Choline kinase
Encyclopedia
Choline Kinase is an enzyme
which catalyzes the first reaction in the choline pathway for phosphatidylcholine
(PC) biosynthesis.This reaction involves the transfer of a phosphate group from ATP to choline in order to form phosphocholine
.
Thus, the two substrates
of this enzyme are ATP
and choline
, whereas its two products
are ADP
and O-phosphocholine.Choline Kinase requires magnesium ions (+2) as a cofactor
for this reaction. This enzyme belongs to the family of transferase
s, specifically those transferring phosphorus-containing groups (phosphotransferase
s) with an alcohol group as acceptor
. The first detailed investigation of the enzyme was conducted by McCamen in 1962, where it was shown that the brain is the richest source of the enzyme in mammalian tissue. A related enzyme, ethanolamine kinase
tends to co-purify with choline kinase leading to a suggestion that the two activities are mediated by two distinct active site
s on a single protein. The systematic name of this enzyme class is ATP:choline phosphotransferase. These enzymes participate in glycine, serine and threonine metabolism and glycerophospholipid metabolism.
In mammalian cells, the enzyme exists as three isoforms,CKα-1,CKα-2 and CKβ. These isoforms are encoded by two separate genes
, CHKA
and CHKB
and are only active in their homodimeric,heterodimeric and oligomeric forms.
have been solved for this class of enzymes, with PDB
accession codes , , , , , and .
CKα-2 originating from C.elegans, is a dimeric enzyme with each monomer being composed of two domains.The active site is located between the two domains. (See figure below) Its overall structure is similar to members of the eukaryotic protein kinase
family. Mammalian choline kinases exists in either dimeric or tetrameric forms in solution. Structural studies carried out on CKα-2, have implied that the conserved residues in the CK family of enzymes could possible play a vital role in substrate binding as well as in the stabilization of catalytically important residues.
An enlarged view of the residues involved in the dimer interface between the S-shaped loop of the yellow subunit and the loop following helix A and strand 4 of the cyan subunit. Only residues that are involved in direct salt bridges, hydrogen bonds, or van der Waals interactions are shown. Salt bridges and hydrogen bonds, dashed lines; labels of residues from the yellow subunit, red; labels of residues from the cyan subunit, blue.]]
phosphotransferase and CK.
Propositions for this mechanism have been made based on mechanistic studies done on eukaryotic protein kinases. It has been proposed that in the CKα-2 mechanism,ATP binds first, followed by Choline, and then the transfer of the phosphoryl group takes place. The product O-phosphocholine is then released, followed by the release of ADP.
in eukaryotic membranes. Phosphatidylcholine is important for a variety of function in eukaryotes such as facilitating the transport of cholesterol
through the organism, acting as a substrate for the production of second messengers and as a cofactor for the activity of several membrane-related enzymes. CK also plays a vital role in the production of sphingomyelin
,another important membrane phospholipid and in the regulation of cell growth.
The production of phosphocholine from CK is necessary for the signal transduction
pathways related to mitogenesis. It has also been found that CK plays a critical role in the proliferation of human mammary epithelial cells.
In vivo
studies carried out using CKα-1 and CKβ isoforms suggest that each isoform might be involved in different biochemical pathways.CKβ plays a major role in the catalysis of the phosphorylation of ethanolamine
while CKα-1 catalyzes the phosphorylation of both choline and ethanolamine.
Studies using colon, human lung and prostate carcinomas also revealed that CK is upregulated by overexpression of CKα-1 in these cells compared to the normal, non-cancerous cells.
One possible explanation for this is that CKα-1 aids in the regulation of Protein Kinase B phoshorylation, particularly at the Serine-473 end.Consequently, high levels of expression and activity of CKα-1 promotes cell growth and survival. Based on the observation that increased activity of CKα-1 is related to cancer,CKα-1 has promising use as a tumor biomarker and in diagnosing and following the progression of tumors. Even more interesting is the critical role that the discovery of inhibitors of CKα-1 could play in the development of novel drugs for cancer treatment. This is key point, given the fact that all human cancer cells have shown increased levels of this particular enzyme.
mice models,that a defect in the CKβ activity leads to a decrease in the Phosphatidylcholine(PC) content in the hindlimb muscle. This, however, does not affect the phoshoethanolamine (PE) content.
The net effect is then that the PC/PE ratio decreases and this is leads to impaired membrane integrity in the liver. This compromised membrane potential leads to malfunctioning of the mitochondria. Although CK is required for the biosynthesis of PC,CK is normally present in excess and so is not generally considered the rate-limiting step. Researchers have concluded, however, that due to the reduced activity of CK seen in the hindlimb muscle of the CKβ knockout mice model,CK is probably the rate-limiting enzyme in skeletal muscles.This suggests that defect in CKβ may lead to a decrease in PC synthesis in the muscles resulting in muscular dystrophy
. These results suggest that CK could possibly play a vital role in the homeostasis
of PC.
