Protein kinase A

In cell biology, Protein kinase A (PKA^{[N 1]}) 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 -^[1] 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.^[2]

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: One of the substrates that are activated by the kinase is a phosphodiesterase, which 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

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

References

1. ^ Hallows KR, Alzamora R, Li H, et al. (April 2009). "AMP-activated protein kinase inhibits alkaline pH- and PKA-induced apical vacuolar H+-ATPase accumulation in epididymal clear cells". Am. J. Physiol., Cell Physiol. 296 (4): C672–81.  
2. ^ Voet, Voet & Pratt (2006). Fundamentals of Biochemistry. Wiley. Pg 492
3. ^ ^{a b c d e} Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone.   Page 172
4. ^ Rodriguez P, Kranias EG. (December 2005). "Phospholamban: a key determinant of cardiac function and dysfunction.". Arch Mal Coeur Vaiss 98 (12): 1239–43.  
5. ^ ^{a b c d e} Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders.   Page 842
6. ^ Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders.   Page 844
7. ^ Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders.   Page 852
8. ^ ^{a b c d} Walter F., PhD. Boron (2003). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. p. 1300.   Page 867

External links

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

Notes