Cyclic AMP-dependent protein kinase A

Protein kinase A (PKA) is a cyclic AMP-dependent protein kinase, a member of a family of protein kinases that are activated by binding of cAMP to their two regulatory subunits, which results in the release of two active catalytic subunits. Targets of PKA include L-type calcium channels (the relevant subunit and site of phosphorylation is still uncertain), phospholam-ban (the regulator of the sarcoplasmic calcium ATPase, SERCA) and key enzymes of glucose and lipid metabolism. 

Figure 14-2. Regulation of cyclic AMP-dependent protein kinase A (PKA) by cyclic AMP. Activation of adenylate cyclase by binding of G( -GTP amplifies the signal by synthesis of many molecules of cyclic AMP. Cyclic AMP binding to PKA causes dissociation of the regulatory subunits from the catalytic subunits, which carry on the signal. Phosphodiesterase regulates the concentration of cyclic AMP by catalyzing its hydrolysis to AMP, which shuts off the signal.

Although the primary mode of action of ACTH is via cyclic AMP and cyclic AMP-dependent protein kinase (A-kinase), calcium probably plays a secondary role in enhancing and modulating the action of ACTH via the A-kinase pathway. This is discussed later in the consideration of the interaction of the A-kinase pathway of activation by ACTH and other second messenger systems involving calcium and protein kinase C. 

Figure 5.3 shows how the second messenger, cAMP, formed fi om ATP by adenylyl cyclase, transmits the signal by activating a second-messenger-dependent serine/ threonine protein kinase, the cyclic AMP-dependent protein kinase A (PKA). 

Steinberg, R. A. A kinase-negative mutant of s49 mouse lymphoma cells is defective in posttranslational maturation of catalytic subunitof cyclic amp-dependent protein kinase. Mol. Cell Biol. 11 (1991) 705-712. 

A number of kinase structures have been determined in various catalytic states. For example, structures of the cyclin-dependent kinase, CDK2, in its inactive state and in a partially active state after cyclin binding have been discussed in Chapter 6. The most thoroughly studied kinase is the cyclic AMP-dependent protein kinase the structure of both the inactive and the active... 

Cyclic AMP-dependent protein kinase is shown complexed with a pseudosubstrate peptide (red). This complex also includes ATP (yellow) and two Mn ions (violet) bound at the active site. 

FIGURE 15.7 Cyclic AMP-dependent protein kinase (also known as PKA) is a 150- to l70-kD R9C9 tetramer in mammalian cells. The two R (regulatory) subunits bind cAMP ( = 3 X 10 M) cAMP binding releases the R subunits from the C (catalytic) subunits. C subunits are enzymatically active as monomers. 

AKAPs are cyclic AMP-dependent protein kinase (PKA)-anchoring proteins, a family of about 30 proteins anchoring PKA at subcellular sites in close vicinity to a certain substrate. 

Sharma, R. and Wang, J. H. Differential regulation of bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase isozzymesdby cyclic AMP-dependent protein kinase and calmodulin-dependent protein phosphatase. Proc. Natl Acad. Sci. U.S.A. 82 2603-2607,1986. 

Couve, A., Thomas, P, Calver, A. R., et al. (2002) Cyclic AMP-dependent protein kinase phosphorylation facilitates GABAb receptor-effector coupling. Nat. Neurosci. 25,25. 

Roth, N. S., Campbell, P. T., Caron, M. G., Lefkowitz, R. J., and Lohse, M. J. (1991) Comparative rates of desensitization of beta-adrenergic receptors by the beta-adrenergic receptor kinase and the cyclic AMP-dependent protein kinase. Proc. Natl. Acad. Sci. U. S. A. 88, 6201-6204. 

Cyclic AMP-dependent protein kinase (protein kinase-A). 

Adrenaline increases the rate of gluconeogenesis it binds to the a-receptor on the surface of the liver cell, which results in an increase in cytosolic concentration of Ca " ions (Chapter 12). This increases the activity of the Ca " -catmodulin-dependent protein kinase which phosphory-lates and causes similar changes in the activities of the enzymes PFK-2 and pyruvate kinase to those resulting from activation of cyclic-AMP-dependent protein kinase. Hence Ca " ions increase the rate of gluconeogenesis. 

Figure 7.15 Inhibition of acetyl-CoA carboxylase by cyclic AMP dependent protein kinase and AMP dependent protein kinase the dual effect of glucagon. Phosphorylation of acetyl-CoA carboxylase by either or both enzymes inactivates the enzyme which leads to a decrease in concentration of malonyl-CoA, and hence an increase in activity of carnitine palmitoyltransferase-I and hence an increase in fatty acid oxidation. Insulin decreases the cyclic AMP concentration maintaining an active carboxylase and a high level of malonyl-CoA to inhibit fatty acid oxidation.

It is instructive to note that the biochemistry of the reactions that initiate the visual cascade and the glycogenolytic cascade is similar. The cyclic AMP-dependent protein kinase complex comprises the regulatory and catalytic components (R and C) for which the regulatory signal is the concentration of cyclic AMP. This binds to the regulatory component of the kinase (the R subunit) which then dissociates from the R-C complex. The C is now catalyti-cally active and catalyses the initial reaction in a cascade sequence which leads to activation of the target protein (phosphorylase). 

Cyclic AMP (adenosine 3, 5 -cyclic monophosphate) is anotter secondary messenger that acts as an intracellular mediator for many different hormones, communicating the signal through the cyclic AMP-dependent protein kinase. This, in turn, phosphorylates other proteins at ine and threonine residues. Certain cell-surfece receptors act by increasing the concentration of intracellular cyclic AMP. A long-duration sudden increase of intracellular cyclic AMP takes place with cholera toxins in intestinal epithelial cells. Other cell-surfece receptors play the opposite role of decreasing the concentration of cyclic AMP. 

Nemeroff CB, Krishnan KRR, Reed D, et al Adrenal gland enlargement in major depression a computed tomographic study. Arch Gen Psychiatry 49 384-387, 1992 Nestler EJ, Terwilliger RZ, Duman RS Chronic antidepressant administration alters the subcellular distribution of cyclic AMP-dependent protein kinase in rat frontal cortex. J Neurochem 53 1644-1647, 1989 Nestor PC, Parasuraman R, Haxby JV, et al Divided attention and metabolic brain dysfunction in mild dementia of the Alzheimer s type. Neuropsychologia 29(5) 379-387, 1991... 

Lent, B.A. Kim, K.-H. Phosphorylation and activation of acetyl-coenzyme A Carboxylase kinase by the catalytic subunit of cyclic AMP-dependent protein kinase. Arch. Biochem. Biophys., 225, 972-978 (1983)... 

Leonard, A. S. and Hell, J. W. Cyclic AMP-dependent protein kinase and protein kinase C phosphorylate N-methyl-D-aspartate receptors at different sites, J. Biol. Chem. 1997, 272, 12107-12115. 

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. 

The yeast enzyme is a homodimer of Mr2 X 102 500 and has 49% sequence identity to the muscle enzyme. The yeast enzyme is more simply regulated feedback inhibition by the allosteric inhibitor glucose-6-phosphate and activation by a 3 -5 -cyclic AMP-dependent protein kinase or a yeast phosphorylase that phosphorylates Thr-10.55... 

Yates A. J., Walters J. D., Wood C. F., and Johnson J. D. (1989). Ganglioside modulation of cyclic AMP-dependent protein kinase and cyclic nucleotide phosphodiesterase in vitro. J. Neurochem. 

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