Circular dichroic and fluorescence spectroscopic study of the conformation of botulinum neurotoxin types A and E

Botulinum neurotoxin (NT) is synthesized by Clostridium botulinum in any of seven antigenically distinct forms, called types A through G. Protease(s) endogenous to the bacteria, or trypsin, nicks the single chain protein to a dichain molecule which generally is more toxic. The conformation of dichain type A (nicked by endogenous protease), single chain type E, and dichain type E NT (nicked by trypsin) have been determined using circular dichroism (CD) and fluorescence spectroscopy. The high degree of ordered secondary structure (alpha helix 28%, beta sheet 42%, total 70%) found in type A NT at pH 6.0 was similar to that found at pH 9.0 (alpha 22%, beta 47%, total 69%). The secondary structure of the single chain type E NT at pH 6.0 (alpha 18%, beta 37%, total 55%) differed somewhat from these values at pH 9.0 (alpha 22%, beta 43%, total 65%). The dichain type E NT at pH 6.0 assumed a secondary structure (alpha 20%, beta 47%, total 67%) more similar to that of dichain type A than the single chain type E NT. Examination with the fluorogenic probe toluidine napthalene sulfonate revealed that the hydrophobicity of the type A and E NTs were higher at pH 9.0 than at pH 6.0. Also, the hydrophobicity of the dichain type E NT was higher than its precursor the single chain protein and appeared similar to that of the dichain type A NT. The CD and fluorescence studies indicate that conversion of the single chain type E NT to the dichain form (i.e. nicking by trypsin) induced changes in conformation.(ABSTRACT TRUNCATED AT 250 WORDS)