Phospholipid enzymatic cleavage

The other phospholipids can be derived from phosphatidates (residue = phosphatidyl). Their phosphate residues are esterified with the hydroxyl group of an amino alcohol choline, ethanolamine, or serine) or with the cyclohexane derivative myo-inositol. Phosphatidylcholine is shown here as an example of this type of compound. When two phosphatidyl residues are linked with one glycerol, the result is cardiolipin (not shown), a phospholipid that is characteristic of the inner mitochondrial membrane. Lysophospholipids arise from phospholipids by enzymatic cleavage of an acyl residue. The hemolytic effect of bee and snake venoms is due in part to this reaction. 

Phosphatidylcholine (lecithin) is the most abundant phospholipid in membranes. Phosphatidylethanolamine (cephalin) has an ethanolamine residue instead of choline, and phosphatidylserine has a serine residue. In phosphatidylinositol, phosphatidate is esterified with the sugarlike cyclic polyalcohol myo-inositol. A doubly phosphorylated derivative of this phospholipid, phosphatidylinositol 4,5-bisphosphate, is a special component of membranes, which, by enzymatic cleavage, can give rise to two second messengers, diacylglycerol (DAG) and inositol l,4,5trisphosphate (InsPsi see p.386). 

The enzymatic cleavage of an acyl group from phospholipids yields lysophospholipids, which are more water soluble than phospholipids and, therefore, useful as emulsifiers for oils in water. Three enzymes have been employed to conduct hydrolysis, including phospholipases A and A, which cleave the fatty acyl group from the 1- and 2-glycerol... 

The first step in the formation of the leukotrienes is cleavage of arachidonic add from membrane phospholipids. 5-I.ipoxygcnasp, which is a calcium-dependent enzyme, catalyzes the introductionof Oi into the arachidonic add and the conversion of the resulting hydroperoxide to an epoxide. Leukotriene A, contains an epoxide group. Leukobiene (LTA ) can be converted to leukotriene B (LTB4) by enzymatic hydrolysis. LTB is a dihydroxy fatty acid it regulates the functions... 

Enzymatic. Another method of lipid cleavage involves the enzymatic hydrolysis of phospholipids. The enzyme phospholipase A2 catalyzes the hydrolytic cleavage of phospholipids into fatty acids and lysolecithins, which disrupts the vesicle bilayer (225,226). However, it does not attack the polsunerizable bisdienoyl... 

As we have just seen, arachidonic acid is stored as an ester with glycerol in membrane phospholipids. Signaling begins with the cleavage of these quiescent arachidonate stores, an enzymatic reaction driven by external stimuli or by cell-damaging insults (discussed in chapter 2). Arachidonate is mobilized, becoming thus free... 

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