The enzyme phospholipase A₂ (EC 3.1.1.4, PLA₂, systematic name phosphatidylcholine 2-acylhydrolase) catalyses the cleavage of

fatty acid In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...

s in position 2 of

phospholipid Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

s, hydrolyzing the bond between the second fatty acid "tail" and the

glycerol Glycerol () is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pha ...

molecule: :phosphatidylcholine + H₂O = 1-acylglycerophosphocholine + a carboxylate This particular

phospholipase A phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C, and D, which are distinguished by the type of reaction which they catalyze: *Phospholipase ...

specifically recognizes the ''sn''₂ acyl bond of

s and catalytically hydrolyzes the bond, releasing

arachidonic acid Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega−6 fatty acid 20:4(ω−6), or 20:4(5,8,11,14). It is a precursor in the formation of leukotrienes, prostaglandins, and thromboxanes. Together with omega−3 fatty acids an ...

and lysophosphatidyl choline, a precursor of lysophosphatidic acid. Upon downstream modification by

cyclooxygenase Cyclooxygenase (COX), officially known as prostaglandin-endoperoxide synthase (PTGS), is an enzyme (specifically, a family of isozymes, ) that is responsible for biosynthesis of prostanoids, including thromboxane and prostaglandins such a ...

s or

lipoxygenase Lipoxygenases () (LOX) are a family of (non- heme) iron-containing enzymes, more specifically oxidative enzymes, most of which catalyze the dioxygenation of polyunsaturated fatty acids in lipids containing a cis,cis-1,4- pentadiene into ce ...

is modified into active compounds called

eicosanoid Eicosanoids are lipid signaling, signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, around 20 carbon units in length. Eicosa ...

s. Eicosanoids include

prostaglandin Prostaglandins (PG) are a group of physiology, physiologically active lipid compounds called eicosanoids that have diverse hormone-like effects in animals. Prostaglandins have been found in almost every Tissue (biology), tissue in humans and ot ...

s and

leukotriene Leukotrienes are a family of eicosanoid inflammation, inflammatory mediators produced in leukocytes by the redox, oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid (EPA) by the enzyme arachidonate 5-lipoxyg ...

s, which are categorized as anti-inflammatory and inflammatory mediators. PLA₂ enzymes are commonly found in mammalian tissues as well as arachnid, insect, and snake venom. Venom from bees is largely composed of

melittin Melittin is the main component (40–60% of the dry weight) and the major pain-producing substance of honeybee (''Apis mellifera'') venom. Melittin is a basic peptide consisting of 26 amino acids. Function The principal function of melittin a ...

, which is a stimulant of PLA₂. Due to the increased presence and activity of PLA₂ resulting from a snake or insect bite,

is released from the phospholipid membrane disproportionately. As a result, inflammation and pain occur at the site. There are also prokaryotic A₂ phospholipases. Additional types of phospholipases include phospholipase A₁, phospholipase B, phospholipase C, and

phospholipase D Phospholipase D (PLD) (EC 3.1.4.4; also known as lipophosphodiesterase II, lecithinase D, choline phosphatase; systematic name: phosphatidylcholine phosphatidohydrolase) is an anesthetic-sensitive and mechanosensitive enzyme of the phospholipa ...

Families

Phospholipases A₂ include several unrelated protein 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).

Secreted phospholipases A₂ (sPLA₂)

The

extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

forms of phospholipases A₂ have been isolated from different

venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...

s (

snake Snakes are elongated limbless reptiles of the suborder Serpentes (). Cladistically squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales much like other members of the group. Many species of snakes have s ...

, bee, and

wasp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which look somewhat like wasps, but are in a separate suborder ...

), and from virtually every studied

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

ian tissue (including

pancreas The pancreas (plural pancreases, or pancreata) is an Organ (anatomy), organ of the Digestion, digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a ...

and

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

) as well as from

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

. They require Ca²⁺ for activity. Pancreatic sPLA₂ serve for the initial

digestion Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into th ...

of phospholipid compounds in dietary

fat In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers specif ...

. Venom phospholipases help to immobilize prey by promoting cell lysis. In mice, group III sPLA₂ are involved in sperm maturation, and group X are thought to be involved in sperm capacitation. sPLA₂ has been shown to promote

inflammation Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

in mammals by catalyzing the first step of the arachidonic acid pathway by breaking down

phospholipids Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typi ...

