Diacylglycerol catalyzed phosphorylation

This enzyme [EC 2.7.S.2] (also known as CDP-choline l,2-diacylglycerol cholinephosphotransferase, diacylglycerol cholinephosphotransferase, phosphoryl-choline glyceride transferase, alkylacylglycerol cholinephosphotransferase, and l-alkyl-2-acetylglycerol cholinephosphotransferase) catalyzes the reaction of CDP-choline with 1,2-diacylglycerol to produce CMP and a phosphatidylcholine. l-Alkyl-2-acylglycerol derivatives can also serve as substrates. 

Phorbol esters are promoters that interact with cellular receptors and activate protein kinase C. Usually protein kinase C is activated by Ca++ and diacylglycerol, both of which result from the hydrolysis of phosphoinositides catalyzed by phospholipase C. Phospholipase C is normally activated by several different growth factors. Thus phorbol esters bypass a tightly regulated step in the control of cell growth. Since protein kinase C phosphorylates various proteins, it is not known how this activity participates in establishing a cancerous line of cells. 

Members of the protein kinase C family promote signal transduction by catalyzing ATP-dependent protein phosphorylation [general EC number 2.7.1.37] in response to various signals that promote lipid hydrolysis. The primary activator is diacylglycerol. See also Cal-cium/Calmodulin-Dependent Protein Kinase... 

Several metabohc pathways lead from phosphatidyl inositol to compounds with second messenger" character (review Liscovitch and Cantley, 1994 Divecha and Irvine, 1995). One main pathway, the formation of diacylglycerol and Ins(l,4,5)P3 from PtdIns(4,5)P2, has already been described in 6.4 and Fig. 6.3. Other compounds of regulatory importance can be formed by phosphorylation at the 3 position of the inositol part of Ptdins (Fig. 6.9). The reaction is catalyzed by a class of enzymes known as phosphatidylinositide 3-kinases (P13-kinases). The P13-kinases phosphorylate various phosphatidyl inositol compoimds at the 3 position. For example, PtdIns(3,4,5)P3, produced by 3 phosphorylation of Ptdlns(4,5)P2, has an important function as an intracellular messenger (see 6.6.2). 

The mechanisms by which antitumor-promoters suppress the tumor promotion are not known, but may be due to the following effects (i) inhibition of polyamine metabolism (ii) inhibition of arachidonic acid metabolism (iii) protease inhibition (iv) induction of differentiation (v) inhibition of oncogene expression (vi) inhibition of PKC and (vii) inhibition of oxidative DNA damage [3,6,91]. The polyamine content of cells is correlated to their proliferative, and often, their neoplastic capabilities. A key enzyme in the polyamine biosynthetic pathway, ornithine decarboxylase (ODC), catalyzes the convertion of ornithine to putrescine. Phorbol ester promoters such as TPA cause increased ODC activity and accumulation of polyamines in affected tissues. Diacylglycerol activated PKC, and the potent tumor promoter, TPA, binds to, and activates PKC, in competition with diacylglycerol. PKC stimulation results in phosphorylation of regulatory proteins that affect cell proliferation. Some chemopreventive agents have inhibitory activity towards PKC. Refer to recent review articles for further discussion [3,6,91]. 

Diacylglycerol was mentioned as an activator of protein kinase C. In addition to diacylglycerol, protein kinase C is also activated by increased cellular calcium levels. Protein kinase C catalyzes, for example, the phosphorylation of G protein components the j8 subunit of the fiy complex and the a-GDP complex. Such phosphorylation apparently prevents the reassociation of the a-GDP with the fiy complex to give the G -GDP protein. It is thus clear that phosphorylation via protein kinase C participates in the regulation of G protein function. 

Likewise, phosphatidyl inositol is formed by the transfer of a diacylglycerol phosphate unit from CDP-diacylglycerol to inositol. Subsequent phosphorylations catalyzed by specific kinases lead to the synthesis of phosphatidyl inositol 4,5-bisphosphate, an important molecule in signal transduction. Recall that hormonal and sensory stimuli activate phospholipase C, an enzyme that hydrolyzes this phospholipid to form two intracellular messengers—diacylglycerol and inositol 1,4,5-trisphosphate (Section 15.2). 

Diacylglycerol binds to protein kinase C and activates it. Activated protein kinase C catalyzes the phosphorylation of a number of target proteins, as discussed in the Diet and Cancer chapter. 

Subsequent phosphorylations catalyzed by specific kinases lead to the synthesis of pkosphatidylinositol 4,5-bisphosphate, the precursor of two intracellular messengers diacylglycerol and inositol l,4,S-trisphosphate (Section 14.2). If the alcohol is phosphatidylglycerol, the products are diphosphatidylglycerol (cardiolipin) and CMP. In eukaryotes, cardiolipin is located exclusively in inner mitochondrial membranes and plays an important role in the organization of the protein components of oxidative phosphorylation. 

The major pathway of phosphatidylcholine (lecithin) synthesis is via preformed choline (Fig. 15.5). Phosphotidylethanolamine can be converted to phosphatidylcholine in a minor pathway by the addition of 3CH3 groups (from methionine). Thus, phosphatidylcholine can be synthesized de novo if choline is not available and there is a source of "CH3" groups. In a major pathway, phosphatidylcholine can also be synthesized more directly starting with choline. Choline can be phosphorylated with ATP to form phosphocholine. Phosphocholine can react, via phosphocholine cytidyltransferase, in the presence of CTP, to form CDP choline + pyrophosphate, which is pulled in the forward direction by hydrolysis of the pyrophosphate. The CDP choline then can react with diacylglycerol to form phosphatidylcholine and CMP. This reaction is catalyzed by the enzyme phosphocholinetransferase. The major role of phospholipids in cell membranes is discussed in Chapter 4. 

Fernandez, F. Shridas, P. Jiang, S. Aebi, M. Waechter, C.J. Expression and characterization of a human cDNA that complements the temperature-sensitive defect in dolichol kinase activity in the yeast sec59-l mutant the enzymatic phosphorylation of dolichol and diacylglycerol are catalyzed by separate CTP-mediated kinase activities in Saccharomyces cerevisiae. Cly-cobiology, 12, 555-562 (2002)... 

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