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 adenosine triphosphate (ATP) to choline in order to form phosphocholine. 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 adenosine triphosphate (ATP) to choline in order to form phosphocholine. Thus, the two substrates of this enzyme are ATP and choline, whereas its two products are adenosine diphosphate (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. As of late 2007, six structures have been solved for this class of enzymes, with PDB accession codes 1NW1, 2CKO, 2CKP, 2CKQ, 2I7Q, and 2IG7. 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 possibly 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. 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. In this figure, there is a similarity between APH(3′)-IIIa, an aminoglycoside 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. 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.