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. (+info)
beta2-adrenergic cAMP signaling is uncoupled from phosphorylation of cytoplasmic proteins in canine heart.
BACKGROUND: Recent studies of beta-adrenergic receptor (beta-AR) subtype signaling in in vitro preparations have raised doubts as to whether the cAMP/protein kinase A (PKA) signaling is activated in the same manner in response to beta2-AR versus beta1-AR stimulation. METHODS AND RESULTS: The present study compared, in the intact dog, the magnitude and characteristics of chronotropic, inotropic, and lusitropic effects of cAMP accumulation, PKA activation, and PKA-dependent phosphorylation of key effector proteins in response to beta-AR subtype stimulation. In addition, many of these parameters and L-type Ca2+ current (ICa) were also measured in single canine ventricular myocytes. The results indicate that although the cAMP/PKA-dependent phosphorylation cascade activated by beta1-AR stimulation could explain the resultant modulation of cardiac function, substantial beta2-AR-mediated chronotropic, inotropic, and lusitropic responses occurred in the absence of PKA activation and phosphorylation of nonsarcolemmal proteins, including phospholamban, troponin I, C protein, and glycogen phosphorylase kinase. However, in single canine myocytes, we found that beta2-AR-stimulated increases in both ICa and contraction were abolished by PKA inhibition. Thus, the beta2-AR-directed cAMP/PKA signaling modulates sarcolemmal L-type Ca2+ channels but does not regulate PKA-dependent phosphorylation of cytoplasmic proteins. CONCLUSIONS: These results indicate that the dissociation of beta2-AR signaling from cAMP regulatory systems is only apparent and that beta2-AR-stimulated cAMP/PKA signaling is uncoupled from phosphorylation of nonsarcolemmal regulatory proteins involved in excitation-contraction coupling. (+info)
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. (+info)
Molecular mode of inhibition of glycogenolysis in rat liver by the dihydropyridine derivative, BAY R3401: inhibition and inactivation of glycogen phosphorylase by an activated metabolite.
The racemic prodrug BAY R3401 suppresses hepatic glycogenolysis. BAY W1807, the active metabolite of BAY R3401, inhibits muscle glycogen phosphorylase a and b. We investigated whether BAY R3401 reduces hepatic glycogenolysis by allosteric inhibition or by phosphatase-catalyzed inactivation of phosphorylase. In gel-filtered liver extracts, racemic BAY U6751 (containing active BAY W1807) was tested for inhibition of phosphorylase in the glycogenolytic (in which only phosphorylase a is active) and glycogen-synthetic (for the evaluation of a:b ratios) directions. Phosphorylase inactivation by endogenous phosphatase was also studied. In liver extracts, BAY U6751 (0.9-36 micromol/l) inhibited glycogen synthesis by phosphorylase b (notwithstanding the inclusion of AMP), but not by phosphorylase a. Inhibition of phosphorylase-a-catalyzed glycogenolysis was partially relieved by AMP (500 micromol/l). BAY U6751 facilitated phosphorylase-a dephosphorylation. Isolated hepatocytes and perfused livers were tested for BAY R3401-induced changes in phosphorylase-a:b ratios and glycogenolytic output. Though ineffective in extracts, BAY R3401 (0.25 micromol/l-0.5 mmol/l) promoted phosphorylase-a dephosphorylation in hepatocytes. In perfused livers exposed to dibutyryl cAMP (100 micromol/l) for maximal activation of phosphorylase, BAY R3401 (125 micromol/l) inactivated phosphorylase by 63% but glucose output dropped by 83%. Inhibition of glycogenolysis suppressed glucose-6-phosphate (G6P) levels. Activation of glycogen synthase after phosphorylase inactivation depended on the maintenance of G6P levels by supplementing glucose (50 mmol/l). We conclude that the metabolites of BAY R3401 suppress hepatic glycogenolysis by allosteric inhibition and by the dephosphorylation of phosphorylase a. (+info)
Hepatic glycogen synthesis is highly sensitive to phosphorylase activity: evidence from metabolic control analysis.
We used metabolic control analysis to determine the flux control coefficient of phosphorylase on glycogen synthesis in hepatocytes by titration with a specific phosphorylase inhibitor (CP-91149) or by expression of muscle phosphorylase using recombinant adenovirus. The muscle isoform was used because it is catalytically active in the b-state. CP-91149 inactivated phosphorylase with sequential activation of glycogen synthase. It increased glycogen synthesis by 7-fold at 5 mm glucose and by 2-fold at 20 mm glucose with a decrease in the concentration of glucose causing half-maximal rate (S(0.5)) from 26 to 19 mm. Muscle phosphorylase was expressed in hepatocytes mainly in the b-state. Low levels of phosphorylase expression inhibited glycogen synthesis by 50%, with little further inhibition at higher enzyme expression, and caused inactivation of glycogen synthase that was reversed by CP-91149. At endogenous activity, phosphorylase has a very high (greater than unity) negative control coefficient on glycogen synthesis, regardless of whether it is determined by enzyme inactivation or overexpression. This high control is attenuated by glucokinase overexpression, indicating dependence on other enzymes with high control. The high control coefficient of phosphorylase on glycogen synthesis affirms that phosphorylase is a strong candidate target for controlling hyperglycemia in type 2 diabetes in both the absorptive and postabsorptive states. (+info)
Fatty acid and amino acid modulation of glucose cycling in isolated rat hepatocytes.
