Molecular Modeling of Folding in Lactam-Modified Conotoxins

Abstract

We report a method to model the conformational folding of α-conotoxins and the factors that affect the synthesis of specific regioisomers by using a combination of molecular dynamics methods to determine the geometric factors (S-S distances and C-N distances in lactam-modified α-conotoxins) and ab initio methods to determine the conformational energy and molecular orbital information. In the literature, the replacement of the Cys2-Cys7 disulfide bridge with a lactam bridge caused a complete loss of activity. However, exchanging the larger Cys3-Cys13 bridge led to analogues that exhibited considerable affinities for the receptor sites. In this work, we examine the effect of the exchange of the latter bridge by replacing Cys3 with an aspartate residue and the Cys13 with a basic amino acid. The results show that thermal fluctuations lead to configurations where a molecular orbital overlap between S-S atoms (Cys2-Cys7) can take place, leading to the proper regioisomer formation. Furthermore, ab initio methods predict adequate orbital overlap between the sulfur atoms. In addition, the amino acid proline appears to generate rigidity in its surrounding amino acids, specifically in at least the region controlling the relative orientation of the Cys2 and Cys7 residues. The length of the methylene chain of the basic amino acid at position 13 affects the probability of forming a lactam bridge between positions 7 and 13. With short chains (one methylene group between the backbone and the amino group), there never is any observed orbital overlap between the carbon and nitrogen atoms, possibly because of the rigidity of the backbone. The probability of robust overlap increases with longer chain size and it is expected to match the efficiency of the Cys2-Cys7 overlap when using lysine at position 13.