Conformational analysis of endothelin-1: effects of solvation free energy.

In order to investigate conformational preferences of the 21-residue peptide hormone endothelin-1 (ET-1), an extensive conformational search was carried out in vacuo using a combination of high temperature molecular dynamics/annealing and a Monte Carlo/minimization search in torsion angle space. Fully minimized conformations from the search were grouped into families using a clustering technique based on rms fitting over the Cartesian coordinates of the atoms of the peptide backbone of the ring region. A wide range of local energy mining were identified even though two disulfide bridges (Cys1-Cys15 and Cys3-Cys11) constrain the structure of the peptide. Low energy conformers of ET-1 as a nonionized species in vacuo are stabilized by intramolecular interaction of the ring region (residues 1-15) with the tail (residues 16-21). Strained conformations for individual residues are observed. Conformational similarity to protein loops is established by matching to protein crystal structures. In order to assess the influence of aqueous environment on conformational preference, the electrostatic contribution to the solvation energy was calculated for ET-1 as a fully ionized species (Asp8, Lys9, Glu10, Asp18, N- and C-terminus) using a continuum electrostatics model (DelPhi) for each of the conformers generated in vacuo, and the total solvation free energy was estimated by adding a hydrophobic contribution proportional to solvent accessible surface area. Solvation dramatically alters the relative energetics of ET-1 conformers from that calculated in vacuo. Conformers of ET-1 favored by the electrostatic solvation energy in water include conformers with helical secondary structure in the region of residues 9-15. Perhaps of most importance, it was demonstrated that the contribution to solvation by an individual charge depends not only on its solvent accessibility but on the proximity of other charges, i.e., it is a cooperative effect. This was shown by the calculation of electrostatic solvation energy as a function of conformation with individual charges systematically turned "on" and "off". The cooperative effect of multiple charges on solvation demonstrated in this manner calls into question models that relate solvation energy simply to solvent accessibility by atom or residue alone.