Protein Kinase A

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The title of this article should be cAMP-dependent protein kinase. The initial letter is capitalized due to technical restrictions.

In cell biology, protein kinase A (PKA, also known as cAMP-dependent protein kinase (cAPK) EC 2.7.11.11), refers to a family of enzymes whose activity is dependent on the level of cyclic AMP (cAMP) in the cell. Protein kinase A has several functions in the cell, including regulation of glycogen, sugar, and lipid metabolism.

Mechanism

Activation

Each PKA is a holoenzyme that consists of two regulatory and two catalytic subunits. Under low levels of cAMP, the holoenzyme remains intact and is catalytically inactive. When the concentration of cAMP rises (e.g. activation of adenylate cyclases by G protein-coupled receptors coupled to G_s, inhibition of phosphodiesterases which degrade cAMP), cAMP binds to the two binding sites on the regulatory subunits, which then undergo a conformational change that releases the catalytic subunits.

Catalyzation

The free catalytic subunits can then catalyse the transfer of ATP terminal phosphates to protein substrates at serine, or threonine residues. This phosphorylation usually results in a change in activity of the substrate. Since PKAs are present in a variety of cells and act on different substrates, PKA and cAMP regulation are involved in many different pathways.

The mechanisms of further effects may be divided into direct protein phosphorylation and protein synthesis:

• In direct protein phosphorylation PKA directly either increases or decreases the activity of a protein.
• In protein synthesis PKA first directly activates CREB, which binds the cAMP response element, altering the transcription and therefore the synthesis of the protein. This mechanism generally takes longer time (hours to days).

Inactivation

PKA is thus controlled by cAMP. Also, the catalytic subunit itself can be regulated by phosphorylation.

Downregulation of protein kinase A occurs by a feedback mechanism: one of the substrates that is activated by the kinase is a phosphodiesterase, which converts quickly cAMP to AMP, thus reducing the amount of cAMP that can activate protein kinase A.

Function

PKA phosphorylates other proteins, altering their function. However, what proteins are available for phosphorylation depends on in what kind of cell the PKA activity is present, since protein composition varies from cell type to cell type. Thus, the effects of PKA varies with cell type:

Overview table

Cell type	Organ/system	Stimulators ligands --> Gs-GPCRs or PDE inhibitors	Inhibitors ligands --> Gi-GPCRs or PDE stimulators	Effects
adipocyte		epinephrine --> β-adrenergic receptor glucagon --> Glucagon receptor		enhance lipolysis stimulate lipase
myocyte (skeletal muscle)	muscular system	epinephrine --> β-adrenergic receptor glucagon --> Glucagon receptor		produce glucose stimulate glycogenolysis phosphorylate glycogen phosphorylase (activating it) phosphorylate Acetyl-CoA carboxylase (inhibiting it) inhibit glycogenesis phosphorylate glycogen synthase (inhibiting it) stimulate gluconeogenesis phosphorylate fructose 2,6-bisphosphatase (stimulating it) inhibit glycolysis phosphorylate pyruvate dehydrogenase (inhibiting it).
hepatocyte	liver	epinephrine --> β-adrenergic receptor glucagon --> Glucagon receptor		produce glucose stimulate glycogenolysis phosphorylate glycogen phosphorylase (activating it) phosphorylate Acetyl-CoA carboxylase (inhibiting it) inhibit glycogenesis phosphorylate glycogen synthase (inhibiting it) stimulate gluconeogenesis phosphorylate fructose 2,6-bisphosphatase (stimulating it) inhibit glycolysis phosphorylate pyruvate dehydrogenase (inhibiting it).
neurons in nucleus accumbens	nervous system	dopamine --> dopamine receptor		Activate reward system
principal cells in kidney	kidney	Vasopressin --> V2 receptor theophylline (PDE inhibitor)		exocytosis of aquaporin 2 to apical membrane. synthesis of aquaporin 2 phosphorylation of aquaporin 2 (stimulating it)
myocyte (smooth muscle)	muscular system	β2 adrenergic agonists --> β-2 adrenergic receptor histamine --> Histamine H2 receptor prostacyclin --> prostacyclin receptor Prostaglandin D2 --> PGD2 receptor Prostaglandin E2 --> PGE2 receptor VIP --> VIP receptor L-Arginine --> imidazoline and α-2 receptor? (Gi-coupled)	muscarinic agonists, e.g. acetylcholine --> muscarinic receptor M2 NPY --> NPY receptor	Vasodilation
Thick ascending limb cell	kidney	Vasopressin --> V2 receptor		stimulate Na-K-2Cl symporter (perhaps only minor effect)
Cortical collecting tubule cell	kidney	Vasopressin --> V2 receptor		stimulate Epithelial sodium channel (perhaps only minor effect)
Inner medullary collecting duct cell	kidney	Vasopressin --> V2 receptor		stimulate urea transporter 1 urea transporter 1 exocytosis
proximal convoluted tubule cell	kidney	PTH --> PTH receptor 1		Inhibit NHE3 --> ↓H+ secretion
juxtaglomerular cell	kidney	adrenergic agonists --> β-receptor agonists --> A2 receptor dopamine --> dopamine receptor glucagon --> glucagon receptor		renin secretion

In adipocytes, myocytes and hepatocytes

Epinephrine and glucagon affect the activity of protein kinase A by changing the levels of cAMP in a cell via the G-protein mechanism, using adenylate cyclase. Protein Kinase A acts to phosphorylate many enzymes important in metabolism. Protein kinase A phosphorylates Acetyl-CoA carboxylase and pyruvate dehydrogenase. Allosteric regulation of these enzymes in such a manner has an inhibitory effect. Insulin will increase the level of phosphorylation of these enzymes, which will divert acetyl-coA down the lipogenesis pathway. Glucagon has an antagonistic effect.

In nucleus accumbens neurons

PKA helps transfer/translate the dopamine signal into cells. It has been found (postmortem) to be elevated in the brains of smokers, in the nucleus accumbens, which mediates reward and motivation: a part of the brain acted on by "virtually all" recreational drugs; as well as "in the area of the midbrain that responds to dopamine, which acts as a 'reward chemical' in smokers and former smokers."

See also

• Protein kinase
• Signal transduction
• G protein-coupled receptor
• Serine/threonine-specific protein kinase
• Myosin light-chain kinase

External links

• MeSH Cyclic+AMP-Dependent+Protein+Kinases