Category:EC 2.7.1
Category:Enzymes of known structure
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
which catalyzes the first reaction in the choline pathway for phosphatidylcholine
Phosphatidylcholine
Phosphatidylcholines are a class of phospholipids that incorporate choline as a headgroup.They are a major component of biological membranes and can be easily obtained from a variety of readily available sources such as egg yolk or soy beans from which they are mechanically extracted or chemically...
(PC) biosynthesis.This reaction involves the transfer of a phosphate group from ATP to choline in order to form phosphocholine
Phosphocholine
Phosphocholine is an intermediate in the synthesis of phosphatidylcholine in tissues. Phosphocholine is made in a reaction, catalyzed by choline kinase, that converts ATP + Choline into Phosphocholine and ADP...
.
- ATP + choline
ADP + O-phosphocholine
Thus, the two substrates
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
of this enzyme are ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
and choline
Choline
Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....
, whereas its two products
Product (chemistry)
Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...
are ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
and O-phosphocholine.Choline Kinase requires magnesium ions (+2) as a cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....
for this reaction. This enzyme belongs to the family of transferase
Transferase
In biochemistry, a transferase is an enzyme that catalyzes the transfer of a functional group from one molecule to another . For example, an enzyme that catalyzed this reaction would be a transferase:In this example, A would be the donor, and B would be the acceptor...
s, specifically those transferring phosphorus-containing groups (phosphotransferase
Phosphotransferase
Phosphotransferases are a category of enzymes that catalyze phosphorylation reactions. The general form of the reactions they catalyze is: A—P + B ⇔ B—P + A...
s) with an alcohol group as acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
. The first detailed investigation of the enzyme was conducted by McCamen in 1962, where it was shown that the brain is the richest source of the enzyme in mammalian tissue. A related enzyme, ethanolamine kinase
Ethanolamine kinase
In enzymology, an ethanolamine kinase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are ATP and ethanolamine, whereas its two products are ADP and O-phosphoethanolamine....
tends to co-purify with choline kinase leading to a suggestion that the two activities are mediated by two distinct active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
s on a single protein. The systematic name of this enzyme class is ATP:choline phosphotransferase. These enzymes participate in glycine, serine and threonine metabolism and glycerophospholipid metabolism.
In mammalian cells, the enzyme exists as three isoforms,CKα-1,CKα-2 and CKβ. These isoforms are encoded by two separate genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
, CHKA
CHKA
Choline kinase alpha is an enzyme that in humans is encoded by the CHKA gene.-Further reading:...
and CHKB
CHKB (gene)
Choline kinase beta is a protein encoded by the CHKB gene. This gene is found on chromosome 22 in humans.Choline kinase and ethanolamine kinase catalyze the phosphorylation of choline/ethanolamine to phosphocholine/phosphoethanolamine. This is the first enzyme in the biosynthesis of...
and are only active in their homodimeric,heterodimeric and oligomeric forms.
Structural Studies
As of late 2007, 6 structuresTertiary structure
In biochemistry and molecular biology, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates.-Relationship to primary structure:...
have been solved for this class of enzymes, with PDB
Protein Data Bank
The Protein Data Bank is a repository for the 3-D structural data of large biological molecules, such as proteins and nucleic acids....
accession codes , , , , , and .
CKα-2 originating from C.elegans, is a dimeric enzyme with each monomer being composed of two domains.The active site is located between the two domains. (See figure below) Its overall structure is similar to members of the eukaryotic protein kinase
Protein kinase
A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them . Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins...
family. Mammalian choline kinases exists in either dimeric or tetrameric forms in solution. Structural studies carried out on CKα-2, have implied that the conserved residues in the CK family of enzymes could possible play a vital role in substrate binding as well as in the stabilization of catalytically important residues.
An enlarged view of the residues involved in the dimer interface between the S-shaped loop of the yellow subunit and the loop following helix A and strand 4 of the cyan subunit. Only residues that are involved in direct salt bridges, hydrogen bonds, or van der Waals interactions are shown. Salt bridges and hydrogen bonds, dashed lines; labels of residues from the yellow subunit, red; labels of residues from the cyan subunit, blue.]]
Mechanism
Although not much is known about the mechanism by which choline kinase reacts, the recent advancement in the elucidation of the structure of the enzyme has provided scientists with much more insight than they had previously.Since the structure of CK is very similar to that of the eukaryotic protein kinase family,the location of ATP and choline binding pockets have been proposed. These are shown in the figures below.Proposed ATP Binding Site
In this figure, we see a striking similarity between APH(3′)-IIIa, an aminoglycosideAminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...
phosphotransferase and CK.
