Conformational flexibility of a scorpion toxin active on mammals and insects: a circular dichroism study

Three scorpion toxins have been analyzed by circular dichroism in water and in 2,2,2-trifluoroethanol (TFE) solutions. These toxins were chosen because they are representative of three kinds of pharmacological activities: (1) toxin AaH IT2, an antiinsect toxin purified from the venom of Androctonus australis Hector, which is able to bind to insect nervous system preparation, (2) toxin Css II, from the venom of Centruroides suffusus suffusus, which is a beta-type antimammal toxin capable of binding to mammal nervous system preparation, and (3) the toxin Ts VII from the venom of Tityus serrulatus, which is able to bind to both types of nervous systems. In order to minimize bias, CD data were analyzed by a predictive algorithm to assess secondary structure content. Among the three molecules, Ts VII presented the most unordered secondary structure in water, but it gained in ordered forms when solubilized in TFE. These results indicated that the Ts VII backbone is the most flexible, which might result in a more pronounced tendency for this toxin molecule to undergo conformational changes. This is consistent with the fact that it competes with both antiinsect and beta-type antimammal toxins for the binding to the sodium channel.