phosphorylase with 8-azidoadenosine 5'-monophosphate, a photoaffinity analog of AMP

The ability of 8-azidoadenosine 5'-monophosphate (N3AMP) to act as a photoaffinity label for the AMP binding site on glycogen phosphorylase (EC 2.4.1.1) was tested. 8-Azidoadenosine 5'-monophosphate can replace AMP as an allosteric modifier of both phosphorylases a and b; the pH optimum and the extent of activation are comparable to that observed with AMP. 8-Azidoadenosine 5'-monophosphate resembles the natural activator in having a higher affinity for phosphorylase a. The effects of 8-azidoadenosine 5'-monophosphate and AMP on phosphorylase b are additive when each is present at a concentration which gives less than 50% activation. Increasing the concentration of the substrate, glucose 1-phosphate, decreases the apparent activation constant (Ka) for the interaction of 8-azidoadenosine 5'-monophosphate with phosphorylase b. Glucose 6-phosphate is an inhibitor of phosphorylase b with either AMP or 8-azidoadenosine 5'-monophosphate. In the presence of ultraviolet light, 8-azidoadenosine 5'-monophosphate is irreversibly incorporated into phosphorylase a; incorporation at the allosteric site can be reduced if AMP is added prior to irradiation. Under the conditions used in the photolysis experiments, 3--5% of the available AMP sites were labeled with 8-azidoadenosine 5'-monophosphate. The data indicate the potential usefulness of 8-azidoadenosine 5'-monophosphate as a probe for the AMP site on phosphorylase.     

Interaction of muscle glycogen phosphorylase B with methotrexate, folic and folinic acids

The interaction of rabbit skeletal muscle glycogen phosphorylase b with methotrexate, folic and folinic acids has been studied. Microscopic dissociation constant for the glycogen phosphorylase b--methotrexate complex determined by analytical ultracentrifugation is 0.43 mM. A subunit of glycogen phosphorylase b is shown to have two sites for methotrexate binding. AMP and FMN diminish the affinity of glycogen phosphorylase b to methotrexate, whereas glycogen does not influence the methotrexate binding to the enzyme. Methotrexate, folic and folinic acids are found to be inhibitors of the muscle glycogen phosphorylase b. The inhibition is reversible and characterized by positive kinetic cooperativity (the Hill coefficient exceeds one unity). The value of the pterin concentration causing two-fold diminishing of the enzymatic reaction rate increased in the order: folic acid (0.65 mM), methotrexate (1.01 mM), folinic acid (3.7 mM). The antagonism between methotrexate, folic and folinic acids, on the one hand, and AMP and FMN, on the other, is revealed for their combined action.     

A tentative mechanism of the ternary complex formation between phosphorylase kinase, glycogen phosphorylase b and glycogen

The kinetics of rabbit skeletal muscle phosphorylase kinase interaction with glycogen has been studied. At pH 6.8 the binding of phosphorylase kinase to glycogen proceeds only in the presence of Mg2+, whereas at pH 8.2 formation of the complex occurs even in the absence of Mg2+. On the other hand, the interaction of phosphorylase kinase with glycogen requires Ca2+ at both pH values. The initial rate of the complex formation is proportional to the enzyme and glycogen concentrations, suggesting the formation of the complex with stoichiometry 1:1 at the initial step of phosphorylase kinase binding by glycogen. According to the kinetic and sedimentation data, the substrate of the phosphorylase kinase reaction, glycogen phosphorylase b, favors the binding of phosphorylase kinase with glycogen. We suggest a model for the ordered binding of phosphorylase b and phosphorylase kinase to the glycogen particle that explains the increase in the tightness of phosphorylase kinase binding with glycogen in the presence of phosphorylase b.     

