The chemical basis for interfacial activation of monomeric phospholipases A2. Autocatalytic derivatization of the enzyme by acyl transfer from substrate

Abstract

A basic monomeric phospholipase A2 from the venom of the American water moccasin, Agkistrodon piscivorus piscivorus, undergoes Ca2+-dependent, autocatalytic acylation during the course of hydrolysis of both model and natural phospholipid substrates. Acylation occurs at 2 lysine residues, Lys-7 and Lys-10, in the NH2-terminal alpha-helical segment of the enzyme, and when both positions are fully derivatized, the stable bisacylphospholipase A2 becomes a dimer in solution. The acylated enzyme is fully activated toward monomolecular layers of lecithins. Similar studies applied to the monomeric phospholipases A2 from porcine pancreas and from the venom of Agkistrodon contortrix contortrix also showed irreversible activation of the enzymes by substrate with the same kinetic consequences and formation of dimers. Acylation thus enables these enzymes to overcome the lag period observed under such conditions with native monomeric phospholipases, a phenomenon referred to as interfacial activation. Activation of the enzyme by acylation potentiates the phospholipase for interfacial recognition via formation of a dimeric enzyme. The naturally occurring phospholipase A2 dimer from Crotalus atrox venom displays no lag in the hydrolysis of lecithin monolayers nor does it undergo substrate level acylation. These facts support our proposal that dimerization concomitant with acylation is responsible for the large rate enhancements seen in the hydrolysis of aggregated phospholipids by monomeric phospholipases. Our findings demonstrate for the first time a chemical mechanism for interfacial activation of and interfacial recognition by phospholipases A2.