Glycogen phosphorylase b kinase

Glycogen phosphorylase b is the less active form of glycogen phosphorylase. It differs from glycogen phosphorylase a in that it is not phosphorylated and that it requires AMP for activity. Glycogen phosporylase b is a substrate for the enzyme glycogen phosphorylase b kinase, which converts the b form to the a form by adding two phosphates. The reaction is stimulated in the presence of calcium via interaction of calmodulin with glycogen phosphorylase b kinase... 

See also Glycogen Phosphorylase a. Glycogen Phosphorylase b. Glycogen, Kinase Cascade, Glycogen Phosphorylase b Kinase, Figure 16.11... 

Proteins phosphorylated by cAPK include glycogen phosphorylase b kinase (activates), PFK-2 (inactivates), acetyl CoA carboxylase (inactivates), hormone-sensitive triacylglycerol lipase... 

Glycogen phosporylase b kinase (also called synthase-phosphorylase kinase (SPK) or simply phosphorylase b kinase) is part of the glycogen kinase cascade regulatory system (Figure 13.18). The enzyme is converted from the inactive form to active by phosphorylation catalyzed by cAMP-dependent protein kinase. Glycogen phosphorylase b kinase, when active, phosphorylates the glycogen phosphorylase b (less active form) to convert it to the more active form (glycogen phosphorylase a). 

Inhibition of phosphoprotein phosphatase (PP-1), whose activity would tend to restore activity of glycogen synthase. PP-1 and other phosphoprotein phosphatases play converse roles in glycogenolysis, in which dephosphorylation of glycogen phosphorylase b kinase (SPK) causes its inactivation. 

The a form is derived from the b form by phosphorylation of the b form by the enzyme glycogen phosphorylase b kinase (Figure 13.18). 

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. 

Glycogen and its enzymes are compartmentalized. Glycogen granules are only present in astrocytes of adult animals but are found in both astrocytes and neurons of immature animals. Of the enzymes involved in glycogen metabolism, glycogen phosphorylase is found in astrocytes only. Under steady-state conditions, it is probable that less than 10% of phosphorylase in brain is in the unphosphorylated b form (requiring AMP). This form is probably not very active at the low AMP concentrations present when intracellular glucose is sufficient to maintain ATP synthesis. Brain phosphorylase b kinase is activated indirectly by cAMP and by the molar concentrations... 

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. 

Glycogen synthase Phosphorylase b kinase RASCTSSS Glycogen synthesis... 

FIGURE 15-24 Regulation of muscle glycogen phosphorylase by covalent modification. In the more active form of the enzyme, phosphorylase a, Ser14 residues, one on each subunit, are phosphorylated. Phosphorylase a is converted to the less active form, phosphorylase b, by enzymatic loss of these phosphoryl groups, catalyzed by phosphorylase a phosphatase (PP1). Phosphorylase b can be reconverted (reactivated) to phosphorylase a by the action of phosphorylase b kinase. 

In contrast to the situation in the adipocyte, hypothyroidism potentiates /3-ad-renergic receptor-mediated cAMP and glycogen phosphorylase response in rat he-patocytes [88]. Thyroid hormones suppress /3-adrenergic-stimulated phosphorylase b kinase and phosphorylase a activities, while enhancing phosphoprotein phosphatase activity in the same cells [89,90]. In other words, thyroid hormones seem to... 

Metabolic Effects of Mutant Enzymes Predict and explain the effect on glycogen metabolism of each of the following defects caused by mutation (a) loss of the cAMP-binding site on the regulatory subunit of protein kinase A (PKA) (b) loss of the protein phosphatase inhibitor (inhibitor 1 in Fig. 15-40) (c) overexpression of phosphorylase b kinase in liver (d) defective glucagon receptors in liver. 

Two key regulatory enzymes involved in the control of glycogen metabolism were first recognized as targets of cAMP and cAMP-dependent protein kinase in liver and skeletal muscle. These are phosphorylase b kinase and glycogen synthase. The molecular details of the phosphorylation and regulation of these enzymes are better understood in muscle than in liver since the liver enzymes have only recently been purified to homogeneity in the native form. However, it appears that they share many key features in common. 

The major substrate of phosphorylase b kinase is phosphorylase b which is phos-phorylated on a single serine residue at position 14, resulting in conversion to the more catalytically active form phosphorylase a [70], Phosphorylation of skeletal muscle phosphorylase also results in conversion of the Mr 200000 dimeric b form to the Mr 400000 tetrameric a form, whereas phosphorylation of the liver enzyme does not alter its dimeric structure [82]. Phosphorylase a is much less dependent than phosphorylase b upon the allosteric activator AMP [82], Since the activity of phosphorylase is rate-limiting for glycogen breakdown, its activation by phosphorylase b kinase results in enhanced glycogenolysis and glucose release from the liver. 

In this laboratory exercise, you will study the effects of phosphorylation and allosteric regulation on the activity of glycogen phosphorylase. In the first experiment, you will phosphorylate glycogen phosphorylase b in vitro using y-[32P]ATP and glycogen phosphorylase kinase. In the second experiment, you will study the effect of phosphorylation on glycogen phosphorylase activity, as well as the effect of AMP on glycogen phosphorylase b activity with the use of a coupled enzymatic (kinetic) assay (Fig. 15-3). 

During exercise, [ATP] falls and [AMP] rises. Recall that AMP is an allosteric activator of glycogen phosphorylase b. Thus, even in the absence of covalent modification by phosphorylase kinase, glycogen is degraded. 

The two errzymes involved in the synthesis and break down of glycogen exist in active and inactive forms. Thus, glycogen synthase is present as glycogen syn-thase-I (independent form or active) and glycogen synthase-D (dependent form or inactive) of which the former can be inactivated by protein kinase which, in turn, is activated by cyclic AMP. Similarly, glycogen phosphorylase exists as phosphorylase-a (highly active) and phosphorylase-b (poorly active) forms. The formation of active phosphorylase-a occurs in the presence of phosphorylase-b kinase which per se is activated by a cAMP dependent protein kinase (Chart-4). 

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