FR258900, a potential anti-hyperglycemic drug, binds at the allosteric site of glycogen phosphorylase

FR258900 has been discovered as a novel inhibitor of human liver glycogen phosphorylase a and proved to suppress hepatic glycogen breakdown and reduce plasma glucose concentrations in diabetic mice models. To elucidate the mechanism of inhibition, we have determined the crystal structure of the cocrystallized rabbit muscle glycogen phosphorylase b-FR258900 complex and refined it to 2.2 A resolution. The structure demonstrates that the inhibitor binds at the allosteric activator site, where the physiological activator AMP binds. The contacts from FR258900 to glycogen phosphorylase are dominated by nonpolar van der Waals interactions with Gln71, Gln72, Phe196, and Val45' (from the symmetry-related subunit), and also by ionic interactions from the carboxylate groups to the three arginine residues (Arg242, Arg309, and Arg310) that form the allosteric phosphate-recognition subsite. The binding of FR258900 to the protein promotes conformational changes that stabilize an inactive T-state quaternary conformation of the enzyme. The ligand-binding mode is different from those of the potent phenoxy-phthalate and acyl urea inhibitors, previously described, illustrating the broad specificity of the allosteric site.     

Interaction of muscle glycogen phosphorylase B with F-actin

The binding of rabbit muscle glycogen phosphorylase b to F-actin has been studied by sedimentation in analytical centrifuge in 10 mM Tris-acetate buffer pH 6.8 at 20 degrees C. The adsorption capacity of F-actin is equal to (7.8 +/- 0.9) X 10(-7) mole of glycogen phosphorylase b per 1 g of F-actin; the microscopic dissociation constant for the glycogen phosphorylase-F-actin complex is (5.4 +/- 0.5) X 10(-7) M. It was found that the allosteric activator, AMP, facilitates the adsorption of glycogen phosphorylase b on F-actin, whereas the substrate, Pi, and the inhibitor, ATP, cause an opposite effect.     

Site-directed mutants of glycogen phosphorylase are altered in their interaction with phosphorylase kinase

Glycogen phosphorylase is found in resting muscle as phosphorylase b, which is inactive without AMP. Phosphorylation by phosphorylase kinase (PhK) produces phosphorylase a, which is active in the absence of AMP. PhK is the only kinase that can phosphorylate phosphorylase b, which in turn is the only physiological substrate for PhK. We have explored the reasons for this specificity and how these two enzymes recognize each other by studying site-directed mutants of glycogen phosphorylase. All mutants were assayed for changes in their interaction with a truncated form of the catalytic subunit of phosphorylase kinase, gamma(1-300). Five mutations (R69K, R69E, R43E, R43E/R69E, and E501A), made at sites that interact with the amino terminus in either phosphorylase b or a, showed little difference in phosphorylation by gamma(1-300) compared to wild-type phosphorylase b. Five mutations, made at three sites in the amino-terminal tail of phosphorylase (K11A, K11E, I13G, R16A, and R16E), however, produced decreases in catalytic efficiency for gamma(1-300), compared to that for phosphorylase b. R16E was the poorest substrate for gamma(1-300), giving a 47-fold decrease in catalytic efficiency. The amino terminus, and especially Arg 16, are very important factors for recognition of phosphorylase by gamma(1-300). A specific interaction between Lys 11 of phosphorylase and Glu 110 of gamma(1-300) was also confirmed. In addition, I13G and R16A were able to be phosphorylated by protein kinase A, which does not recognize native phosphorylase.     

Effect of osmolytes on the interaction of flavin adenine dinucleotide with muscle glycogen phosphorylase b

The effect of three osmolytes, trimethylamine N-oxide (TMAO), betaine and proline, on the interaction of muscle glycogen phosphorylase b with allosteric inhibitor FAD has been examined. In the absence of osmolyte, the interaction is described by a single intrinsic dissociation constant (17.8 microM) for two equivalent and independent binding sites on the dimeric enzyme. However, the addition of osmolytes gives rise to sigmoidal dependencies of fractional enzyme-site saturation upon free inhibitor concentration. The source of this cooperativity has been shown by difference sedimentation velocity to be an osmolyte-mediated isomerization of phosphorylase b to a smaller dimeric state with decreased affinity for FAD. These results thus have substantiated a previous inference that the tendency for osmolyte-enhanced self-association of dimeric glycogen phosphorylase b in the presence of AMP was being countered by the corresponding effect of molecular crowding on an isomerization of dimer to a smaller, nonassociating state.     

