Phosphorylase b kinase

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. 

Both liver and muscle phosphorylase b kinase are large Mt (1.3 million) proteins with a tetrameric structure (afiy8)4 [70,71]. The molecular weights of the subunits are a, 140000-145000 (3, 116000-130000 y, 4100045000 S, 17000. White and red muscles contain isozymic forms differing in the Mr of the a-subunit. The S sub- 

The activation of skeletal muscle phosphorylase b kinase by cAMP-dependent protein kinase is concomitant with an initial phosphorylation of the /8-subunit [77,78], After 2 mol phosphate per (a/3yS)4 have been incorporated into this subunit, phos- 

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. 

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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... 

In the 1940s Carl and Gertrude Cori isolated and purified an active form (phosphorylase a) and an inactive form (phosphorylase b) of an enzyme from muscle. Phosphorylase b is activated by AMP (see page 64). In 1955, Fischer Krebs found an enzyme that catalysed the conversion of phosphorylase b to phosphorylase a, together with hydrolysis of ATP to ADP. Thus it appeared to bring about phosphorylation of the enzyme. The enzyme was termed phosphorylase b kinase, was partially purified and the interconversion was established as... 

In GSD IX (phosphorylase b-kinase deficiency), amylo-l,6-glucosidase activity in RBC is usually elevated. 

Phosphorylase b-kinase is the enzyme deficient in glycogenosis type IX (GSD IX, formerly GSD VIII, MIM 306 000). 

Phosphorylase b-kinase converts the enzyme phosphorylase from its low activity form (phosphorylase b) to the high activity form (phosphorylase a) by phosphorylation. The enzyme activity of phosphorylase b-kinase is correlated with the increase of phosphorylase activity in the probands sample and is measured by determining the amount of P, produced by phosphorylase activity. 

Table 4.6.19 Enzyme activities for phosphorylase b-kinase assay... 

Besley GTN (1987) Phosphorylase b kinase deficiency in glycogenosis type VIII differentiation of different phenotypes and heterozygotes by erythrocyte enzyme assay. J Inherit Metab Dis 10 115-118... 

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. 

First, the binding of one hormone molecule to one receptor catalytically activates several Gs molecules. Next, by activating a molecule of adenylyl cyclase, each active Gsa molecule stimulates the catalytic synthesis of many molecules of cAMP. The second messenger cAMP now activates PKA, each molecule of which catalyzes the phosphorylation of many molecules of the target protein—phosphorylase b kinase in Figure 12-16. This... 

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. 

Preliminary data from a number of laboratories suggests that the calmodulin-binding domains of several other enzymes might contain basic, amphiphilic a helices. The catalytic subunit (y subunit) of phosphorylase b kinase is known to bind extremely tightly to calmodulin (the 8 subunit ... 

Responses to various hormones can cross-talk by means of multiple interactions/modifications. One example has already been discussed Phosphorylase b kinase is responsive both to cAMP and to Ca2+ ions. Receptors can themselves be phosphorylated by protein... 

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. 

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