Spider venom phospholipase D toxin structure: Interfacial binding site, mechanism, activation, and head group preference

bound to a micelle-like agglomeration of product and substrate sphingolipids. Each enzyme subunit binds three sphingolipid molecules, one in the active site and two at adjacent noncatalytic sites, generating an interface that approximates the IBS predicted by molecular dynamics. The conformations of substrate and cyclic product in the active site definitively confirm our previously proposed catalytic mechanism. Comparisons with lipid-free structures show conformational changes in two loops that suggest a mechanism for allosteric/surface activation. Docking studies suggest that the variable preference of these toxins for phosphocholine and phosphoethanolamine head groups involves subtle changes in size and shape of the active-site pocket. The structures reveal key facets of the molecular basis of loxoscelism and show that in favorable cases crystallography can illuminate the IBS of peripheral membrane proteins.