Inositol (1,4,5) trisphosphate 3-kinase (), abbreviated here as ITP3K, is an enzyme that facilitates a phospho-group transfer from

adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...

to 1D-myo-inositol 1,4,5-trisphosphate. This enzyme belongs to the family of

transferase A transferase is any one of a class of enzymes that catalyse the transfer of specific functional groups (e.g. a methyl or glycosyl group) from one molecule (called the donor) to another (called the acceptor). They are involved in hundreds of di ...

s, specifically those transferring phosphorus-containing groups (

phosphotransferase Phosphotransferases are a category of enzymes ( EC number 2.7) that catalyze phosphorylation reactions. The general form of the reactions they catalyze is: :A-P + B \rightleftharpoons B-P + A Where ''P'' is a phosphate group and A and B are the do ...

s) with an alcohol group as acceptor. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

of this enzyme class is ATP:1D-myo-inositol-1,4,5-trisphosphate 3-phosphotransferase. ITP3K catalyzes the transfer of the gamma-phosphate from ATP to the 3-position of

inositol 1,4,5-trisphosphate Inositol trisphosphate or inositol 1,4,5-trisphosphate abbreviated InsP3 or Ins3P or IP3 is an inositol phosphate signaling molecule. It is made by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that is located in the ...

to form inositol 1,3,4,5-tetrakisphosphate. ITP3K is highly specific for the 1,4,5-

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

of IP₃, and it exclusively

phosphorylates In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, whi ...

the 3-OH position, producing Ins(1,3,4,5)P4, also known as inositol tetrakisphosphate or IP4. In biology, the enzyme ITP3K is abbreviated a number of different ways, including 1D-myo-inositol-trisphosphate 3-kinase, ITP3K, ITPK, IP3-kinase, IP3-3-kinase, Ins(1,4,5)P3 3-kinase. In addition the enzyme may be named as the product of one of 3 genes in humans

ITPKA Inositol-trisphosphate 3-kinase A is an enzyme that in humans is encoded by the ''ITPKA'' gene. Structure ITPKA is one of three inositol-trisphosphate 3-kinase (ITP3K) genes in humans. ITP3K proteins regulate inositol phosphate metabolism by p ...

ITPKB Inositol-trisphosphate 3-kinase B is an enzyme that in humans is encoded by the ''ITPKB'' gene. Function The protein encoded by the ITPKB gene is one of 3 isoforms of Inositol-trisphosphate 3-kinase expressed in humans. ITPKB protein regulates ...

, and

ITPKC ITPKC is one of 3 human genes that encode for an Inositol-trisphosphate 3-kinase. This gene that has been associated with Kawasaki disease. Kawasaki disease is an acute febrile illness that involves the inflammation of blood vessels throughout th ...

, or one of two in fruit flies
IP3K1
an

��a mutant known to geneticists as ''wavy''. The nematode genome has one form of the enzyme, coded by th
LFE-2 gene
ITP3K enzymes are expressed only in

metazoans Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

; they are not expressed in yeast or plants. All ITP3Ks belong to a larger structural family, the inositol polyphosphate kinases, or IPKs. Note however, that the human genome also contains a gene for a different kinase known as ITPK1, which is an
inositol 1, 3, 4-trisphosphate 5/6-kinase
and is not a member of the IPK family. The ITP3K enzyme family is sometimes confused with a different enzyme family that has a similar name, that is, the phosphatidyl inositol 3-kinases or

phosphoinositide 3-kinase Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which i ...

(PI3-K),whose substrates are inositol lipids, not the soluble second messenger

inositol trisphosphate Inositol trisphosphate or inositol 1,4,5-trisphosphate abbreviated InsP3 or Ins3P or IP3 is an inositol phosphate signaling molecule. It is made by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that is located in the p ...

Discovery and characterization

Scientific interest in the inositol phosphates intensified in the years following the 1983 discovery that

was an intracellular messenger that releases calcium from intracellular stores in the endoplasmic reticulum. By the end of the decade, a large number of inositol phosphate kinases and phosphatases had been discovered, including ITP3K in 1986. Biochemical and molecular studies in the 1990s led to the purification of the enzyme from rat brain and it molecular cloning, and these studies revealed various feedback mechanisms by which the enzyme is regulated by calcium and protein kinases. In 1999, ITP3K was identified as being a member of a larger family of Inositol polyphosphate kinases, which share a similar structure and catalytic mechanism. ITP3K enzymes share common structural features including a conserved catalytic core which binds ATP located near the C-terminus, and various regulatory domains nearer to the N-terminus.