Proposed Choline Binding Site
ile:Proposed Choline Binding Site.png|thumb|center|450px|Proposed Choline Binding Site-The loops are colored as follows: the ATP binding loop (residues 81–88), red; residue Asp301, magenta; choline pocket loop 1 (residues 322–343), green;choline pocket loop 2 (residues 396–405), blue. On the right, the surface is colored on the basis of the electrostatic potential of the molecule.Propositions for this mechanism have been made based on mechanistic studies done on eukaryotic protein kinases. It has been proposed that in the CKα-2 mechanism,ATP binds first, followed by Choline, and then the transfer of the phosphoryl group takes place. The product O-phosphocholine is then released, followed by the release of ADP.
Evolution
After closely studying the structurally similar enzymes,CKα-2, APH(3′)-IIIa, and PKA,researchers observed that PKA had less insertions to its structural core compared to the other enzymes. Against this background,it is believed that CKα-2 have evolved from PKA to have more structural elements attached to it.Biological Function
Choline Kinase catalyzes the formation of phoshocholine,the committed step in phosphatidylcholine biosynthesis. Phosphatidylcholine is the major phospholipidPhospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
in eukaryotic membranes. Phosphatidylcholine is important for a variety of function in eukaryotes such as facilitating the transport of cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
through the organism, acting as a substrate for the production of second messengers and as a cofactor for the activity of several membrane-related enzymes. CK also plays a vital role in the production of sphingomyelin
Sphingomyelin
Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide...
,another important membrane phospholipid and in the regulation of cell growth.
The production of phosphocholine from CK is necessary for the signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
pathways related to mitogenesis. It has also been found that CK plays a critical role in the proliferation of human mammary epithelial cells.
In vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
studies carried out using CKα-1 and CKβ isoforms suggest that each isoform might be involved in different biochemical pathways.CKβ plays a major role in the catalysis of the phosphorylation of ethanolamine
Ethanolamine
Ethanolamine, also called 2-aminoethanol or monoethanolamine , is an organic chemical compound that is both a primary amine and a primary alcohol . Like other amines, monoethanolamine acts as a weak base...
while CKα-1 catalyzes the phosphorylation of both choline and ethanolamine.
Oncogenic Activity and CKα-1
Overexpression of CKα-1 has been found to be associated with cancer.Recent studies carried out on cancer cell lines have shown that CKα-1 is overexpressed in breast cancer cells. This leads to an accumulation of phosphocholine in the breast and causes malignancy.Studies using colon, human lung and prostate carcinomas also revealed that CK is upregulated by overexpression of CKα-1 in these cells compared to the normal, non-cancerous cells.
One possible explanation for this is that CKα-1 aids in the regulation of Protein Kinase B phoshorylation, particularly at the Serine-473 end.Consequently, high levels of expression and activity of CKα-1 promotes cell growth and survival. Based on the observation that increased activity of CKα-1 is related to cancer,CKα-1 has promising use as a tumor biomarker and in diagnosing and following the progression of tumors. Even more interesting is the critical role that the discovery of inhibitors of CKα-1 could play in the development of novel drugs for cancer treatment. This is key point, given the fact that all human cancer cells have shown increased levels of this particular enzyme.
Muscular Dystrophy and CKβ
It has been shown, using CKβ knockoutKnockout
A knockout is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, Muay Thai, mixed martial arts, Karate and others sports involving striking...
mice models,that a defect in the CKβ activity leads to a decrease in the Phosphatidylcholine(PC) content in the hindlimb muscle. This, however, does not affect the phoshoethanolamine (PE) content.
The net effect is then that the PC/PE ratio decreases and this is leads to impaired membrane integrity in the liver. This compromised membrane potential leads to malfunctioning of the mitochondria. Although CK is required for the biosynthesis of PC,CK is normally present in excess and so is not generally considered the rate-limiting step. Researchers have concluded, however, that due to the reduced activity of CK seen in the hindlimb muscle of the CKβ knockout mice model,CK is probably the rate-limiting enzyme in skeletal muscles.This suggests that defect in CKβ may lead to a decrease in PC synthesis in the muscles resulting in muscular dystrophy
Muscular dystrophy
Muscular dystrophy is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.In the 1860s, descriptions of boys who...
. These results suggest that CK could possibly play a vital role in the homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
of PC.
Further reading
Category:Articles created via the Article WizardCategory:EC 2.7.1
Category:Enzymes of known structure