Mammals contain 9–10 secreted phospholipases A 2 (sPLA 2s) that display widely different affinities for membranes, depending on the phospholipid composition. The much higher enzymatic activity of human group X sPLA 2 (hGX) compared with human group IIA sPLA 2 (hGIIA) on phosphatidylcholine (PC)-rich vesicles is due in large part to the higher affinity of the former enzyme for such vesicles; this result also holds when vesicles contain cholesterol and sphingomyelin. The inclusion of anionic phosphatidylserine in PC vesicles dramatically enhances interfacial binding and catalysis of hGIIA but not of hGX. This is the result of the large number of lysine and arginine residues scattered over the entire surface of hGIIA, which cause the enzyme to form a supramolecular aggregate with multipl
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