Calcium phosphorylase kinase activity

Phosphorylase kinase activity has an absolute requirement for Ca +, which binds to the 5-subunit. The amino acid sequence of this subunit is nearly identical to that of calmodulin, with four calcium binding sites, but unlike calmodulin, the 5-subunit is an integral part of the enzyme and does not dissociate from it in the absence of Ca +. In the presence of Ca +, kinase activity is further increased by phosphorylation of the a- and j6-subunits, catalyzed by cAMP-dependent protein kinase and several other kinases. Phosphorylation may activate the enzyme by disinhibiting... 

SIGNAL INTEGRATION by phosphorylase kinase. Phosphorylase kinase eventually phosphorylates and activates glycogen phosphorylase. Either (or both) phosphorylation and calcium signaling pathways converge at phosphorylase kinase. 

Calcium ions released from the SR are required to initiate contraction and activate glycogen phosphorylase kinase... 

Correct answer = D. Ca2+ released from the sarcoplasmic reticulum during exercise binds to the calmodulin subunit of phosphorylase kinase, thereby activating this enzyme. The other choices are not caused by an elevation of cytosolic calcium. 

Some protein kinases are cAMP-independent and respond instead to increased free Ca2+ levels. Some of these kinases contain the calcium-binding subunit calmodulin. Binding of Ca2+ to calmodulin induces an allosteric activation analogous to the activation of protein kinases by cAMP. Some protein kinases, such as phosphorylase kinase, are regulated by both cAMP and Ca2+. 

Phosphorylase kinase is one of the best characterized enzyme systems to illustrate the role of calcium ions in regulation of intermediary metabolism. Phosphorylase kinase is composed of four different subunits termed a (Mr 145000), /3 (MT 128000), y (A/r 45000) and 5 (Mr 17000) and has the structure (a/3y8)A [106]. Only one of its four subunits actually catalyses the phosphorylation reaction the other three subunits are regulatory and enable the enzyme complex to be activated both by calcium and cyclic AMP. The y subunit carries the catalytic activity the 8 subunit is the calcium binding protein calmodulin and is responsible for the calcium dependence of the enzyme. The a and /3 subunits are the targets for cyclic-AMP mediated regulation, both being phosphorylated by the cyclic-AMP dependent protein kinase. Calmodulin appears to interact with phosphorylase kinase in a different manner from other enzymes, since it is an integral component of the enzyme. Phosphorylase kinase has an absolute requirement for calcium, and is inactive in its absence. 

Since phosphorylase kinase not only activates phosphorylase, but also phospho-rylates glycogen synthase thereby decreasing its activity, the regulation of phosphorylase kinase by calcium may also provide a mechanism for co-ordinating the rates of glycogenolysis and glycogen synthesis during muscle contraction. 

Ninety-nine percent of the calcinm and 85% of the phosphoms in the body are fonnd in bones, mostly a poorly crystalline hydroxyapatite Caio(P04)6(OH)2. In addition, calcium is involved in blood coagulation and is an intermediary factor between impulses and muscle contraction. Calcium has an activating effect on enzymes such as phosphorylase kinase and pyruvate dehydrogenase (lipoamide)-phosphatase. 

Phosphorylase Kinase Is Activated by Phosphorylation and Calcium Ions... 

Phosphorylase kinase can also be partly activated by Ca2+ levels of the order of 1 iM. Its 8 subunit is calmodulin, a calcium sensor that stimulates many enzymes in eukaryotes (Section 15.3.2). This mode of activation of the kinase is important in muscle, where contraction is triggered by the release of Ca2+ from the sarcoplasmic reticulum. Phosphorylase kinase attains maximal activity only after both phosphorylation of the P subunit and activation of the 8 subunit by Ca2+ binding. 

Finally, one does not always have a warning, that is tinit to get the endocrine system going to produce adrenalin, thus the release of Calcium in the muscle cells bypasses much of the cascade, activ ng the normally inactive o-Phosphorylase kinase b, which then acts on both the phosphorylase and the synthase. 

Initiation of muscle contraction by action potential from an a motor neuron depolarizes the muscle-cell membrane and causes an increase in the myoplasmic [Ca " ] (Chapter 21). The increase in calcium activates phosphorylase kinase and Ca +/calmoduIin-dependent kinase, which inactivate glycogen synthase and active glycogen phosphorylase (Figure 15-11). This step coordinates muscle contraction with glycogenolysis. These steps are reversed by one or more phosphatases at the end of contraction. 

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. 

Another advantage of a cascade is the possibility of intervening anywhere along the cascade, not only at the initiation. In the case of the phosphorylase cascade, phosphorylase kinase in the inactive (i.e., nonphosphorylated) form can be activated allosterically by calcium, thus causing phosphorylase kinase to catalyze the phosphorylation of phosphorylase to the active form, causing increased glycogenolysis. One of the subunits of phosphorylase kinase is a polypeptide known as calmodulin. This polypeptide occurs in many proteins that require or have Ca2+ as an effector. This polypeptide in phosphorylase kinase results in the ca2+ activation of this enzyme. The binding of Ca2+ by the calmodulin subunit of phosphorylase kinase also facilitates a more rapid phosphorylation of this enzyme by the cAMP-dependent protein kinase. 

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