In cell biology, protein kinase A (PKA) is a household of enzymes whose exercise depends on mobile ranges of cyclic AMP (cAMP). PKA is also called cAMP-dependent protein kinase (EC 2.7.11.11). PKA has a number of features within the cell, together with regulation of glycogen, sugar, and lipid metabolism.
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Historical past[edit]
Protein kinase A, extra exactly referred to as adenosine 3′,5′-monophosphate (cyclic AMP)-dependent protein kinase, abbreviated to PKA, was found by chemists Edmond H. Fischer and Edwin G. Krebs in 1968. They received the Nobel Prize in Physiology or Medication in 1992 for his or her work on phosphorylation and dephosphorylation and the way it pertains to PKA exercise.[1]
PKA is likely one of the most generally researched protein kinases, partly due to its uniqueness; out of 540 completely different protein kinase genes that make up for human kinome, just one different protein kinase, casein kinase 2, is thought to exist in a physiological tetrameric advanced, which means it’s made up of 4 subunits.[2]
The variety of mammalian PKA subunits was realized after Dr. Stan Knight and others recognized 4 doable catalytic subunit genes and 4 regulatory subunit genes. In 1991, Susan Taylor and colleagues crystallized the PKA Cα subunit, which revealed the bi-lobe construction of the protein kinase core for the very first time, offering a blueprint for all the opposite protein kinases in a genome (the kinome).[3]
Construction[edit]
When inactive, the PKA holoenzyme exists as a tetramer which consists of two regulatory subunits and two catalytic subunits. The catalytic subunit incorporates the energetic website, a sequence of canonical residues present in protein kinases that bind and hydrolyse ATP, and a site to bind the regulatory subunit. The regulatory subunit has domains to bind to cyclic AMP, a site that interacts with catalytic subunit, and an auto inhibitory area. There are two main types of regulatory subunit; RI and RII.[4]
Mammalian cells have at the least two varieties of PKAs: kind I is principally within the cytosol, whereas kind II is sure by way of its regulatory subunits and particular anchoring proteins, described within the anchorage part, to the plasma membrane, nuclear membrane, mitochondrial outer membrane, and microtubules. In each varieties, as soon as the catalytic subunits are freed and energetic, they will migrate into the nucleus (the place they will phosphorylate transcription regulatory proteins), whereas the regulatory subunits stay within the cytoplasm.[5]
The next human genes encode PKA subunits:
Mechanism[edit]
Activation[edit]
PKA can also be generally referred to as cAMP-dependent protein kinase, as a result of it has historically been regarded as activated by means of launch of the catalytic subunits when ranges of the second messenger known as cyclic adenosine monophosphate, or cAMP, rise in response to quite a lot of indicators. Nevertheless, latest research evaluating the intact holoenzyme complexes, together with regulatory AKAP-bound signalling complexes, have steered that the native sub mobile activation of the catalytic exercise of PKA would possibly proceed with out bodily separation of the regulatory and catalytic elements, particularly at physiological concentrations of cAMP.[6][7] In distinction, experimentally induced supra physiological concentrations of cAMP, which means greater than usually noticed in cells, are capable of trigger separation of the holoenzymes, and launch of the catalytic subunits.[6]
Extracellular hormones, similar to glucagon and epinephrine, start an intracellular signalling cascade that triggers protein kinase A activation by first binding to a G protein–coupled receptor (GPCR) on the goal cell. When a GPCR is activated by its extracellular ligand, a conformational change is induced within the receptor that’s transmitted to an hooked up intracellular heterotrimeric G protein advanced by protein area dynamics. The Gs alpha subunit of the stimulated G protein advanced exchanges GDP for GTP in a response catalyzed by the GPCR and is launched from the advanced. The activated Gs alpha subunit binds to and prompts an enzyme known as adenylyl cyclase, which, in flip, catalyzes the conversion of ATP into cAMP, immediately rising the cAMP stage. 4 cAMP molecules are capable of bind to the 2 regulatory subunits. That is executed by two cAMP molecules binding to every of the 2 cAMP binding websites (CNB-B and CNB-A) which induces a conformational change within the regulatory subunits of PKA, inflicting the subunits to detach and unleash the 2, now activated, catalytic subunits.[8]
As soon as launched from inhibitory regulatory subunit, the catalytic subunits can go on to phosphorylate quite a lot of different proteins within the minimal substrate context Arg-Arg-X-Ser/Thr.,[9] though they’re nonetheless topic to different layers of regulation, together with modulation by the warmth secure pseudosubstrate inhibitor of PKA, termed PKI.[7][10]
Under is an inventory of the steps concerned in PKA activation:
Catalysis[edit]
The liberated catalytic subunits can then catalyze the switch of ATP terminal phosphates to protein substrates at serine, or threonine residues. This phosphorylation often ends in a change in exercise of the substrate. Since PKAs are current in quite a lot of cells and act on completely different substrates, PKA regulation and cAMP regulation are concerned in many various pathways.
