Phospholipase C
Phospholipase C (PLC) is a class of membrane-associated
There are thirteen kinds of mammalian phospholipase C that are classified into six isotypes (β, γ, δ, ε, ζ, η) according to structure. Each PLC has unique and overlapping controls over expression and subcellular distribution. However, PLC is not limited to mammals, and is present in bacteria and Chromadorea as well.
Phospholipase C | |||||||||
---|---|---|---|---|---|---|---|---|---|
Identifiers | |||||||||
ExPASy | NiceZyme view | ||||||||
KEGG | KEGG entry | ||||||||
MetaCyc | metabolic pathway | ||||||||
PRIAM | profile | ||||||||
PDB structures | RCSB PDB PDBe PDBsum | ||||||||
Gene Ontology | AmiGO / QuickGO | ||||||||
|
Variants
Mammalian variants
The extensive number of functions exerted by the PLC reaction requires that it be strictly regulated and able to respond to multiple extra- and intracellular inputs with appropriate kinetics. This need has guided the evolution of six isotypes of PLC in animals, each with a distinct mode of regulation. The pre-mRNA of PLC can also be subject to differential splicing such that a mammal may have up to 30 PLC enzymes.[2]
- beta: PLCB1, PLCB2, PLCB3, PLCB4
- gamma: PLCG1, PLCG2
- delta: PLCD1, PLCD3, PLCD4
- epsilon: PLCE1
- eta: PLCH1, PLCH2
- zeta: PLCZ1
- phospholipase C-like: PLCL1, PLCL2
Bacterial variants
Most of the bacterial variants of phospholipase C are characterized into one of four groups of structurally related proteins. The toxic phospholipases C are capable of interacting with eukaryotic cell membranes and hydrolyzing phosphatidylcholine and sphingomyelin, leading to cell lysis.[3]
- Zinc-metallophospholipases C: Clostridium perfringens alpha-toxin, Bacillus cereus PLC (BC-PLC)
- Sphingomyelinases: B. cereus, Staphylococcus aureus
- Phosphatidylinositol-hydrolyzing enzymes: B. cereus, B. thuringiensis, L. monocytogenes (PLC-A)
- Pseudomonad phospholipases C: Pseudomonas aeruginosa (PLC-H and PLC-N)
Chromadorea
The class of Chromadorea also utilizes the enzyme phospholipase C to regulate the releases of calcium. The enzyme releases inositol 1,4,5-trisphosphate (IP3) that denotes a signaling pathway involved in activating ovulation, the propelling of the oocyte into the spermatheca. This gene is involved in various activities like controlling GTPase, breaking down certain molecules, and binding to small GTPase. It helps in fighting bacteria and regulating protein movement in cells. It's found in the excretory system, intestines, nerves, and reproductive organs. The expression of the enzyme in the spermatheca is controlled by the transcription factors FOS-1 and JUN-1.[4]
Enzyme structure
In mammals, PLCs share a conserved core structure and differ in other domains specific to each family. The core enzyme includes a split triosephosphate isomerase (TIM) barrel, pleckstrin homology (PH) domain, four tandem EF hand domains, and a C2 domain.[1] The TIM barrel contains the active site, all catalytic residues, and a Ca2+ binding site. It has an autoinhibitory insert that interrupts its activity called an X-Y linker. The X-Y linker has been shown to occlude the active site, and with its removal, PLC is activated.[5]
The genes encoding
Enzyme mechanism
The primary catalyzed reaction of PLC occurs on an insoluble substrate at a lipid-water interface. The residues in the active site are conserved in all PLC isotypes. In animals, PLC selectively catalyzes the hydrolysis of the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) on the glycerol side of the phosphodiester bond. There is the formation of a weakly enzyme-bound intermediate, inositol 1,2-cyclic phosphodiester, and release of diacylglycerol (DAG). The intermediate is then hydrolyzed to inositol 1,4,5-trisphosphate (IP3).[7] Thus the two end products are DAG and IP3. The acid/base catalysis requires two conserved histidine residues and a Ca2+ ion is needed for PIP2 hydrolysis. It has been observed that the active-site Ca2+ coordinates with four acidic residues and if any of the residues are mutated then a greater Ca2+ concentration is needed for catalysis.[8]
Signaling Pathway
Phosphoinositide-specific phospholipase C (PLC) is a key player in cell signaling processes. When cells encounter signals like hormones or growth factors, PLC breaks down a molecule called PIP2 to produce new signaling molecules. PIP2 is a type of molecule found in cell membranes. When cells receive certain signals from outside, an enzyme called PLC breaks down PIP2 into smaller molecules, which then send messages within the cell. Various types of PLC are activated differently, contributing to cells' ability to respond to their surroundings.
