Dynamics of saxitoxin binding to saxiphilin c-lobe reveals conformational change

Thermodynamic parameters (ΔG, ΔH, ΔS, ΔCp) have been determined to evaluate the dynamics of binding of saxitoxin to the c-lobe of saxiphilin. We have developed an improved method to rapidly express and purify recombinant saxiphilin c-lobe, and fully characterized it by mass spectrometry for the first time. Surface plasmon resonance (SPR) was used to characterize the interaction between saxitoxin and immobilized c-lobe. At 298 K, c-lobe binds saxitoxin with KD=1.2 nM, ΔH°=−11.7±0.8 kcal/mol, and ΔS°=1.17±0.07 cal/mol K. Analysis of ΔCp of toxin association at several temperatures suggests that hydrophobic forces contribute to the binding event. Additionally, changes in 8-anilino-1-naphthalene sulfonic acid (ANS) fluorescence upon binding to c-lobe in the presence and absence of saxitoxin support a conformational change in c-lobe upon saxitoxin binding.