Glycogen Inactive

PHOSPHOPROTEINS. These proteins have phosphate groups esterified to the hydroxyls of serine, threonine, or tyrosine residues. Casein, the major protein of milk, contains many phosphates and serves to bring essential phosphorus to the growing infant. Many key steps in metabolism are regulated between states of activity or inactivity, depending on the presence or absence of phosphate groups on proteins, as we shall see in Chapter 15. Glycogen phospho-rylase a is one well-studied example. 

Glycogen phosphorylase conforms to the Monod-Wyman-Changeux model of allosteric transitions, with the active form of the enzyme designated the R state and the inactive form denoted as the T state (Figure 15.17). Thus, AMP promotes the conversion to the active R state, whereas ATP, glucose-6-P, and caffeine favor conversion to the inactive T state. 

Glycogenosis type VIII (phosphorylase b kinase deficiency) gives rise to myopathy and liver disease, either singly or in combination. Phosphorylase b kinase (PBK) converts the inactive b form of both muscle and liver phosphorylases to the active a forms of the enzymes. The ischemic lactate test sometimes shows a flat result as in McArdle s disease, but is more likely to be normal. Histochemical demonstration of myophosphorylase activity in tissue sections shows a near-normal reaction due to the presence of phosphorylase a. Accumulation of glycogen is modest and found mainly in type 2 (fast-twitch glycolytic) muscle fibers. 

Not only is phosphorylase activated by a rise in concentration of cAMP (via phosphorylase kinase), but glycogen synthase is at the same time converted to the inactive form both effects are mediated via cAMP-dependent protein kinase. Thus, inhibition of glycogenolysis enhances net glycogenesis, and inhibition of glycogenesis enhances net glycogenolysis. Furthermore,... 

In contrast to inhibitor 1, DARPP-32 and NIPP1, which regulate signal transduction, the function of inhibitor 2 appears to be different. There is evidence that inhibitor 2 associates with PP1, as the phosphatase is newly synthesized and contributes to the proper folding of the enzyme [40]. Inhibitor 2 can thus be considered a chaperone protein. The inactive PP 1-inhibitor 2 complex can then be activated upon phosphorylation of inhibitor 2 by glycogen synthase kinase-3. Whether this process is regulated in neurons in association with synaptic activity remains unknown. 

Glycogen phosphorylase isoenzymes have been isolated from liver, brain and skeletal muscle. All forms are subject to covalent control with conversion of the inactive forms (GP-b) to the active forms (GP-a) by phosphorylation on specific serine residues. This phosphorylation step, mediated by the enzyme phosphorylase kinase, is initiated by glucagon stimulation of the hepatocyte. Indeed, the same cAMP cascade which inhibits glycogen synthesis simultaneously stimulates glycogenolysis, giving us an excellent example of reciprocal control. 

The next key point is to realize that each enzyme in the pathway exists in both active and inactive forms. cAMP initiates a cascade of reactions by activating protein kinase A (PK-A)," the active form of which activates the next enzyme in the sequence, and so on. At the end of the day, glycogen phosphorylase is activated and glucose or ATP is produced. This signaling pathway is a marvelous amplification system. A few molecules of glucagon or adrenaline may induce formation of many molecules of cAMP, which may activate many of PK-A, and so on. The catalytic power of enzymes is magnified in cascades of this sort. 

Since the change in rate of glycogen degradation, catalysed by glycogen phosphorylase, depends on a balance between the two activities, which constimte a cycle, the relationship is also known as an interconversion cycle. By convention, the active form is the a form and the inactive form is the b form (Figure 3.12). 

Fig. 2.1L Model of allosteric and covalent activation of glycogen phosphorylase of muscle. The R-form of the subunits are represented by circles, the T-form by squares. The active state of glycogen phosphorylase (GP) is characterized by a high affinity, the inactive state by low affinity for the substrate PI.

Fig. 2.12. Structural changes at the N-terminus of glycogen phosphorylase as a result of phosphorylation. a) R-form of the dimer of glycogen phosphorylase a. b) T form of the dimer of glycogen phosphorylase b. Phosphorylation at Serl3 near the N-terminus transforms the inactive glycogen phosphorylase b into the active glycogen phosphorylase a. The N-terminus rearranges significantly as a result of phosphorylation. In the inactive T-state the N-terminus interacts with the same subunit, while in the R-form it forms interactions with the other subunit. After Barford and Johnson (1991), with permission.

Glycogen synthase is regulated via the same pathway. Glycogen synthase is inactive in the phosphorylated form whereas in the dephosphorylated form, it is active. Three key enzymes of glycogen metabohsm are thus controlled with the help of reversible protein phosphorylation. 

Fig. 7.18. Regulation of glycogen metabolism in muscle. Phosphorylase kinase stands at the center of regulation of glycogen metabolism. Phosphorylase kinase may exist in an active, phosphorylated form and an inactive, unphosphorylated form. Phosphorylation of phosphorylase kinase is triggered by hormonal signals (e.g. adrenahne) and takes place via an activation of protein kinase A in the cAMP pathway. In the absence of hormonal stimulation, phosphorylase kinase can also be activated by an increase in cytosolic Ca. The active phosphorylase kinase stimulates glycogen degradation and inhibits glycogen synthesis, in that, on the one side, it activates glycogen phosphorylase by phosphorylation, and on the other side, it inactivates glycogen synthase by phosphorylation.

GSK3, inactivated by phosphorylation, cannot convert glycogen synthase (GS) to its inactive form by phosphorylation, so GS remains active. 

One downstream effect of epinephrine is to activate glycogen phosphorylase b. This conversion is promoted by the enzyme phosphorylase b kinase, which catalyzes the phosphorylation of two specific Ser residues in phosphorylase b, converting it to phosphorylase a (see Fig. 6-31). Cyclic AMP does not affect phosphorylase b kinase directly. Rather, cAMP-dependent protein kinase, also called protein kinase A or PKA, which is allosterically activated by cAMP (Fig. 12-12, step (5)), catalyzes the phosphorylation of inactive phosphorylase b kinase to yield the active form. 

Fig. 12-23). Glycogen synthase kinase 3 (GSK3) is inactive when phosphorylated on a Ser residue in its auto-inhibitory domain (Fig. 12-23b). Dephosphorylation of that domain frees the enzyme to bind and phosphory-late its target proteins. Similarly, the polar head group of the phospholipid PIP3, protruding from the inner leaflet of the plasma membrane, provides points of attachment for proteins that contain SH3 and other domains. 

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