Regulation
Activation
Receptors that activate this pathway are mainly G protein-coupled receptors coupled to the Gαq subunit, including:
- 5-HT2 serotonergic receptors
- adrenergic receptors[9]
- Calcitonin receptors
- histamine receptors
- Metabotropic glutamate receptors, Group I
- muscarinic receptors
- Thyroid-Releasing Hormone receptor in anterior pituitary gland
Other, minor, activators than Gαq are:
- growth hormone-releasing hormone.[10]
- Cannabinoid receptors
Inhibition
- Small molecule U73122: aminosteroid, putative PLC inhibitor.[11][12] However, the specificity of U73122 has been questioned.[13] It has been reported that U73122 activates the phospholipase activity of purified PLCs.[14]
- Edelfosine: lipid-like, anti-neoplastic agent (ET-18-OCH3)[15]
- Autoinhibition of the X-Y linker in mammalian cells: It is proposed that the X-Y linker consists of long stretches of acidic amino acids that form dense areas of negative charge. These areas could be repelled by the negatively charged membrane upon binding of the PLC to membrane lipids. The combination of repulsion and steric constraints is thought to remove the X-Y linker from near the active site and relieve auto-inhibition.[1]
- Compounds containing the morpholinobenzoic acid scaffold belong to a class of drug-like phosphatidylcholine-specific PLC inhibitors.[16][17][18]
- o-phenanthroline: heterocyclic organic compound, known to inhibit zinc-metalloenzymes[19]
- EDTA: molecule that chelates Zn2+ ions and effectively inactivates PLC, known to inhibit zinc-metalloenzymes[20]
Biological function
PLC cleaves the
The two products of the PLC catalyzed reaction, DAG and IP3, are important second messengers that control diverse cellular processes and are substrates for synthesis of other important signaling molecules. When PIP2 is cleaved, DAG remains bound to the membrane, and IP3 is released as a soluble structure into the
Both DAG and IP3 are substrates for the synthesis of regulatory molecules. DAG is the substrate for the synthesis of phosphatidic acid, a regulatory molecule. IP3 is the rate-limiting substrate for the synthesis of inositol polyphosphates, which stimulate multiple protein kinases, transcription, and mRNA processing.[26] Regulation of PLC activity is thus vital to the coordination and regulation of other enzymes of pathways that are central to the control of cellular physiology.
Additionally, phospholipase C plays an important role in the inflammation pathway. The binding of agonists such as
The bacterial variant Clostridium perfringens type A produces alpha-toxin. The toxin has phospholipase C activity, and causes hemolysis, lethality, and dermonecrosis. At high concentrations, alpha-toxin induces massive degradation of phosphatidylcholine and sphingomyelin, producing diacylglycerol and ceramide, respectively. These molecules then participate in signal transduction pathways.[6] It has been reported that the toxin activates the arachidonic acid cascade in isolated rat aorta.[28] The toxin-induced contraction was related to generation of thromboxane A2 from arachidonic acid. Thus it is likely the bacterial PLC mimics the actions of endogenous PLC in eukaryotic cell membranes.
See also
- Glycosylphosphatidylinositol diacylglycerol-lyase EC 4.6.1.14 A trypanosomal enzyme.
- Phosphatidylinositol diacylglycerol-lyase EC 4.6.1.13 Another related bacterial enzyme
- Phosphoinositide phospholipase C EC 3.1.4.11 The main form found in eukaryotes, especially mammals.
- Zinc-dependent phospholipase C family of bacterial enzymes EC 3.1.4.3 that includes the alpha toxins of C. perfringens (also known as lecithinase), P. aeruginosa, and S. aureus.
References
- ^ PMID 23140367.
- PMID 18593525.
- PMID 8336671.
- PMC 3566351.
- PMID 18691970.
- ^ PMID 15632295.
- PMID 9048554.
- PMID 9565585.
- ^ ISBN 978-1-4160-2328-9. Page 104
- ^ GeneGlobe -> GHRH Signaling[permanent dead link] Retrieved on May 31, 2009
- PMID 2147038.
- PMID 20590621.
- PMID 23297405.
- PMID 21266572.
- PMID 16129772.
- PMID 34328856.
- S2CID 208813280.
- S2CID 211536972.
- PMID 807246.
- ^ "Phospholipase C, Phosphatidylinositol-specific from Bacillus cereus" (PDF). Product Information. Sigma Aldrich.
- S2CID 29839094.
- PMID 25633344.
- PMID 20519312.
- ^ ISBN 978-0-8153-3218-3.
- PMID 10669417.
- PMID 22403074.
- PMID 7685181.
- PMID 2497921.