Choline kinase

Choline Kinase (also known as CK,ChoK and Choline Phosphokinase) 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.
 * ATP + choline $$\rightleftharpoons$$ ADP + O-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 transferases, specifically those transferring phosphorus-containing groups (phosphotransferases) 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 sites 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.

Structural Studies
As of late 2007, 6 structures 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. ]]

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 aminoglycoside phosphotransferase and CK.

Proposed Choline Binding Site
[[File: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 phospholipid 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.

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 knockout 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.