Protein kinase A

In cell biology, protein kinase A refers to a family of enzymes whose activity is dependent on the level of cyclic AMP (cAMP) in the cell. PKA is also known as cAMP-dependent protein kinase ). 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 that degrade cAMP), cAMP binds to the two binding sites on the regulatory subunits, which leads to the release of the catalytic subunits.

Catalysis

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:

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 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.

Anchorage

The 2 regulatory subunits of protein kinase A is important for localizing the kinase inside the cell, with the aid of A-kinase anchoring protein (AKAP), AKAP binds both to the regulatory subunits and to either a component of cytoskeleton structure or a membrane of an organelle, anchoring the enzyme complex to a particular subcellular compartment.

The catalytic function of protein kinase A would sometimes couple with the AKAP, binding PKA together with phosphodiesterase to form a complex that functions as a signal module. For example, an AKAP locating near the nucleus of a heart muscle cell, would bind to both PKA and phosphodiesterase that hydrolyzes cAMP. As phosphodiesterase contributes to the steady low concentration of cAMP in unstimulated cells, as the cell is stimulated, PKA is then responsible for the activation of phosphodiesterase (adjacent to PKA) in order to lower the concentration of cAMP. In this condition, as PKA and phosphodiesterase have formed a complex, the proximity increases the efficiency of PKA's activity.

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 --> G_s-GPCRs or PDE inhibitors	Inhibitors ligands --> G_i-GPCRs or PDE stimulators	Effects
adipocyte		epinephrine --> ?-adrenergic receptor glucagon --> Glucagon receptor		enhance lipolysis stimulate lipase^[1]
myocyte (skeletal muscle)	muscular system	epinephrine --> ?-adrenergic receptor		produce glucose stimulate glycogenolysis phosphorylate glycogen phosphorylase (activating it)^[1] phosphorylate Acetyl-CoA carboxylase (inhibiting it) inhibit glycogenesis phosphorylate glycogen synthase (inhibiting it)^[1] stimulate gluconeogenesis phosphorylate fructose 2,6-bisphosphatase (stimulating it) inhibit glycolysis phosphorylate pyruvate dehydrogenase (inhibiting it).
hepatocyte	liver	epinephrine --> ?-adrenergic receptor glucagon --> Glucagon receptor		produce glucose stimulate glycogenolysis phosphorylate glycogen phosphorylase (activating it)^[1] phosphorylate Acetyl-CoA carboxylase (inhibiting it) inhibit glycogenesis phosphorylate glycogen synthase (inhibiting it)^[1] 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. ^[2] synthesis of aquaporin 2 ^[2] phosphorylation of aquaporin 2 (stimulating it)^[2]
myocyte (smooth muscle)	muscular system	?2 adrenergic agonists --> ?-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 ?-2 receptor? (G_i-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) ^[2]
Cortical collecting tubule cell	kidney	Vasopressin --> V2 receptor		stimulate Epithelial sodium channel (perhaps only minor effect) ^[2]
Inner medullary collecting duct cell	kidney	Vasopressin --> V2 receptor		stimulate urea transporter 1 urea transporter 1 exocytosis^[3]
proximal convoluted tubule cell	kidney	PTH --> PTH receptor 1		Inhibit NHE3 --> ↓H⁺ secretion^[4]
juxtaglomerular cell	kidney	adrenergic agonists --> ?-receptor^[5] agonists --> A2 receptor^[5] dopamine --> dopamine receptor^[5] glucagon --> glucagon receptor^[5]		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. For example, protein kinase A phosphorylates acetyl-CoA carboxylase and pyruvate dehydrogenase. Such allosteric regulation 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.

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." ^[6]