Molecular Modeling of Folding in Lactam-Modified (Glutamate+Lysine Analog) 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 causes a complete loss of activity. However, exchanging the larger Cys3-Cys13 bridge leads to analogues that exhibit considerable affinities for the receptor sites. In previous work [Osysko et al. Biophys. J. 2011, 100, 155a], we studied the effect of side-chain length at position 13 by replacing this amino acid with modified lysines, where one to four methylene units would separate the alpha carbon and the ammonium group. Cys 3 was then replaced by aspartate. In this work, we examine a similar effect but replacing Cys3 with a glutamate residue instead. 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. 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. Surprisingly, we find that the best overlap conditions are achieved for very short chains (only one methylene group separating the amino group and the alpha carbon) and, to a slightly smaller extent, for the longest chains (four methylene groups, i.e. lysine). Few overlap arrangements were observed in the simulations with two or three methylene groups.