Structural, functional, and bioinformatics studies reveal a new snake venom homologue phospholipase A 2 class

Phospholipases A 2 (PLA 2 s) are enzymes responsible for membrane disruption through Ca 2+ ‐dependent hydrolysis of phospholipids. Lys49‐PLA 2 s are well‐characterized homologue PLA 2 s that do not show catalytic activity but can exert a pronounced local myotoxic effect. These homologue PLA 2 s were first believed to present residual catalytic activity but experiments with a recombinant toxin show they are incapable of catalysis. Herein, we present a new homologue Asp49‐PLA 2 (BthTX‐II) that is also able to exert muscle damage. This toxin was isolated in 1992 and characterized as presenting very low catalytic activity. Interestingly, this myotoxic homologue Asp49‐PLA 2 conserves all the residues responsible for Ca 2+ coordination and of the catalytic network, features thought to be fundamental for PLA 2 enzymatic activity. Previous crystallographic studies of apo BthTX‐II suggested this toxin could be catalytically inactive since a distortion in the calcium binding loop was observed. In this article, we show BthTX‐II is not catalytic based on an in vitro cell viability assay and time‐lapse experiments on C2C12 myotube cell cultures, X‐ray crystallography and phylogenetic studies. Cell culture experiments show that BthTX‐II is devoid of catalytic activity, as already observed for Lys49‐PLA 2 s. Crystallographic studies of the complex BthTX‐II/Ca 2+ show that the distortion of the calcium binding loop is still present and impairs ion coordination even though Ca 2+ are found interacting with other regions of the protein. Phylogenetic studies demonstrate that BthTX‐II is more phylogenetically related to Lys49‐PLA 2 s than to other Asp49‐PLA 2 s, thus allowing Crotalinae subfamily PLA 2 s to be classified into two main branches: a catalytic and a myotoxic one. Proteins 2010. © 2010 Wiley‐Liss, Inc.