In cell biology, protein kinase A (PKA) is a family of serine-threonine kinases 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). PKA has several functions in the cell, including regulation of glycogen, sugar, and lipid metabolism. It should not be confused with 5'-AMP-activated protein kinase (AMP-activated protein kinase).
History Protein kinase A, more precisely known as adenosine 3',5'-monophosphate (cyclic AMP)-dependent protein kinase, abbreviated to PKA, was discovered by chemists Edmond H. Fischer and Edwin G. Krebs in 1968. They won the Nobel Prize in Physiology or Medicine in 1992 for their work on phosphorylation and dephosphorylation and how it relates to PKA activity. PKA is one of the most widely researched protein kinases, in part because of its uniqueness; out of 540 different protein kinase genes that make up the human kinome, only one other protein kinase, casein kinase 2, is known to exist in a physiological tetrameric complex, meaning it consists of four subunits. The diversity of mammalian PKA subunits was realized after Dr. Stan McKnight and others identified four possible catalytic subunit genes and four regulatory subunit genes. In 1991, Susan Taylor and colleagues crystallized the PKA Cα subunit, which revealed the bi-lobe structure of the protein kinase core for the very first time, providing a blueprint for all the other protein kinases in a genome (the kinome).
Structure When inactive, the PKA apoenzyme exists as a tetramer which consists of two regulatory subunits and two catalytic subunits. The catalytic subunit contains the active site, a series of canonical residues found in protein kinases that bind and hydrolyse ATP, and a domain to bind the regulatory subunit. The regulatory subunit has domains to bind to cyclic AMP, a domain that interacts with catalytic subunit, and an auto inhibitory domain. There are two major forms of regulatory subunit; RI and RII. Mammalian cells have at least two types of PKAs: type I is mainly in the cytosol, whereas type II is bound via its regulatory subunits and special anchoring proteins, described in the anchorage section, to the plasma membrane, nuclear membrane, mitochondrial outer membrane, and microtubules. In both types, once the catalytic subunits are freed and active, they can migrate into the nucleus (where they can phosphorylate transcription regulatory proteins), while the regulatory subunits remain in the cytoplasm. The following human genes encode PKA subunits:
catalytic subunit – PRKACA, PRKACB, PRKACG regulatory subunit type I - PRKAR1A, PRKAR1B regulatory subunit type II - PRKAR2A, PRKAR2B
Mechanism
Activation PKA is also commonly known as cAMP-dependent protein kinase, because it has traditionally been thought to be activated through release of the catalytic subunits when levels of the second messenger called cyclic adenosine monophosphate, or cAMP, rise in response to a variety of signals. However, recent studies evaluating the intact holoenzyme complexes, including regulatory AKAP-bound signalling complexes, have suggested that the local sub cellular activation of the catalytic activity of PKA might proceed without physical separation of the regulatory and catalytic components, especially at physiological concentrations of cAMP. In contrast, experimentally induced supra physiological concentrations of cAMP, meaning higher than normally observed in cells, are able to cause separation of the holoenzymes, and release of the catalytic subunits. Extracellular hormones, such as glucagon and epinephrine, begin an intracellular signalling cascade that triggers protein kinase A activation by first 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 by protein domain dynamics. The Gs alpha subunit of the stimulated G protein complex exchanges GDP for GTP in a reaction catalyzed by the GPCR 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 cAMP, directly increasing the cAMP level. Four cAMP molecules are able to bind to the two regulatory subunits. This is done by two cAMP molecules binding to each of the two cAMP binding sites (CNB-B and CNB-A) which induces a conformational change in the regulatory subunits of PKA, causing the subunits to detach and unleash the two, now activated, catalytic subunits. Once released from inhibitory regulatory subunit, the catalytic subunits can go on to phosphorylate a number of other proteins in the minimal substrate context Arg-Arg-X-Ser/Thr., although they are still subject to other layers of regulation, including modulation by the heat stable pseudosubstrate inhibitor of PKA, termed PKI. Below is a list of the steps involved in PKA activation:
Cytosolic cAMP increases Two cAMP molecules bind to each PKA regulatory subunit The regulatory subunits move out of the active sites of the catalytic subunits and the R2C2 complex dissociates The free catalytic subunits interact with proteins to phosphorylate Ser or Thr residues.
Catalysis The liberated 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 (CRE), altering the transcription and therefore the synthesis of the protein. In general, this mechanism takes more time (hours to days).
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