Structural Characterization of Myotoxic Ecarpholin S from Echis carinatus Venom
Xingding Zhou 1, Tien-Chye Tan 1, Suresh Valiyaveettil 1, Mei-Lin Go 1, R. Manjunatha Kini 1, Adrian Velazquez-Campoy 2 and Jayaraman Sivaraman 3*
1 National University of Singapore
2 Institute of Biocomputation and Physics of Complex Systems, University of Zaragoza
3 National University of Sigapore
* To whom correspondence should be addressed. E-mail: dbszhoux{at}nus.edu.sg.
Submitted on July 18, 2007
Revised on August 13, 2007
Accepted on 22 May 2008
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Abstract |
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Phospholipase A2 (PLA2), a common toxic component of snake venoms, has been implicated in various pharmacological effects. Ecarpholin S, isolated from the snake venom Echis carinatus sochureki, is a phospholipase PLA2 (PLPLA2) belonging to the Ser49-PLPLA2 subgroup. It has been characterized as having low enzymatic and yet potent myotoxic activities. Crystal structures of the native ecarpholin S and its complexes with lauric acid, and its inhibitor suramin have been elucidated. This is the first report of the structure of a member of the Ser49-PLPLA2 subgroup. We have also examined interactions of ecarpholin S with phosphatidyglycerol and lauric acid using Surface Plasmon Resonance, and suramin with Isothermal Titration Calorimetry studies. Most Ca2+-dependent PLPLA2 enzymes have Asp in the position 49 which plays a crucial role in Ca2+ binding. The three-dimensional structure of ecarpholin S reveals a unique conformation of the Ca2+-binding loop that is not favorable for Ca2+ coordination. Furthermore, the endogenously bound fatty acid (lauric acid) in the hydrophobic channel may also interrupt the catalytic cycle. These two observations may account for its low enzymatic activity, despite the full retention of the catalytic machinery. They may also be applicable to other non-Asp49-PLPLA2 enzymes. The interaction of suramin in its complex with ecarpholin S is quite different than that reported for the Lys49-PLPLA2/suramin complex, where the interfacial recognition face (i-face), C-terminal and N-terminal regions of ecarpholin S play important roles. The present study provides significant structural and functional insights into the myotoxic activity of ecarpholin S and, also in general, for non-Asp49-PLA2 enzymes.
Key Words:
Crystal structure, Isothermal Titration Calorimetry, PLA2, Protein-ligand interaction, Ser49-PLA2, Snake venom
