STRUCTURE OF A BASIC PHOSPHOLIPASE A(2) FROM AGKISTRODON HALYS PALLASAT 2.13 ANGSTROM RESOLUTION

The basic phospholipase A(2) isolated from the venom of Agkistrodon ha lys Pallas (Agkistrodon blomhoffii Brevicaudus) is a hemolytic toxin a nd one of the few PLA(2)'s capable of hydrolyzing the phospholipids of E. coil membranes in the presence of a bactericidal/permeability-incr easing protein (BPI) of neutrophils. The crystal structure has been de termined and refined at 2.13 Angstrom to a R factor of 16.5% (F > 3 si gma) with excellent stereochemistry. A superposition of the two molecu les in the asymmetric unit gives an r.m.s. deviation of 0.326 Angstrom for all C alpha atoms. The refined structure allowed a detailed compa rison with other PLA(2) species of known structures. The overall archi tecture is similar to those of other PLA(2)'s with a few significant d ifferences. One of which is in the region connecting the N-terminal he lix and the Ca2+-binding loop. Unexpectedly, the conformation of the p eptide plane Cys29-Gly30 in the Ca2+-binding loop is very different to that of other PLA(2)'s. The amide NH of Gly30 does not point toward t he proposed site for stabilization of the tetrahedral intermediate oxy anion of the substrate analogue. The structure includes four residues which occur less frequently in other PLA(2)'s. His1, Arg6 and Trp70 lo cated at the interfacial recognition site may play an important role i n the interaction with aggregated substrates, while Trp77 contributes to the hydrophobic interactions between the beta-wing and the main bod y of the molecule. This structure analysis reveals that two clusters o f basic residues are located at or near the interfacial recognition si te, forming an asymmetric positively charge distribution. In contrast to the acidic isoform, the present enzyme is a dimer in the crystallin e state. The special phospholipid hydrolysis behaviors are discussed i n the light of the structure determined.