[A priori calculations of spatial structure of neurotoxin. Fragment Leu1-Cys23]

Conformational properties of a linear and cyclic Leu1-Cys23 neurotoxine fragment have been investigated. Starting from a linear array, the most favourable cyclic form with the Cys3-Cys23 disulfide bond is shown to arise from one of the low-energy extended structures. The formation of the disulfide bond does not interfere with stabilising contacts: the proximation of S-atoms to the valence distance can be effected by small changes of the labile fragment geometry. The results obtained are indicative of the strictly predetermined mechanism of the protein folding into the native conformation with the proximal Cys residues, forming a disulfide bridge. The calculated low-energy Leu1-Cys23 neurotoxine form coincides with the X-ray structure of the corresponding fragment of erabutoxine b. For the neurotoxine II homologies, neurotoxines alpha, D, 4, CM-14 and beta, optimal orientations of side chains are determined. The amino acid changes in the 1--23 fragment are shown not to disturb the backbone form established for the neurotoxine II, and thus not to change the geometry necessary for the formation of the Cys3-Cys23 bridge.