Control of phosphorylase kinase in the isolated glycogen particle by Ca2+-Mg2+ synergistic activation and cAMP-dependent phosphorylation

Role of glycogen in cardiac metabolic stress

Metabolic stress in the myocardium arises from a diverse array of acute and chronic pathophysiological contexts. Glycogen mishandling is a key feature of metabolic stress, while maladaptation in energy-stress situations confers functional deficits. Cardiac glycogen serves as a pivotal reserve for myocardial energy, which is classically described as an energy source and contributes to glucose homeostasis during hypoxia or ischemia. Despite extensive research activity, how glycogen metabolism affects cardiovascular disease remains unclear. In this review, we focus on its regulation across myocardial energy metabolism in response to stress, and its role in metabolism, immunity, and autophagy. We further summarize the cardiovascular-related drugs regulating glycogen metabolism. In this way, we provide current knowledge for the understanding of glycogen metabolism in the myocardium.

Quantification of the glycogen cascade system: the ultrasensitive responses of liver glycogen synthase and muscle phosphorylase are due to distinctive regulatory designs

Background

Signaling pathways include intricate networks of reversible covalent modification cycles. Such multicyclic enzyme cascades amplify the input stimulus, cause integration of multiple signals and exhibit sensitive output responses. Regulation of glycogen synthase and phosphorylase by reversible covalent modification cycles exemplifies signal transduction by enzyme cascades. Although this system for regulating glycogen synthesis and breakdown appears similar in all tissues, subtle differences have been identified. For example, phosphatase-1, a dephosphorylating enzyme of the system, is regulated quite differently in muscle and liver. Do these small differences in regulatory architecture affect the overall performance of the glycogen cascade in a specific tissue? We address this question by analyzing the regulatory structure of the glycogen cascade system in liver and muscle cells at steady state.

Results

The glycogen cascade system in liver and muscle cells was analyzed at steady state and the results were compared with literature data. We found that the cascade system exhibits highly sensitive switch-like responses to changes in cyclic AMP concentration and the outputs are surprisingly different in the two tissues. In muscle, glycogen phosphorylase is more sensitive than glycogen synthase to cyclic AMP, while the opposite is observed in liver. Furthermore, when the liver undergoes a transition from starved to fed-state, the futile cycle of simultaneous glycogen synthesis and degradation switches to reciprocal regulation. Under such a transition, different proportions of active glycogen synthase and phosphorylase can coexist due to the varying inhibition of glycogen-synthase phosphatase by active phosphorylase.

Conclusion

The highly sensitive responses of glycogen synthase in liver and phosphorylase in muscle to primary stimuli can be attributed to distinctive regulatory designs in the glycogen cascade system. The different sensitivities of these two enzymes may exemplify the adaptive strategies employed by liver and muscle cells to meet specific cellular demands.

Binary affinity chromatography for the purification of glycogen phosphorylase

A binary affinity chromatography medium was prepared and found to be useful for the purification and quantitative isolation of glycogen phosphorylase from rabbit skeletal muscle and liver. Glycogen is used as the binary ligand as it has affinity toward both the column matrix and the enzyme. Agarose beads derivatized with concanavalin A bound glycogen to the level of 35 mg/ml. The glycogen-impregnated beads were able to bind 9 mg/ml of phosphorylase a or b. The phosphorylase is tightly bound so that the column can be washed free of contaminants before quantitative elution of the phosphorylase by 2 M glucose, which releases the glycogen-phosphorylase complex. It appears that binary affinity chromatography may have general utility for the isolation and purification of enzymes and other specific binding agents.

Scanning tunneling microscopy of the enzymes of muscle glycogenolysis

Scanning tunneling microscopy (STM) has been used to examine the structures of the skeletal muscle enzymes phosphorylase and phosphorylase kinase. The interaction of these two proteins represents the last step in the process of signal transduction which results in muscle glycogen being converted into metabolic energy for use in muscle contraction. Phosphorylase b has a molecular weight of 97,000 and the dimer is seen by STM to have dimensions of 11 X 5.7 nm. Phosphorylase b has a tendency to form linear arrays of dimers on the graphite surface used as the support for STM imaging. Phosphorylase kinase is imaged as a butterfly-like object with lateral dimensions of 36 X 27 nm. The molecular thicknesses given by scanning tunneling microscopy for these two non-conducting molecules is significantly less than expected. The height measurement in STM is dependent not only on the surface topology of the object being imaged, but also on the electronic work function of the object compared to that of the graphite surface on which it lies. In addition to the individual proteins, a complex between phosphorylase and phosphorylase kinase has been observed by scanning tunneling microscopy.

