Structure of agkistrodotoxin in an orthorhombic crystal form with six molecules per asymmetric unit

The structure of agkistrodotoxin crystallized under basic conditions has been determined at 2.8 Å resolution by the molecular-replacement technique and refined to a crystallographic R factor of 0.194 and a free R factor of 0.260 with good stereochemistry. The molecular packing in the crystal differs from other PLA₂s. The six molecules in the asymmetric unit form three dimers linked by Ca²⁺ ions in a near-perfect six-ligand octahedral coordinating system. Extensive intermolecular hydrophobic interactions occur at the interfacial recognition site of each neurotoxin molecule, which provides an insight into phospholipase A₂-membrane interactions. This hydrophobic interaction-induced molecular association along the interfacial recognition site suggests a self-protection mechanism of agkistrodotoxin.