Catalytic domain

Evidence for this exquisite specificity and for the catalytic mechanism was found when the apo-enzyme, substrate-bound complex, and product-bound complex X-ray crystal structures of

were determined. The figure to the right depicts the catalytic mechanism, whereby the 3'OH of IP₃ attacks the gamma-phosphate of ATP, and amino acid residues of ITPK important for stabilizing the substrates and products in the active site. ITP3K Catalytic Mechanism

The structure of the catalytic domain of the human ITP3KA has been shown to be divided into three subdomains. These subdomains are displayed as the N lobe, which is a N-terminal domain, the C lobe, which is a C-terminal subdomain and a third alpha-only subdomain. The ITP3K catalytic domain varies somewhat from the protein kinase superfamily, and it has a novel four-helix substrate binding domain. In this

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

, the two domains are in an open conformation, which indicates that the two domains are both accessible at the same time. This suggests that substrate recognition and

catalysis Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

by ITP3K involves a dynamic conformational cycle. Additionally, this unique helical domain of ITPK blocks access to the

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...

by membrane-bound phosphoinositides, explaining the structural basis for

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...

inositol polyphosphate specificity. Another feature of the catalytic core is the ATP binding site. Here, one molecule of ADP is bound in the cleft of the major domain, which indicates the active site of the kinase. In further detail, the larger domain of the

protein structure Protein structure is the three-dimensional arrangement of atoms in an amino acid-chain molecule. Proteins are polymers specifically polypeptides formed from sequences of amino acids, the monomers of the polymer. A single amino acid monomer ma ...

has an α/β-class structure. The domain has an N-terminal and a C-terminal lobe with a cleft in between and each of these lobes is built around an antiparallel β-sheet. In the N-terminal, the sheet has three strands, whereas in the C-terminal there is a five-stranded sheet. The second domain, is α-helical and consists of four α helices linked by long loops. The helices are loosely packed against each other and the entire domain is highly mobile as compared to the large α/β domain. The helical domain is juxtaposed against one end of the cleft in the large domain.

Regulation

ITP3K is regulated by various

post-translational Post-translational modification (PTM) is the covalent and generally enzymatic modification of proteins following protein biosynthesis. This process occurs in the endoplasmic reticulum and the golgi apparatus. Proteins are synthesized by ribosom ...

mechanisms. ITP3Ks are stimulated directly by calcium/calmodulin (Ca2+/CaM) binding. Generally, mammalian ITP3Ks are activated by calcium and calmodulin to varying degrees. The method in which this works is calmodulin recognizes sequences which contain amphiphilic alpha-helices with clusters of positively charged and

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

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

. Certain sequences are required for CaM binding and

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

activation and this level of stimulation appears to be specific to cell, tissue, or isoform. ITP3Ks from

nematodes The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

and ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...

'' lack the CaM-binding sites and therefore are insensitive to calcium and calmodulin. Another major post-translational modification that is important for ITP3K regulation is

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

. ITP3K activity is indirectly stimulated by phosphorylation by calcium/calmodulin-dependent kinase II (CaMKII). In addition, there is evidence that ITP3Ks may be activated upon phosphorylation by protein kinase C (PKC) and inhibited upon phosphorylation by protein kinase A (PKA).

Isoforms

There are three ITP3Ks which are encoded by the human genome:

, and

. All share a conserved C-terminal catalytic domain, but differ in mechanisms of regulation as well as tissue expression.

is predominant in

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

and in the

testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoster ...

. It is localized to

dendritic spines A dendritic spine (or spine) is a small membranous protrusion from a neuron's dendrite that typically receives input from a single axon at the synapse. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical si ...

by an association with filamentous

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...

which is consistent with its probable role in memory functions.

is expressed more widely, but it is often enriched in immune tissues, and it has different intracellular localizations that depend on tissue, interaction with actin filaments, and proteolysis at the N-terminal regions.

is also expressed in many different tissues and it is more enriched in the nucleus compared to the other isoforms.

Functions in Calcium Signaling

ITP3K plays a role in regulating or cooperating with intracellular calcium signals that occur following the liberation of

. In this pathway, either a G-protein coupled receptor (GPCR) or receptor tyrosine kinase (RTK) is activated by an extracellular ligand-binding event. Initiation of the pathway leads to an activated G-alpha subunit of a

heterotrimeric G protein Heterotrimeric G protein, also sometimes referred to as the ''"large" G proteins'' (as opposed to the subclass of smaller, monomeric small GTPases) are membrane-associated G proteins that form a Heteromer, heterotrimeric complex. The biggest no ...

(in the case of GPCR-mediated

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellula ...

) or autophoshorylation of RTK cytoplasmic domains (in the case of RTK-mediated signal transduction). These intracellular events eventually lead to activation of phospholipase C (PLC), which cleaves the phospholipid

PIP2 Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)''P''2, also known simply as PIP2 or PI(4,5)P2, is a minor phospholipid component of cell membranes. PtdIns(4,5)''P''2 is enriched at the plasma membrane where it is a substrate for a number of ...

into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃). DAG remains associated with the

plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

, while IP₃ is released into the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

. IP₃ then diffuses through the cytosol and binds to IP₃ receptors on the

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium in bio ...