, resulting in the formation of

fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

including

. This arachidonic acid is then metabolized to form several inflammatory and thrombogenic molecules. Excess levels of sPLA₂ is thought to contribute to several inflammatory diseases, and has been shown to promote vascular inflammation correlating with coronary events in

coronary artery disease Coronary artery disease (CAD), also called coronary heart disease (CHD), or ischemic heart disease (IHD), is a type of cardiovascular disease, heart disease involving Ischemia, the reduction of blood flow to the cardiac muscle due to a build-up ...

and acute coronary syndrome, and possibly leading to

acute respiratory distress syndrome Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin co ...

and progression of

tonsillitis Tonsillitis is inflammation of the tonsils in the upper part of the throat. It can be acute or chronic. Acute tonsillitis typically has a rapid onset. Symptoms may include sore throat, fever, enlargement of the tonsils, trouble swallowing, and en ...

. In children, excess levels of sPLA₂ have been associated with inflammation thought to exacerbate

asthma Asthma is a common long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wh ...

and ocular surface inflammation ( dry eye). Increased sPLA₂ activity is observed in the

cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless Extracellular fluid#Transcellular fluid, transcellular body fluid found within the meninges, meningeal tissue that surrounds the vertebrate brain and spinal cord, and in the ventricular system, ven ...

of humans with

Alzheimer's disease Alzheimer's disease (AD) is a neurodegenerative disease and the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems wit ...

and

multiple sclerosis Multiple sclerosis (MS) is an autoimmune disease resulting in damage to myelinthe insulating covers of nerve cellsin the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to Action potential, transmit ...

, and may serve as a marker of increases in permeability of the blood-cerebrospinal fluid barrier. There are atypical members of the phospholipase A₂ family, such as ''PLA2G12B'', that have no phospholipase activity with typical phospholipase substrate. The lack of enzymatic activity of ''PLA2G12B'' indicates that it may have unique function distinctive from other sPLA₂. It has been shown that in ''PLA2G12B'' null mice, VLDL levels were greatly reduced, suggesting it could have an effect in lipoprotein secretion.

Cytosolic phospholipases A₂ (cPLA₂)

The

intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

, group IV PLA₂ are also Ca-dependent, but they have a different 3D structure and are significantly larger than secreted PLA₂ (more than 700 residues). They include a C2 domain and a large catalytic domain. These phospholipases are involved in

cell signaling In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...

processes, such as

inflammatory response Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

. They release

from membrane phospholipids. Arachidonic acid is both a

signaling molecule In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...

and the precursor for the synthesis of other signaling molecules termed

s. These include

s and

s. Some eicosanoids are synthesized from diacylglycerol, released from the

lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organisms and many viruses a ...

by phospholipase C (see below). Phospholipases A₂ can be classified based on sequence homology.

Lipoprotein-associated PLA₂ (Lp-PLA₂)

Increased levels of Lp-PLA₂ are associated with cardiac disease, and may contribute to

atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis, characterized by development of abnormalities called lesions in walls of arteries. This is a chronic inflammatory disease involving many different cell types and is driven by eleva ...

. 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.

Mechanism

Two reaction mechanisms have been suggested for sPLA₂s, the single-water mechanism and the assisted water mechanism. The single-water

catalytic Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...

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 the

water molecule, w5, is unclear. His-48 improves the nucleophilicity of the

water via a bridging second water molecule, w6. In the assisted-water mechanism, two water molecules are necessary to traverse the distance between the catalytic

histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an Amine, α-amino group (which is in the protonated –NH3+ form under Physiological condition, biological conditions), a carboxylic ...

and the

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

. The

basicity In chemistry, there are three definitions in common use of the word "base": ''Arrhenius bases'', ''Brønsted bases'', and ''Lewis bases''. All definitions agree that bases are substances that react with acids, as originally proposed by Guilla ...

of His-48 is thought to be enhanced through

hydrogen bonding In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

with Asp-99. An

asparagine Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...

substitution for His-48 maintains wild-type activity, as the

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...

functional group on

can also function to lower the p''K''_a, or

acid dissociation constant In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative property, quantitative measure of the acid strength, strength of an acid in Solution (chemistry), solution. I ...

, 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, 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 coordinated

oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...

. The role of calcium can also be duplicated by other relatively small cations like cobalt and nickel. Before becoming active in digestion, the proform of PLA₂ is activated by trypsin. PLA₂ can also be characterized as having a channel featuring a

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

wall in which hydrophobic amino acid residues such as Phe, Leu, and Tyr serve to bind the substrate. Another component of PLA₂ is the seven disulfide bridges that are influential in regulation and stable protein folding.

Biological effects

PLA₂ action can release histamine from rat peritoneal mast cells. It also causes histamine release in human basophils.