The mechanisms of additional results could also be divided into direct protein phosphorylation and protein synthesis:
Phosphorylation mechanism[edit]
The Serine/Threonine residue of the substrate peptide is oriented in such a means that the hydroxyl group faces the gamma phosphate group of the sure ATP molecule. Each the substrate, ATP, and two Mg2+ ions type intensive contacts with the catalytic subunit of PKA. Within the energetic conformation, the C helix packs in opposition to the N-terminal lobe and the Aspartate residue of the conserved DFG motif chelates the Mg2+ ions, aiding in positioning the ATP substrate. The triphosphate group of ATP factors out of the adenosine pocket for the switch of gamma-phosphate to the Serine/Threonine of the peptide substrate. There are a number of conserved residues, embody Glutamate (E) 91 and Lysine (Ok) 72, that mediate the positioning of alpha- and beta-phosphate teams. The hydroxyl group of the peptide substrate’s Serine/Threonine assaults the gamma phosphate group on the phosphorus by way of an SN2 nucleophilic response, which leads to the switch of the terminal phosphate to the peptide substrate and cleavage of the phosphodiester bond between the beta-phosphate and the gamma-phosphate teams. PKA acts as a mannequin for understanding protein kinase biology, with the place of the conserved residues serving to to tell apart the energetic protein kinase and inactive pseudokinase members of the human kinome.
Inactivation[edit]
Downregulation of protein kinase A happens by a suggestions mechanism and makes use of quite a lot of cAMP hydrolyzing phosphodiesterase (PDE) enzymes, which belong to the substrates activated by PKA. Phosphodiesterase rapidly converts cAMP to AMP, thus lowering the quantity of cAMP that may activate protein kinase A. PKA can also be regulated by a posh sequence of phosphorylation occasions, which might embody modification by autophosphorylation and phosphorylation by regulatory kinases, similar to PDK1.[7]
Thus, PKA is managed, partly, by the degrees cAMP. Additionally, the catalytic subunit itself may be down-regulated by phosphorylation.
Anchorage[edit]
The regulatory subunit dimer of PKA is necessary for localizing the kinase contained in the cell. The dimerization and docking (D/D) area of the dimer binds to the A-kinase binding (AKB) area of A-kinase anchor protein (AKAP). The AKAPs localize PKA to varied places (e.g., plasma membrane, mitochondria, and many others.) throughout the cell.
AKAPs bind many different signaling proteins, creating a really environment friendly signaling hub at a sure location throughout the cell. For instance, an AKAP situated close to the nucleus of a coronary heart muscle cell would bind each PKA and phosphodiesterase (hydrolyzes cAMP), which permits the cell to restrict the productiveness of PKA, for the reason that catalytic subunit is activated as soon as cAMP binds to the regulatory subunits.
Operate[edit] – “protein kinase a”
PKA phosphorylates proteins which have the motif Arginine-Arginine-X-Serine uncovered, in flip (de)activating the proteins. Many doable substrates of PKA exist; an inventory of such substrates is on the market and maintained by the NIH.[11]
As protein expression varies from cell kind to cell kind, the proteins which might be obtainable for phosphorylation will depend on the cell wherein PKA is current. Thus, the consequences of PKA activation range with cell kind:
Overview desk[edit]
In adipocytes and hepatocytes[edit]
Epinephrine and glucagon have an effect on the exercise of protein kinase A by altering the degrees of cAMP in a cell by way of the G-protein mechanism, utilizing adenylate cyclase. Protein kinase A acts to phosphorylate many enzymes necessary in metabolism. For instance, protein kinase A phosphorylates acetyl-CoA carboxylase and pyruvate dehydrogenase. Such covalent modification has an inhibitory impact on these enzymes, thus inhibiting lipogenesis and selling web gluconeogenesis. Insulin, then again, decreases the extent of phosphorylation of those enzymes, which as an alternative promotes lipogenesis. Recall that gluconeogenesis doesn’t happen in myocytes.
In nucleus accumbens neurons[edit]
PKA helps switch/translate the dopamine sign into cells within the nucleus accumbens, which mediates reward, motivation, and process salience. The overwhelming majority of reward notion entails neuronal activation within the nucleus accumbens, some examples of which embody intercourse, leisure medication, and meals. Protein Kinase A sign transduction pathway helps in modulation of ethanol consumption and its sedative results. A mouse research reviews that mice with genetically diminished cAMP-PKA signalling outcomes into much less consumption of ethanol and are extra delicate to its sedative results.[18]
In skeletal muscle[edit]
PKA is directed to particular sub-cellular places after tethering to AKAPs. Ryanodine receptor (RyR) co-localizes with the muscle AKAP and RyR phosphorylation and efflux of Ca2+ is elevated by localization of PKA at RyR by AKAPs.[19]
In cardiac muscle[edit]
In a cascade mediated by a GPCR referred to as β1 adrenoceptor, activated by catecholamines (notably norepinephrine), PKA will get activated and phosphorylates quite a few targets, particularly: L-type calcium channels, phospholamban, troponin I, myosin binding protein C, and potassium channels. This will increase inotropy in addition to lusitropy, rising contraction pressure in addition to enabling the muscle tissues to chill out sooner.[20][21]
In reminiscence formation[edit]
PKA has at all times been thought-about necessary in formation of a reminiscence. Within the fruit fly, reductions in expression exercise of DCO (PKA catalytic subunit encoding gene) could cause extreme studying disabilities, center time period reminiscence and quick time period reminiscence. Long run reminiscence depends on the CREB transcription issue, regulated by PKA. A research executed on drosophila reported that a rise in PKA exercise can have an effect on quick time period reminiscence. Nevertheless, a lower in PKA exercise by 24% inhibited studying talents and a lower by 16% affected each studying capability and reminiscence retention. Formation of a traditional reminiscence is extremely delicate to PKA ranges.[22]
See additionally[edit]
References[edit]
“protein kinase a”