The binding of beta- and gamma-cyclodextrins to glycogen phosphorylase b: kinetic and crystallographic studies

A number of regulatory binding sites of glycogen phosphorylase (GP), such as the catalytic, the inhibitor, and the new allosteric sites are currently under investigation as targets for inhibition of hepatic glycogenolysis under high glucose concentrations; in some cases specific inhibitors are under evaluation in human clinical trials for therapeutic intervention in type 2 diabetes. In an attempt to investigate whether the storage site can be exploited as target for modulating hepatic glucose production, alpha-, beta-, and gamma-cyclodextrins were identified as moderate mixed-type competitive inhibitors of GPb (with respect to glycogen) with K(i) values of 47.1, 14.1, and 7.4 mM, respectively. To elucidate the structural basis of inhibition, we determined the structure of GPb complexed with beta- and gamma-cyclodextrins at 1.94 A and 2.3 A resolution, respectively. The structures of the two complexes reveal that the inhibitors can be accommodated in the glycogen storage site of T-state GPb with very little change of the tertiary structure and provide a basis for understanding their potency and subsite specificity. Structural comparisons of the two complexes with GPb in complex with either maltopentaose (G5) or maltoheptaose (G7) show that beta- and gamma-cyclodextrins bind in a mode analogous to the G5 and G7 binding with only some differences imposed by their cyclic conformations. It appears that the binding energy for stabilization of enzyme complexes derives from hydrogen bonding and van der Waals contacts to protein residues. The binding of alpha-cyclodextrin and octakis (2,3,6-tri-O-methyl)-gamma-cyclodextrin was also investigated, but none of them was bound in the crystal; moreover, the latter did not inhibit the phosphorylase reaction.     

Amylin activates glycogen phosphorylase and inactivates glycogen synthase via a cAMP-independent mechanism.

Although the novel pancreatic peptide amylin has been shown to induce insulin resistance and decrease glucose uptake, the mechanism of amylin's actions is unknown. The following study evaluated the effect of amylin on glycogen metabolism in isolated soleus muscles in the presence and absence of insulin (200 microU/ml). Total glycogen, glycogen phosphorylase and glycogen synthases activities, and cAMP levels were measured. Total glycogen levels were significantly decreased by amylin (100 nM) in fed or fasted muscles under conditions of insulin stimulation. Amylin (100 nM) activated glycogen phosphorylase by as much as 100% and decreased glycogen synthase activity by over 60%, depending on the metabolic state of the muscles. These effects where comparable to those of the beta adrenergic agonist isoproterenol. A lower concentration of amylin (1 nM) did not significantly affect glycogen levels, glycogen phosphorylase, or glycogen synthase activity. Cyclic AMP levels were increased two-fold by isoproterenol but were unaffected by amylin. In conclusion, amylin induces glycogenolysis by decreasing glycogen synthesis and increasing breakdown. The effect of amylin on enzyme activity is consistent with a phosphorylation-dependent mechanism. It is likely that these events are mediated via a cAMP independent protein kinase.     

The amino-terminal tail of glycogen phosphorylase is a switch for controlling phosphorylase conformation, activation, and response to ligands

Glycogen phosphorylase is a muscle enzyme which metabolizes glycogen, producing glucose-1-phosphate, which can be used for the production of ATP. Phosphorylase activity is regulated by phosphorylation/dephosphorylation, and by the allosteric binding of numerous effectors. In this work, we have studied 10 site-directed mutants of glycogen phosphorylase (GP) in its amino-terminal regulatory region to characterize any changes that the mutations may have made on its structure or function. All of the GP mutants had normal levels of activity in the presence of the allosteric activator AMP. Some of the mutants were observed to have altered AMP-binding characteristics, however. R16A and R16E were activated at very low AMP concentration and crystallized at low temperature, like the phosphorylated form of GP, phosphorylase a, and unlike the dephospho-form, phosphorylase b. This indicates that even without phosphorylation, the structures of these mutants are more like phosphorylase a than phosphorylase b. These mutants were also very poorly phosphorylated in the presence of the inhibitor glucose, while phosphorylase b was phosphorylated normally with this inhibitor present. In contrast to R16A and R16E, four other mutants behaved like phosphorylase b after phosphorylation. R69E was only partially activated by phosphorylation, and I13G, R43E, and R43E/R69E were completely inactive after phosphorylation. We propose a model for the many functions of the amino terminus to explain the many varied effects of these mutations.     

