| phospholipase A2 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 Phospholipase cleavage sites. Note that an enzyme that displays both PLA1 and PLA2 activities is called aphospholipase B. | |||||||||
| Identifiers | |||||||||
| EC no. | 3.1.1.4 | ||||||||
| CAS no. | 9001-84-7 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDBPDBePDBsum | ||||||||
| Gene Ontology | AmiGO /QuickGO | ||||||||
| |||||||||
| Phospholipase A2 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 Bee venom phospholipase A2 sPLA2.Middle plane of the lipid bilayer - black dots. Boundary of the hydrocarbon core region - red dots (extracellular side). Layer of lipid phosphates - yellow dots. | |||||||||
| Identifiers | |||||||||
| Symbol | Phospholip_A2_1 | ||||||||
| Pfam | PF00068 | ||||||||
| InterPro | IPR001211 | ||||||||
| PROSITE | PDOC00109 | ||||||||
| SCOP2 | 1bbc /SCOPe /SUPFAM | ||||||||
| OPM superfamily | 82 | ||||||||
| OPM protein | 1g4i | ||||||||
| |||||||||
The enzymephospholipase A2 (EC 3.1.1.4,PLA2, systematic name phosphatidylcholine 2-acylhydrolase) catalyses the cleavage offatty acids in position 2 ofphospholipids,hydrolyzing the bond between the second fatty acid "tail" and theglycerol molecule:
This particularphospholipase specifically recognizes thesn2acyl bond ofphospholipids and catalyticallyhydrolyzes the bond, releasingarachidonic acid andlysophosphatidyl choline, a precursor oflysophosphatidic acid. Upon downstream modification bycyclooxygenases orlipoxygenases,arachidonic acid is modified into active compounds calledeicosanoids. Eicosanoids includeprostaglandins andleukotrienes, which are categorized as anti-inflammatory and inflammatory mediators.[1]
PLA2 enzymes are commonly found in mammalian tissues as well as arachnid, insect, and snake venom.[2] Venom from bees is largely composed ofmelittin, which is a stimulant of PLA2. Due to the increased presence and activity of PLA2 resulting from a snake or insect bite,arachidonic acid is released from thephospholipid membrane disproportionately. As a result, inflammation and pain occur at the site.[3] There are alsoprokaryotic A2 phospholipases.
Additional types ofphospholipases includephospholipase A1,phospholipase B,phospholipase C, andphospholipase D.[4]
Phospholipases A2 include several unrelatedprotein families with common enzymatic activity. Two most notable families are secreted and cytosolic phospholipases A2. Other families include Ca2+ independent PLA2 (iPLA2) and lipoprotein-associated PLA2 (Lp-PLA2), also known as platelet activating factor acetylhydrolase (PAF-AH).
Theextracellular forms of phospholipases A2 have been isolated from differentvenoms (snake,[5]bee, andwasp), and from virtually every studiedmammaliantissue (includingpancreas andkidney) as well as frombacteria. They requireCa2+ for activity.
Pancreatic sPLA2 serve for the initialdigestion of phospholipid compounds in dietaryfat. Venom phospholipases help to immobilize prey by promoting celllysis.[citation needed]
In mice, group III sPLA2 are involved in sperm maturation,[6] and group X are thought to be involved in spermcapacitation.[7]
sPLA2 has been shown to promoteinflammation in mammals by catalyzing the first step of thearachidonic acid pathway by breaking downphospholipids, resulting in the formation offatty acids includingarachidonic acid. This arachidonic acid is then metabolized to form several inflammatory andthrombogenic molecules. Excess levels of sPLA2 is thought to contribute to severalinflammatory diseases, and has been shown to promote vascular inflammation correlating with coronary events incoronary artery disease andacute coronary syndrome,[8] and possibly leading toacute respiratory distress syndrome[9] and progression oftonsillitis.[10]
In children, excess levels of sPLA2 have been associated with inflammation thought to exacerbateasthma[11] and ocular surface inflammation (dry eye).[12]
Increased sPLA2 activity is observed in thecerebrospinal fluid of humans withAlzheimer's disease andmultiple sclerosis, and may serve as a marker of increases in permeability of theblood-cerebrospinal fluid barrier.[13]
There are atypical members of the phospholipase A2 family, such asPLA2G12B, that have no phospholipase activity with typical phospholipase substrate.[14] The lack of enzymatic activity ofPLA2G12B indicates that it may have unique function distinctive from other sPLA2. It has been shown that inPLA2G12B null mice, VLDL levels were greatly reduced, suggesting it could have an effect in lipoprotein secretion.[15][16]
Theintracellular, group IV PLA2 are also Ca-dependent, but they have a different3D structure and are significantly larger than secreted PLA2 (more than 700 residues). They include aC2 domain and a large catalytic domain.
