Phosphorylcholine:diacylglycerol

The nature of the reaction catalyzed by PLC i( in which phosphatidylcholine is split into diacylglycerol and phosphorylcholine (Fig. 11) requires two proton transfer steps The first is the deprotonation of an active site water to generate the attacking hydroxide nucleophile, and the second is the protonation of the alkoxide leaving group. Although analyses of the X-ray structures of PLCSc and... 

Diacylglycerol has long been known to be a weak competitive inhibitor of PLC/fc, whereas phosphorylcholine shows very little inhibition [40, 49, 116]. Recent kinetic assays of PLCB(. activity in the presence of DAG indicate that it is a competitive inhibitor with a Kl of the order of 10 mM, whereas phosphorylcholine was found to be an extremely weak (K = 30-50 mM), mixed inhibitor of PLC/J( [34]. Because diacylglycerol is a competitive inhibitor of the enzyme, the nature of the catalytic cycle dictates that it must be the last product to leave the enzyme active site. 

Fig. 17. Catalytic cycle for PLQ,. After substrate binding, hydrolysis occurs in the rate-determining step, followed by the sequential release of phosphorylcholine and diacylglycerol. Amino acids known to be involved in substrate binding are shown, and zinc ions appear as filled circles...

Phosphatidy Icholine rac-l,3-Diacylglycerol-2-phosphorylcholine 2-Methyl derivatives of acyl chain... 

Phosphatidylcholine can be completely hydrolysed with aqueous acid to produce fatty acids, glycerol, phosphoric acid and choline. With alkali, the fatty esters are preferentially hydrolysed, leaving the glycero-phosphoryl-cholines, which can in turn hydrolyse slowly into glycerophosphoric acid and choline. Enzymatic hydrolysis occurs selectively at different ester sites by several phospholipases to produce a range of products. Phospholipases A1 and A2 are obtained commercially from snake venom. They hydrolyse the fatty acid at the -1 and sn-2 positions respectively, to produce lysophosphatidyl-choline. Phospholipase C catalyses the hydrolysis of the phosphate moiety to give 1,2-diacylglycerols and phosphorylcholine. Phospholipase D found in plant tissues mainly catalyses hydrolysis of the phosphate ester to produce choline and phosphatidic acids (1). 

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