Reduced pH induces an inactive non-native conformation of the monomeric bothropstoxin-I (Lys49-PLA2)

Bothropstoxin-I (BthTx-I), a Lys49-PLA2 from Bothrops jararacussu venom, permeabilizes membranes by a non-hydrolytic Ca2+-independent mechanism. The BthTx-I showed activity against liposomes including 10% and 50% negatively charged lipids at pH 7.0, but not at pH 5.0. Nevertheless, ultracentrifugation and FRET demonstrated that at pH 5.0 the BthTx-I is bound to 50% negatively charged membranes. ANS binding identified a non-native monomeric conformation at pH 5.0, suggesting that tertiary structure alterations result in activity loss of the BthTx-I at low pH.