Adenovirus-mediated transfer of the muscle glycogen phosphorylase gene into hepatocytes confers altered regulation of glycogen metabolism.

The muscle isozyme of glycogen phosphorylase is potently activated by the allosteric ligand AMP, whereas the liver isozyme is not. In this study we have investigated the metabolic impact of expression of muscle phosphorylase in liver cells. To this end, we constructed a replication-defective, recombinant adenovirus containing the muscle glycogen phosphorylase cDNA (termed AdCMV-MGP) and used this system to infect hepatocytes in culture. AMP-activatable glycogen phosphorylase activity was increased 46-fold 6 days after infection of primary liver cells with AdCMV-MGP. Despite large increases in phosphorylase activity, glycogen levels were only slightly reduced in AdCMV-MGP-infected liver cells compared to uninfected cells or cells infected with wild-type adenovirus. The lack of correlation of phosphorylase activity and glycogen content suggests that the liver cell environment can inhibit the muscle phosphorylase isozyme. This inhibition can be overcome, however, by addition of carbonyl cyanide m-chlorophenylhydrazone (CCCP), which increases AMP levels by 30-fold and causes a much larger decrease in glycogen levels in AdCMV-MGP-infected cells than in uninfected or wild-type adenovirus-infected controls. CCCP treatment also caused a preferential decrease in glycogen content relative to glucagon treatment in AdCMV-MGP-infected hepatocytes (74% versus 11%, respectively), even though the two drugs caused equal increases in phosphorylase a activity. Introduction of muscle phosphorylase into hepatocytes therefore confers a capacity for glycogenolytic response to effectors that is not provided by the endogenous liver phosphorylase isozyme. The remarkable efficiency of adenovirus-mediated gene transfer into primary hepatocytes and the demonstration of altered regulation of glycogen metabolism as a consequence of expression of a non-cognate phosphorylase isozyme may have implications for gene therapy of glycogen storage diseases.     

A new allosteric site in glycogen phosphorylase b as a target for drug interactions

BACKGROUND: In muscle and liver, glycogen concentrations are regulated by the coordinated activities of glycogen phosphorylase (GP) and glycogen synthase. GP exists in two forms: the dephosphorylated low-activity form GPb and the phosphorylated high-activity form GPa. In both forms, allosteric effectors can promote equilibrium between a less active T state and a more active R state. GP is a possible target for drugs that aim to prevent unwanted glycogen breakdown and to stimulate glycogen synthesis in non-insulin-dependent diabetes. As a result of a data bank search, 5-chloro-1H-indole-2-carboxylic acid (1-(4-fluorobenzyl)-2-(4-hydroxypiperidin-1-yl)-2-oxoethy l)amide, CP320626, was identified as a potent inhibitor of human liver GP. Structural studies have been carried out in order to establish the mechanism of this unusual inhibitor. RESULTS: The structure of the cocrystallised GPb-CP320626 complex has been determined to 2.3 A resolution. CP320626 binds at a site located at the subunit interface in the region of the central cavity of the dimeric structure. The site has not previously been observed to bind ligands and is some 15 A from the AMP allosteric site and 33 A from the catalytic site. The contacts between GPb and CP320626 comprise six hydrogen bonds and extensive van der Waals interactions that create a tight binding site in the T-state conformation of GPb. In the R-state conformation of GPa these interactions are significantly diminished. CONCLUSIONS: CP320626 inhibits GPb by binding at a new allosteric site. Although over 30 A from the catalytic site, the inhibitor exerts its effects by stabilising the T state at the expense of the R state and thereby shifting the allosteric equilibrium between the two states. The new allosteric binding site offers a further recognition site in the search for improved GP inhibitors.