These phospholipases are involved incell signaling processes, such asinflammatory response. They releasearachidonic acid from membrane phospholipids. Arachidonic acid is both asignaling molecule and the precursor for the synthesis of other signaling molecules termedeicosanoids. These includeleukotrienes andprostaglandins. Some eicosanoids are synthesized fromdiacylglycerol, released from thelipid bilayer by phospholipase C (see below).
Phospholipases A2 can be classified based on sequence homology.[17]
Increased levels of Lp-PLA2 are associated with cardiac disease, and may contribute toatherosclerosis.[18] However, the role of Lp-PLA2 in atherosclerosis may depend on its carrier in plasma, and several lines of evidence suggest that HDL-associated Lp-PLA2 may substantially contribute to the HDL antiatherogenic activities.[19]
Two reaction mechanisms have been suggested for sPLA2s, the single-water mechanism and the assisted water mechanism.[20] The single-watercatalytic mechanism is initiated by a His-48/Asp-99/calcium complex within the active site. The calcium ion polarizes the sn-2 carbonyl oxygen. Whether it can simultaneously coordinate with thecatalytic water molecule, w5, is unclear. His-48 improves thenucleophilicity of thecatalytic water via a bridging second water molecule, w6. In the assisted-water mechanism, two water molecules are necessary to traverse the distance between the catalytichistidine and theester. Thebasicity of His-48 is thought to be enhanced throughhydrogen bonding with Asp-99. Anasparagine substitution for His-48 maintains wild-type activity, as theamide functional group onasparagine can also function to lower the pKa, oracid dissociation constant, of the bridging water molecule. computer simulations of the reaction mechanism of the human synovial and viper venom sPLA2s suggest that both the single-water and assisted-water mechanisms are viable,[21][22] as the difference in the free energy of the rate-limiting step is small. The latter is characterized as the degradation of the tetrahedral intermediate composed of a calcium coordinatedoxyanion. The role of calcium can also be duplicated by other relatively small cations like cobalt and nickel.[23] Before becoming active in digestion, the proform of PLA2 is activated by trypsin.
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PLA2 can also be characterized as having a channel featuring ahydrophobic wall in which hydrophobic amino acid residues such asPhe,Leu, andTyr serve to bind the substrate. Another component of PLA2 is the sevendisulfide bridges that are influential in regulation and stable protein folding.[23]
PLA2 action can release histamine from rat peritoneal mast cells.[25] It also causes histamine release in human basophils.[26]
Due to the importance of PLA2 ininflammatory responses, regulation of the enzyme is essential. cPLA2 is regulated byphosphorylation and calcium concentrations. cPLA2 is phosphorylated by aMAPK atSerine-505. When phosphorylation is coupled with an influx of calcium ions, cPLA2 becomes stimulated and can translocate to the membrane to begincatalysis.[27]
Phosphorylation of cPLA2 may be a result of ligand binding to receptors, including:
In the case of an inflammation, the application of glucocorticoids up-regulate (mediated at the gene level) the production of the proteinlipocortin which may inhibit cPLA2 and reduce the inflammatory response.
In normal brain cells, PLA2 regulation accounts for a balance betweenarachidonic acid's conversion into proinflammatory mediators and its reincorporation into the membrane. In the absence of strict regulation of PLA2 activity, a disproportionate amount of proinflammatory mediators are produced. The resulting inducedoxidative stress and neuroinflammation is analogous to neurological diseases such asAlzheimer's disease,epilepsy,multiple sclerosis,ischemia.Lysophospholipids are another class of molecules released from the membrane that are upstream predecessors ofplatelet activating factors (PAF). Abnormal levels of potent PAF are also associated with neurological damage. An optimalenzyme inhibitor would specifically target PLA2 activity on neural cell membranes already underoxidative stress and potentinflammation. Thus, specific inhibitors of brain PLA2 could be a pharmaceutical approach to treatment of several disorders associated with neural trauma.[29]
Increase in phospholipase A2 activity is anacute-phase reaction that rises during inflammation, which is also seen to be exponentially higher in low backdisc herniations compared torheumatoid arthritis.[citation needed] It is a mixture of inflammation andsubstance P that are responsible for pain.[citation needed]
Increased phospholipase A2 has also been associated with neuropsychiatric disorders such asschizophrenia andpervasive developmental disorders (such asautism), though the mechanisms involved are not known.[30][31]
Human phospholipase A2isozymes include:
In addition, the following human proteins contain the phospholipase A2domain:
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