Cyclic AMP activated protein

MLCK itself is phosphorylated by cyclic-AMP activated protein kinase, (protein kinase A) and cyclic-GMP activated protein kinase, (protein kinase G). Protein kinase A will phosphorylate MLCK at two sites and protein kinase G at one in some cases and two in others. These differences seem to be important in how the individual smooth muscle cells are regulated. 

Figure 6.34 Effects of glucagon and insulin on the cyclic AMP level. Glucagon increases the activity of adenylate cyclase, which increases the concentration of cyclic AMP whereas insulin activates the phosphodiesterase which hydrolyses cyclic AMP to form AMP. Cyclic AMP activates protein kinase A.

Figure 12.13 Action and effects of glucagon. Glucagon binds to its receptor on the plasma membrane of the liver which activates adenyl cyclase. The resultant cyclic AMP activates protein kinase which results in phosphorylation and activation of ...

D. hormone- sensitive lipase is phosphorylated by a cyclic AMP-activated protein kinase. 

FIGURE 26-1 Mechanism of action of beta agonists on respiratory smooth muscle. Beta agonists facilitate bronchodilation by stimulating adenyl cyclase activity, which in turn increases intracellular cyclic AMP production. Cyclic AMP activates protein kinase, which appears to add an inhibitory phosphate group to contractile proteins, thus causing muscle relaxation and bronchodilation. 

The G protein-GTP complexes related to receptors for these hormones activate adenylyl cyclase, which synthesizes the second messenger cAMP. Cyclic AMP activates protein kinases, which phosphorylate certain intracellular proteins (eg, enzymes), thus producing the hormonal effect. Conversely, dopamine binding to lactotroph receptors causes conformational changes in its G protein that reduce the activity of adenylyl cyclase and inhibit the secretion of prolactin. 

Cyclic AMP Activates Protein Kinase A by Altering the Quaternary Structure... 

The elevated cytosolic level of cyclic AMP activates protein kinase A through the binding of cyclic AMP to the regulatory subunits, which then dissociate from the catalytic subunits. The free catalytic subunits are now active. 

A partially purified cyclic GMP-activated protein kinase from lobster muscle was later found to have a for cyclic GMP of about 0.08 /x.M and for cyclic AMP of about 4 /aM. A cyclic AMP-activated protein kinase isolated from the same tissue had an apparent Ka for cyclic AMP of about 0.02 /J.M and for cyclic GMP of about 1.2 ptM [76]. Cyclic GMP-dependent phosphorylation of endogenous protein has been demonstrated in membranes of mammalian smooth muscle [78]. In the presence of 10 mM Mn, a half-maximal increase in the phosphorylation of these proteins occurred with 20-30 nM cyclic GMP, but ten-fold higher concentrations of cyclic AMP were required to produce the same increase in phosphorylation. 

A protein inhibitor of cyclic AMP-activated protein kinase has been isolated from mammalian tissue and this promotes a five-fold increase in the binding constant of cyclic AMP to protein kinase [79]. The inhibitor is assumed to interact with the catalytic subunit at the regulatory subunit binding site. Thus, inhibitor protein and regulatory subunit modulate the catalytic subunit activity by an identical mechanism. However, it may be more complex, as a protein kinase modulator protein has been isolated which alters the substrate specificity both of cyclic AMP and of cyclic GMP-activated protein kinases, increasing the phosphorylation of some protein substrates and decreasing that of others [80]. 

The method is based upon the ability of low concentrations of cyclic nucleotides to activate protein kinases which catalyse the phosphorylation of protein substrates, such as histone, by ATP [156,157]. The extent of phosphorylation is proportional to the amount of the cyclic nucleotides. The limits of sensitivity of the method are about 0.3 pmol for cyclic AMP and 0.5 pmol for cyclic GMP. Purification on a Dowex 50 column separates the two cyclic nucleotides from each other and removes any substances, such as ATP, which might interfere with the assay. Cyclic GMP is further purified by column chromatography on aluminum oxide and Dowex 1. Cyclic AMP-activated protein kinase is prepared from bovine heart and cyclic GMP-activated protein kinase from lobster tail. 

For the cyclic AMP assay [y- P]ATP (10 uM) is incubated with 10 mM magnesium acetate, cyclic AMP standard (0-10 pmol) or water, histone mixture, cyclic AMP-activated protein kinase, and 0.1 M sodium acetate buffer, pH 6.0, for 5 min at 30°C. The reaction is terminated by the addition of trichloroacetic-acid-tungstate-sulfuric acid, the precipitate is dissolved in N NaOH, and the radioactivity counted in scintillation fluid. 

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. 

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