The Interaction of Pancreatic Phospholipase A2 with Negatively Charged Substrates — Application: The Transformation of Soluble Phospholipase A2 into a Highly Penetrating “Membrane-Bound” Form
This study deals with some kinetic properties of porcine pancreatic phospholipase A2 (PLA2) acting on neutral and anionic substrates. Short—chain diacylglycerosulfates possess a much higher affinity to the enzyme than the corresponding lecithins. At alkaline pH, micellar solutions of the negatively charged sulfates are hydrolyzed ten times faster than the zwitterionic lecithins. With both substrate classes, maximal enzymatic activity is always found in multimolecular aggregates containing several PLA2 molecules and a number of substrate monomers. The formation of these “high molecular weight” complexes, however, is different for lecithins and the anionic sulfates: with neutral phosphatidylcholine, the enzyme interacts with preformed micelles at substrate concentrations above the CMC. The zymogen has no affinity for these zwitterionic interfaces. The anionic lipids, however, induce enzyme aggregation already at substrate concentrations well below the CMC and the resulting complexes contain a premicellar aggregate of substrate monomers. In these rather unstable aggregates PLA2 displays very high enzymatic activity. Although the zymogen is also able to form similar high molecular weight complexes, this protein does not become activated.
These differences in the interaction of pancreatic PLA2 with neutral and negatively-charged substrates are attributed to the presence of two cationic amino acids in the hydrophobic lipid binding domain: Arg-6 and Lys-116. Specific acylation of Lys-116 with a long-chain fatty acid enables the enzyme to penetrate densily-packed monolayers and to attack liological membranes.
KeywordsAnionic Lipid Detergent Concentration Pancreatic Phospholipase Pancreatic PLA2 Naja Naja
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