We studied the influence of glucose/glucose 6-phosphate cycling on glycogen deposition from glucose in fasted-rat hepatocytes using S4048 and CP320626, specific inhibitors of glucose-6-phosphate translocase and glycogen phosphorylase respectively. The effect of amino acids and oleate was also examined. The following observations were made: (1) with glucose alone, net glycogen production was low. Inhibition of glucose-6-phosphate translocase increased intracellular glucose 6-phosphate (3-fold), glycogen accumulation (5-fold) without change in active (dephosphorylated) glycogen synthase (GSa) activity, and lactate production (4-fold). With both glucose 6-phosphate translocase and glycogen phosphorylase inhibited, glycogen deposition increased 8-fold and approached reported in vivo rates of glycogen deposition during the fasted-->fed transition. Addition of a physiological mixture of amino acids in the presence of glucose increased glycogen accumulation (4-fold) through activation of GS and inhibition of glucose-6-phosphatase flux. Addition of oleate with glucose present decreased glycolytic flux and increased the flux through glucose 6-phosphatase with no change in glycogen deposition. With glucose 6-phosphate translocase inhibited by S4048, oleate increased intracellular glucose 6-phosphate (3-fold) and net glycogen production (1.5-fold), without a major change in GSa activity. It is concluded that glucose cycling in hepatocytes prevents the net accumulation of glycogen from glucose. Amino acids activate GS and inhibit flux through glucose-6-phosphatase, while oleate inhibits glycolysis and stimulates glucose-6-phosphatase flux. Variation in glucose 6-phosphate does not always result in activity changes of GSa. Activation of glucose 6-phosphatase flux by fatty acids may contribute to the increased hepatic glucose production as seen in Type 2 diabetes. (+info)
The regulatory alpha subunit of phosphorylase kinase may directly participate in the binding of glycogen phosphorylase.
The yeast two-hybrid screen has been used to identify potential regions of interaction of the largest regulatory subunit, alpha, of phosphorylase kinase (PhK) with two fragments of its protein substrate, glycogen phosphorylase b (Phb). One fragment, corresponding to residues 17-484 (PhbN'), contained the regulatory domain of the protein, but in missing the first 16 residues was devoid of the sole phosphorylation site of Phb, Ser14; the second fragment corresponded to residues 485-843 (PhbC) and contained the catalytic domain of Phb. Truncation fragments of the alpha subunit were screened for interactions against these two substrate fragments. PhbC was not found to interact with any alpha constructs; however, PhbN' interacted with a region of alpha (residues 864-1014) that is near the phosphorylatable region of that subunit. PhbN' was also screened for interactions against a variety of fragments of the catalytic gamma subunit of PhK; however, no interactions were detected, even with full-length gamma. Our results support the idea that amino acid residues proximal to the convertible serine of Phb are important for its specific interaction with the catalytic subunit of PhK, but that regions distinct from the convertible serine residue of Phb and from the catalytic domain of PhK may also be involved in the interaction of these two proteins. (+info)
Contraction-mediated glycogenolysis in mouse skeletal muscle lacking creatine kinase: the role of phosphorylase b activation.
Skeletal muscle that is deficient in creatine kinase (CK-/-) exhibits accelerated glycogenolysis during contraction. Understanding this phenomenon could provide insight into the control of glycogenolysis during contraction. Therefore, glycogen breakdown was investigated in isolated extensor digitorum longus CK-/- muscle. Muscles were stimulated to produce repeated tetani for 20 s in the presence of sodium cyanide to block mitochondrial respiration. Accumulation of lactate after stimulation was similar in wild-type (WT) and CK-/- muscles, whereas accumulation of glucose-6-phosphate was twofold higher in CK-/- muscles, indicating greater glycogenolysis in CK-/- muscles. Total phosphorylase activity was decreased by almost 30 % in CK-/- muscle (P < 0.001). Phosphorylase fractional activity (-/+ 3.3 mM AMP) was similar in both groups in the basal state (about 10 %), but increased to a smaller extent in CK-/- muscles after stimulation (39 +/- 4 % vs. 52 +/- 4 % in WT, P < 0.05). Inorganic phosphate, the substrate for phosphorylase, increased marginally in CK-/- muscles after stimulation (basal = 25.3 +/- 2.2 micromol (g dry muscle)-1; stimulated = 33.9 +/- 2.3 micromol (g dry muscle)-1), but substantially in WT muscles (basal = 11.4 +/- 0.7 micromol (g dry muscle)-1; stimulated = 54.2 +/- 4.5 micromol (g dry muscle)-1). Kinetic studies of phosphorylase b (dephosphorylated enzyme) from muscle extracts in vitro demonstrated higher relative activities in CK-/- muscles (60-135 %) in response to low AMP concentrations (up to 50 microM) in both the basal state and after stimulation (P < 0.05), whereas no differences in activity between CK-/- and WT muscles were observed at high AMP concentrations (> 100 microM). These data indicate that allosteric activation of phosphorylase b accounts for the accelerated glycogenolysis in CK-/- muscle during contraction. (+info)