Structural analysis of botulinum neurotoxin types A and E in aqueous and nonpolar solvents by Fourier transform infrared, second derivative UV absorption, and circular dichroic spectroscopies

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Abstract
Two pharmacologically similar but antigenically distinct botulinum neurotoxins, types A and E with a 1000-fold difference in their toxicity, were examined for nonpolar solvent-induced changes in secondary structures and polypeptide foldings to understand their structural differences and their comparative responsiveness/susceptibility to solvent perturbation. Analysis of far UV circular dichroic spectra in aqueous buffer for types A and E neurotoxins yielded the following: the α-helix contents were 27 and 20%; the β-sheets were 36 and 44%, the β-turns were 6.0 and 0%, and the random coils were 31 and 36%, respectively. Fourier transform infrared spectra, obtained by using attenuated total reflection technique, indicated high content of α-helix and β-pleated sheet structures for both neurotoxins as judged by strong bands at 1651 and 1633 cm−1 in the amide I frequency region and bands at 1314 and 1245 cm−1 in the amide III frequency region. The peak height ratio of 1314 and 1245 cm−1 bands, suggests that the type A neurotoxin has slightly higher α-helical content than the type E neurotoxin. These observations are consistent with the secondary structures estimated from far UV circular dichroic spectra. Fourier transform infrared spectra of the neurotoxins, exposed to methanol, showed sharp increases of the 1651 cm−1 band and a significant increase in the height of the 1314 cm−1 band, suggesting increases in the α-helical contents of the proteins. The changes were more in the type A than in the type E neurotoxin. The changes were reversible upon reexposure of the proteins to the aqueous buffer. Second derivative absorption spectroscopy demonstrated that methanol also induced changes in the degree of Tyr exposure to solvent. The results are discussed in terms of structural differences between the single and dichain neurotoxins and in terms of their mode of action.