Dynamics of the active loop of snake toxins as probed by time-resolved polarized tryptophan fluorescence

The local environment and dynamics of the single tryptophan residue in the respective active loops of cardiotoxin and alpha-neurotoxin from Naja nigricollis and of erabutoxin b from Laticauda semifasciata have been studied by steady-state and time-resolved polarized fluorescence and analyzed with distributions of decay times. Trp11 in loop I of cardiotoxin exhibits a very broad and complex distribution of fluorescence lifetimes at 20 degrees C. Despite its relatively external location in the toxin, the residue appears to be partly shielded from water and shows restricted but significant conformational fluctuations on the picosecond and nanosecond time scales. The thermal stability of cardiotoxin allowed a study of its static and dynamic fluorescence properties over a large range of temperatures. Interconversions in the intermediate nanosecond range lead to a thorough reorganization of the cardiotoxin fluorescence lifetime distribution with temperature. On the contrary, the fluorescence kinetics of Trp29 in loop II of the two neurotoxins is dominated by about 80% of a major decay time, which suggests that a nearly unique local conformation of the toxin is maintained over all time scales above the sub-nanosecond range. The fluorescence anisotropy decays show that the residue also has extremely limited rotational freedom down to the picosecond time scale. These findings are in good agreement with structural and dynamic information previously reported on the different toxins from NMR and X-ray crystallographic studies. The different dynamic properties around the tryptophan residue of the cardiotoxin and neurotoxin active loops can be analyzed within the frame of their different respective mechanisms of toxicity.