, resulting in the opening of a membrane channel and an influx of

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

ions into the cytoplasm. Calcium serves as a

second messenger Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules—the first messengers. (Intercellular signals, a non-local form or cell signaling, encompassing both first me ...

for various downstream cellular events including

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one o ...

metabolism,

muscle contraction Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...

neurotransmitter release Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + ''cytosis''). As an active transport mechanism, exocytosis requires the use ...

, and

transcriptional regulation In molecular biology and genetics, transcriptional regulation is the means by which a cell regulates the conversion of DNA to RNA (transcription), thereby orchestrating gene activity. A single gene can be regulated in a range of ways, from alt ...

. Therefore, calcium homeostasis is essential for proper cell function and response to extracellular signals. In order to prepare the cell for a future signaling event, the calcium pathway must be tightly regulated. ITP3K seems to play an important role in termination of the signal. As mentioned, ITP3K catalyzes the phosphorylation of IP₃ to make IP₄. Unlike IP₃, IP₄ does not cause opening of calcium channels on the endoplasmic reticulum or sarcoplasmic reticulum. By decreasing the concentration of IP₃ in the cytoplasm, ITP3K terminates propagation of the calcium signaling pathway.

Additional roles

ITP3K is not the only enzyme responsible for clearing IP₃ from the cytoplasm. A second enzyme called inositol 5-phosphatase catalyzes the

dephosphorylation In biochemistry, dephosphorylation is the removal of a phosphate (PO43−) group from an organic compound by hydrolysis. It is a reversible post-translational modification. Dephosphorylation and its counterpart, phosphorylation, activate and deac ...

of IP₃ to create IP₂. Typically, nature does not favor the evolution of a second enzyme to perform an already-existing, identical function. A closer inspection of the evolutionary history of inositol 5-phosphatase and ITP3K gives rise to several interesting hypotheses about the roles of these enzymes in the cell. Inositol 5-phosphatase existed before ITP3K evolved in the mammalian cell. Like other

phosphatase In biochemistry, a phosphatase is an enzyme that uses water to cleave a phosphoric acid Ester, monoester into a phosphate ion and an Alcohol (chemistry), alcohol. Because a phosphatase enzyme catalysis, catalyzes the hydrolysis of its Substrate ...

s, inositol 5-phosphatase is an energy-independent enzyme that cleaves a

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

group off of a substrate. In contrast, ITP3K (like all

s) is energy-dependent, meaning that it requires an ATP molecule to perform the phosphoryl transfer chemistry. If nature already had an energy-independent mechanism for termination of the calcium signaling pathway, why was the evolution of ITP3K advantageous? This apparent redundancy of function, or "waste" of energy by the cell, suggests that ITP3K may have a more important function in the cell than simply clearing the IP₃ second messenger from the cytoplasm. Current hypotheses about additional roles for ITPK are explained in the following two subsections.

Product of ITPK may be a second messenger

As mentioned previously, ITP3K catalyzes a phosphoryl transfer reaction that converts IP₃ to IP₄. IP₄ does not stimulate calcium influx through IP₃ receptor channels on the endoplasmic or sarcoplasmic reticulum. However, it has been shown that IP₄ stimulates calcium channel opening on the plasma membrane. In this way, IP₄ may actually serve to prolong the calcium signal by activating the influx of calcium stores from the extracellular space. In addition, there is evidence that IP₄ binds two

GTPase-activating protein GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are a ...

s, GAP1IP4BP and GAP1m. GAPs are often used in signal transduction as on/off switches. IP₄ binding to GAPs suggests that ITPK may be involved in a parallel signal transduction pathway. The exact role of IP₄ binding to these GAPs has not been determined, though, so additional research in this area will be needed to gain a more complete understanding.

Role in inositol phosphate metabolism

In addition to its potential roles as a

, IP₄ may also function as an essential

precursor Precursor or Precursors may refer to: *Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of unr ...

for other more highly phosphorylated inositol phosphates such as IP₅, IP₆, IP₇, and IP₈. Such maintenance is necessary to prepare the cell for a future incoming signal.

Relevance to physiology and human disease

protein is highly enriched in dendritic spines. ITPKA participates in learning and memory process in neuronal cells, both via its catalytic activity and its interaction with filamentous actin. Although

is expressed physiologically in neurons and testis, the gene becomes expressed in a number of cancer cell types. In most cases, ITP3K expression causes the cancer to be more aggressive.

is implicated in physiologic immune function.

has been linked to Kawasaki Disease, an autoimmune disorder.

References

{{Portal bar, Biology, border=no EC 2.7.1 Enzymes of known structure