Protein kinase A

CAMP-dependent protein kinase
Identifiers
EC number	2.7.11.11
CAS number	Template:CAS
Databases
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BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
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PDB structures	RCSB PDB PDBe PDBsum
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In cell biology, Protein kinase A (PKA) is a family of enzymes whose activity is dependent on cellular levels of cyclic AMP (cAMP). PKA is also known as cAMP-dependent protein kinase (EC 2.7.11.11). Protein kinase A has several functions in the cell, including regulation of glycogen, sugar, and lipid metabolism.

It should not be confused with AMP-activated protein kinase - which, although being of similar nature, may have opposite effects - nor be confused with cyclin-dependent kinases (Cdks), nor be confused with the acid dissociation constant pKa.

Mechanism

Overview: Activation and inactivation mechanisms of PKA

Activation

The PKA enzyme is also known as cAMP-dependent enzyme because it gets activated only if cAMP is present. Hormones such as glucagon and epinephrine begin the activation cascade (that triggers protein kinase A) by binding to a G protein-coupled receptor (GPCR) on the target cell. When a GPCR is activated by its extracellular ligand, a conformational change is induced in the receptor that is transmitted to an attached intracellular heterotrimeric G protein complex. The Gs alpha subunit of the stimulated G protein complex exchanges GDP for GTP and is released from the complex. The activated Gs alpha subunit binds to and activates an enzyme called adenylyl cyclase, which, in turn, catalyzes the conversion of ATP into cyclic adenosine monophosphate (cAMP) -- increasing cAMP levels. Four cAMP molecules are required to activate a single PKA enzyme. This is done by two cAMP molecules binding to each of the two regulatory subunits on a PKA enzyme causing the subunits to detach exposing the two (now activated) catalytic subunits. Next the catalytic subunits can go on to phosphorylate other proteins.

Below is a list of the steps involved in PKA activation:

1. Cytosolic cAMP increases

2. Two cAMP molecules bind to each PKA regulatory subunit

3. The regulatory subunits move out of the active sites of the catalytic subunits and the R2C2 complex dissociates

4. The free catalytic subunits interact with proteins to phosphorylate Ser or Thr residues.

Catalysis

The free catalytic subunits can then catalyze 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 regulation 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. In general, this mechanism takes more time (hours to days).

Inactivation

cAMP

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

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

Anchorage

The regulatory subunit dimer of PKA is important for localizing the kinase inside the cell. The dimerization and docking (D/D) domain of the dimer binds to the A-kinase binding (AKB) domain of A-kinase anchor protein (AKAP). The AKAPs localize PKA to various locations (e.g., plasma membrane, mitochondria, etc.) within the cell.

AKAPs bind many other signaling proteins, creating a very efficient signaling hub at a certain location within the cell. For example, an AKAP located near the nucleus of a heart muscle cell would bind both PKA and phosphodiesterase (hydrolyzes cAMP), which allows the cell to limit the productivity of PKA, since the catalytic subunit is activated once cAMP binds to the regulatory subunits.

Function

PKA phosphorylates proteins that have the motif Arginine-Arginine-X-Serine exposed, in turn (de)activating the proteins. As protein expression varies from cell type to cell type, the proteins that are available for phosphorylation will depend upon the cell in which PKA is present. Thus, the effects of PKA activation vary 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		produce glucose stimulate glycogenolysis phosphorylate glycogen phosphorylase via phosphorylase kinase (activating it) phosphorylate Acetyl-CoA carboxylase (inhibiting it) inhibit glycogenesis phosphorylate glycogen synthase (inhibiting it) stimulate glycolysis phosphorylate phosphofructokinase 2 (stimulating it, cardiomyocytes only)
myocyte (cardiac muscle)	cardiovascular	norepinephrine --> β-adrenergic receptor		sequester Ca2+ in sarcoplasmic reticulum phosphorylates phospholamban
myocyte (smooth muscle)	cardiovascular	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
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 phosphofructokinase-2 (inactivating it) phosphorylate fructose 2,6-bisphosphatase (stimulate it) phosphorylate pyruvate kinase (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)
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 and hepatocytes

Adrenaline 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. For example, protein kinase A phosphorylates acetyl-CoA carboxylase and pyruvate dehydrogenase. Such covalent modification has an inhibitory effect on these enzymes, thus inhibiting lipogenesis and promoting net gluconeogenesis. Insulin, on the other hand, decreases the level of phosphorylation of these enzymes, which instead promotes lipogenesis. Recall that gluconeogenesis does not occur in myocytes.

In nucleus accumbens neurons

PKA helps transfer/translate the dopamine signal into cells. In the nucleus accumbens, which mediates reward, motivation, and task salience. The vast majority of reward perception involves neuronal activation in the nucleus accumbens, some examples of which include sex, recreational drugs, and food.

See also

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

External links

• Cyclic AMP-Dependent Protein Kinases at the US National Library of Medicine Medical Subject Headings (MeSH)
• Drosophila cAMP-dependent protein kinase 1 - The Interactive Fly
• cAMP-dependent protein kinase: PDB Molecule of the Month