Interfacial recognition by bee venom phospholipase A2:: Insights into nonelectrostatic molecular determinants by charge reversal mutagenesis

The basis for tight binding of bee venom phospholipase A(2) (bvPLA2) to anionic versus zwitterionic phospholipid interfaces is explored by charge reversal mutagenesis of basic residues (lysines/ arginines to glutamates) on the putative membrane binding surface, Single-site mutants and, surprisingly, multisite mutants (2-5 of the 6 basic residues mutated) are fully functional on anionic vesicles. Mutants bind tightly to anionic vesicles, and active-site substrate and Ca2+ binding are not impaired. Multisite mutants undergo intervesicle exchange slightly faster than wild type, especially in the presence of salt, It is estimated that electrostatic contribution to interfacial binding is modest, perhaps 2-3 kcal/mol of the estimated 15 kcal/mol. Elution properties of bvPLA2 from HPLC columns containing solid phases of tightly packed monolayers of phosphocholine amphiphiles suggest that ionic effects provide a modest portion of the interfacial binding energy and that this contribution decreases as the number of cationic residues mutated is increased. These results are consistent with the observation that Gila monster venom PLA2 (Pa2), which is homologous to bvPLA2, has high activity on anionic vesicles despite the fact that it has only a single basic residue on its putative interfacial recognition face. Results with bvPLA2 mutants show that manoalogue and 12-epi-scalaradial inactivate bvPLA2 by modification of K94. Also, deletion of the large beta-loop (residues 99-118) is without consequence for interfacial binding and catalysis of bvPLA2. AU together, the preferential binding of bvPLA2 to anionic vesicles versus phosphatidylcholine vesicles is mainly due to factors other than electrostatics. Therefore hydrogen-bonding and hydrophobic interactions must provide a major portion of the interfacial binding energy, and this is consistent with recent spectroscopic studies.