Activities of phosphorylase kinase

Glycogenolysis increases in muscle several hundred-fold immediately after the onset of contraction. This involves the rapid activation of phosphorylase by activation of phosphorylase kinase by Ca +, the same signal as that which initiates contraction in response to nerve stimulation. Muscle phosphorylase kinase has four... 

NARDINI M, SCACCINI c, PACKER L and VIRGILI F (2000) In vivo inhibition of the activity of phosphorylase kinase, protein kinase C and protein kinase A by caffeic acid and a procyanidin rich pine bark (Pinus maritima) extract Biochimica Biophysica Acta 1474, 219-25. 

Fig. 7.19. Subunit structure and regulation of phosphorylase kinase of muscle. Phosphorylase kinase is - according to the excitation state of the muscle - regulated by two pathways. On nervous stimulation of the muscle, voltage-controlled Ca channels are opened, the cytosolic Ca concentration increases and Ca binds to calmoduhn, activating phosphorylase kinase. In resting muscle, activation of phosphorylase kinase is triggered by a hormonal signal. A hormonal signal initiates phosphorylation of the a and P subunits of phosphorylase kinase. In the phosphorylated form, Ca binding affinity of the calmodulin subunit (8) is strongly increased and activation is also possible at low Ca concentrations.

Activation of phosphorylase kinase Phosphorylase kinase exists in two forms an inactive "b" form and an active "a" form. Active cAMP-dependent protein kinase phosphorylates the inactive form of phosphorylase kinase, resulting in its activation (see Figure... 

Hormonal control of the activity of phosphorylase kinase. Just as the activity of phosphorylase is increased by phosphorylation, so is the activity of its phosphorylase kinase (which may be phosphorylated on two serine residues, one in an a subunit and one in a /3 subunit). Hormonal stimulation (/3-adrenergic) leads to the production of 3, 5 -cyclic AMP ( second messenger ), which stimulates the activity of the cyclic-AMP-dependent protein kinase that catalyzes the phosphorylation of phosphorylase kinase. 

Neural control of the activity of phosphorylase kinase. The electrical stimulation of muscle is mediated by the release of Ca2+ ions. These ions also... 

GDP production and the inactivation of the G protein. If hormone remains bound, the G protein can be activated again. Meanwhile, the cAMP produced binds to the R subunits of cAMP-dependent protein kinase, leading to the dissociation of two catalytic subunits, which can then phosphorylate critical proteins in this case the activation of phosphorylase kinase is shown. 

Figure 21.13. Activation of Phosphorylase Kinase. Phosphorylase kinase is activated by hormones that lead to the phosphorylation of the P subunit and by Ca2+ binding of the 8 subunit. Both types of stimulation are required for maximal enzyme activity. 

Phosphorylase kinase has been known for a longer time than protein kinase. It exists in resting muscle in a form which has little activity at or below the normal intracellular pH and exhibits less than maximal activity even at higher pH values. In the presence of ATP and Mg, it is converted to a form (activated phosphorylase kinase) having 25-50 times as much activity at pH 6.8 and about twice as much activity at pH 8.2. The ratio of kinase activity at pH 6.8 to that at pH 8.2 has been widely used as an index of kinase activation. Activation of phosphorylase kinase is exceedingly rapid in response to an increase in the level of cyclic AMP. 

ATP and activation by inorganic phosphate, 5 -AMP, and ADP (Chapter 13), and 3. Skeletal muscle phosphorylase b is allosterically activated by 5 -AMP. Conversion of phosphorylase b to phosphorylase a is affected by epinephrine, through increased levels of cAMP, in a sequence of reactions similar to that indicated in Figure 22-13. Following depolarization of the muscle cell membrane, calcium is released into the sarcoplasm from the sarcoplasmic reticulum, resulting in the calcium-dependent activation of phosphorylase kinase and conversion of phosphorylase to the a form. 

The answer is b. (Murray, pp 199-207. Scriver, pp 1521-1552. Sack, pp 121-138. Wilson, pp 287-31 77) In the presence of low blood glucose, epinephrine or norepinephrine interacts with specific receptors to stimulate adenylate cyclase production of cyclic AMP Cyclic AMP activates protein kinase, which catalyzes phosphorylation and activation of phosphorylase kinase. Activated phosphorylase kinase activates glycogen phosphorylase, which catalyzes the breakdown of glycogen. Phosphorylase kinase can be activated in two ways. Phosphorylation leads to complete activation of phosphorylase kinase. Alternatively, in muscle, the transient increases in levels of Ca" associated with contraction lead to a partial activation of phosphorylase kinase. Ca" " binds to calmodulin, which is a subunit of phosphorylase kinase. Calmodulin regulates many enzymes in mammalian cells through Ca" binding. 

There are allosteric activators of phosphorylase kinase (Ca2+) and muscle phosphorylase (AMP), which can cause activation of the appropriate enzyme without the covalent modification. 

The liver is also affected by epinephrine in this case, the a-1 receptors are activated by epinephrine. This will cause an increase in intracellular liver Ca2+, via IP3, an allosteric activator of phosphorylase kinase which will catalyze the phosphorylation of phosphorylase and thereby activation. This increase in active phosphorylase will rapidly increase glycogenolysis and provide hexosphosphates, which in the case of liver are not used primarily for glycolysis but, via the action of glucose-6-phosphatase, augment the blood glucose levels (Fig. 16.10). 

King, M.M. Carslon, G.M. Synergistic effect of Ca and Mg " " in promoting an activity of phosphorylase kinase that is insensitive to ethylene glycol bis(jd-aminoethyl ether)-N,N-tetraacetic acid. Arch. Biochem. Biophys., 209, 517-523 (1981)... 

Ashida, M. Wyatt, G.R. Properties and activation of phosphorylase kinase from silkmoth fat body. Insect Biochem., 9, 403-409 (1979)... 

It is well known that glycogen disappears from muscle during contraction, presumably from the activation of phosphorylase kinase through a mechanism different from that put into gear by epinephrine. Indeed, during muscle contraction, kinase appears to be stimulated by the effect of Ca ions rather than by that of cyclic AMP. 

In rat liver, 3-blockers do not completely abolish the response to adrenaline due to the presence of the second type of adrenaline receptor, the a-receptor. a-Receptors are characterized by their sensitivity to phenoxybenzamine. The binding of adrenaline to a-receptors does not increase the cAMP concentration, but nevertheless it stimulates glycogen breakdown by causing an increase in the cytosolic concentration of free Ca ions. This calcium is initially released from the endoplasmic reticulum. The non-phosphorylated form of phos-phorylase kinase is activated by calcium ions so that an increase in cytoplasmic calcium concentration increases the activity of phosphorylase kinase, and hence glycogen breakdown, independently of a change in cAMP. Thus Ca ions act as a second messenger for the a-receptor-mediated effects of adrenaline. In most tissues there are more than a receptors but the ratio varies with both the tissue and the species of animal and also with age. The significance of such variation is not understood. 

We have already seen how the two products of phosphatidylinositol bisphosphate hydrolysis can interact through their cellular effects. A good example is liver, where the inositol 1,4,5-triphosphate/Ca pathway controls the activity of phosphorylase kinase whereas the diacylglycerol/ protein kinase C pathway switches off glycogen synthetase. 

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