Regulation

Due to the importance of PLA₂ in

s, regulation of the enzyme is essential. cPLA₂ is regulated by

phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

and calcium concentrations. cPLA₂ is phosphorylated by a

MAPK A mitogen-activated protein kinase (MAPK or MAP kinase) is a type of serine/threonine-specific protein kinases involved in directing cellular responses to a diverse array of stimuli, such as mitogens, osmotic stress, heat shock and proinflamm ...

Serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − ...

-505. When phosphorylation is coupled with an influx of calcium ions, cPLA₂ becomes stimulated and can translocate to the membrane to begin

catalysis Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

. Phosphorylation of cPLA₂ may be a result of ligand binding to receptors, including: * 5-HT₂ receptors * mGLUR1 *

bFGF Fibroblast growth factor 2 (FGF-2), also known as basic fibroblast growth factor (bFGF) and FGF-β, is a growth factor and signaling protein encoded by the ''FGF2'' gene. It binds to and exerts effects via specific fibroblast growth factor rece ...

receptor * IFN-α receptor * IFN-γ receptor In the case of an inflammation, the application of glucocorticoids up-regulate (mediated at the gene level) the production of the protein

lipocortin Annexin is a common name for a group of cellular proteins. They are mostly found in eukaryotic organisms (animals, plants and fungi). In humans, the annexins are found inside the cell (biology), cell. However some annexins (Annexin A1, Annexin A ...

which may inhibit cPLA₂ and reduce the inflammatory response.

Relevance in neurological disorders

In normal brain cells, PLA₂ regulation accounts for a balance between

'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 induced

oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...

and neuroinflammation is analogous to neurological diseases such as

epilepsy Epilepsy is a group of Non-communicable disease, non-communicable Neurological disorder, neurological disorders characterized by a tendency for recurrent, unprovoked Seizure, seizures. A seizure is a sudden burst of abnormal electrical activit ...

ischemia Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...

. Lysophospholipids are another class of molecules released from the membrane that are upstream predecessors of platelet activating factors (PAF). Abnormal levels of potent PAF are also associated with neurological damage. An optimal

enzyme inhibitor An enzyme inhibitor is a molecule that binds to an enzyme and blocks its Enzyme activity, activity. Enzymes are proteins that speed up chemical reactions necessary for life, in which Substrate (biochemistry), substrate molecules are converted ...

would specifically target PLA₂ activity on neural cell membranes already under

and potent

. Thus, specific inhibitors of brain PLA₂ could be a pharmaceutical approach to treatment of several disorders associated with neural trauma. Increase in phospholipase A₂ activity is an acute-phase reaction that rises during inflammation, which is also seen to be exponentially higher in low back

disc herniation A disc herniation or spinal disc herniation is an injury to the intervertebral disc between two vertebrae, usually caused by excessive strain or trauma to the spine. It may result in back pain, pain or sensation in different parts of the body, ...

s compared to

rheumatoid arthritis Rheumatoid arthritis (RA) is a long-term autoimmune disorder that primarily affects synovial joint, joints. It typically results in warm, swollen, and painful joints. Pain and stiffness often worsen following rest. Most commonly, the wrist and h ...

. It is a mixture of inflammation and

substance P Substance P (SP) is an undecapeptide (a peptide composed of a chain of 11 amino acid residues) and a type of neuropeptide, belonging to the tachykinin family of neuropeptides. It acts as a neurotransmitter and a neuromodulator. Substance P ...

that are responsible for pain. Increased phospholipase A₂ has also been associated with neuropsychiatric disorders such as

schizophrenia Schizophrenia () is a mental disorder characterized variously by hallucinations (typically, Auditory hallucination#Schizophrenia, hearing voices), delusions, thought disorder, disorganized thinking and behavior, and Reduced affect display, f ...

and

pervasive developmental disorders The diagnostic category pervasive developmental disorders (PDD), as opposed to specific developmental disorders (SDD), was a group of mental illness, disorders characterized by delays in the development of multiple basic functions including soc ...

(such as

autism Autism, also known as autism spectrum disorder (ASD), is a neurodevelopmental disorder characterized by differences or difficulties in social communication and interaction, a preference for predictability and routine, sensory processing d ...

), though the mechanisms involved are not known.

Isozymes

Human phospholipase A₂

isozyme In biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. di ...

s include: * Group I: '' PLA2G1B'' * Group II: '' PLA2G2A'', , '' PLA2G2D'', , * Group III: * Group IV: '' PLA2G4A'', '' PLA2G4B'', '' PLA2G4C'', , , * Group V: '' PLA2G5'' * Group VI: PLA2G6 * Group VII: ''PLA2G7'' * Group X: '' PLA2G10'' * Group XII: '' PLA2G12A'', In addition, the following human proteins contain the phospholipase A₂ domain: *

References

External links

* Peripheral membrane proteins