Fractionation of particulate glycogen and bound enzymes using high-performance liquid chromatography

During a study aimed at the specificity of binding of phosphorylase and glycogen synthase to glycogen particles we investigated the separation of these particles with the technique of high-performance liquid chromatography using several media. The technique allowed relatively rapid fractionations with little, if any, loss of enzyme activity. The basis of some separations was molecular weight of the particle. Of the molecular exclusion media used, Superose 6 had an exclusion limit large enough to accommodate most glycogens but excluded mouse liver and bovine liver glycogen particles. Toyopearl HW-75 F was a medium that could accommodate liver glycogen and gave us reasonably well-defined separation of the particles that carried phosphorylase and glycogen synthase. Separations on ion-exchange columns, that were obviously based on the overall charge of proteins associated with the glycogen particles, were not very effective in terms of allowing the separation of different types of particle. Of all the media used the hydrophobic interaction column was best in terms of separating what appeared to be distinct populations of particles that had definite preferences in terms of binding phosphorylase and glycogen synthase. As expected, these separations were based on the hydrophobic interaction of the proteins associated with the glycogen particles, since particles that had been deproteinized by a cold water extraction procedure did not bind to the column.

Phosphorylase a formation in protein-glycogen particles isolated from fast-twitch muscle of euthyroid and hypothyroid rats

A fraction containing a protein-glycogen complex was isolated from rat skeletal muscle in order to study the effect of hypothyroidism on phosphorylase activation in this structural and functional unit of the glycogenolytic process. The total activities of phosphorylase and phosphorylase phosphatase in euthyroids and hypothyroids were the same in the fraction containing the protein-glycogen complex (P2 suspension). Hypothyroidism selectively lowered the maximal phosphorylase kinase activity in glycogen particles in the P2 suspension by 40%. Addition of Mg2+ (10 mM), ATP (2 mM), and Ca2+ (5 mM) rapidly stimulated phosphorylase b to a conversion resulting from phosphorylase kinase activation. Hypothyroidism reduced the rate of phosphorylase a formation by 50-70% in the P2 suspension. Glucose 6-phosphate (0.4-1.4 mM) inhibited the rate of phosphorylase a formation and this inhibition was similar for eu- and hypothyroids. There was a shift from 5.2 to 5.8 in the free Ca2+ concentration (pCaF) for half-maximal activation of phosphorylase in the P2 suspension of hypothyroids. A sixfold higher steady-state level of phosphorylase in euthyroids compared to hypothyroids was observed at a pCaF of 5.5. The Ca2+ sensitivity of the phosphorylase kinase, however, was not changed by hypothyroidism. These results provide further insight into the different time course of the phosphorylase activation in skeletal muscle during tetanic stimulation observed in euthyroidism and hypothyroidism (W. J. Leijendekker et al. (1985) Metabolism 34, 437-441).

Mechanism of calmodulin inhibition of cAMP-dependent protein kinase activation of phosphorylation kinase

The activation of phosphorylase kinase (EC 2.7.1.38; ATP:phosphorylase b phosphotransferase) by the catalytic subunit of cAMP-dependent protein kinase (EC 2.7.1.37; ATP:protein phosphotransferase) is inhibited by calmodulin. The mechanism of that inhibition has been studied by kinetic measurements of the interactions of the three proteins. The binding constant for calmodulin with phosphorylase kinase was found to be 90 nM when measured by fluorescence polarization spectroscopy. Glycerol gradient centrifugation studies indicated that 1 mol of calmodulin was bound to each phosphorylase kinase. Phosphorylation of the phosphorylase kinase did not reduce the amount of calmodulin bound. Kinetic studies of the activity of the catalytic subunit of cAMP-dependent protein kinase on phosphorylase kinase as a function of phosphorylase kinase and calmodulin concentrations were performed. The results of those studies were compared with mathematical models of four different modes of inhibition: competitive, noncompetitive, substrate depletion, and inhibition by a complex between phosphorylase kinase and calmodulin. The data conform best to the model in which the inhibitory species is a complex of phosphorylase kinase and calmodulin. The complex apparently competes with the substrate, phosphorylase kinase, which does not have exogenous calmodulin bound to it. In contrast, the phosphorylation of the synthetic phosphate acceptor peptide, Kemptide, is not inhibited by calmodulin.