CONFORMATIONAL-ANALYSIS OF ENDOTHELIN-1 - EFFECTS OF SOLVATION FREE-ENERGY

In order to investigate conformational preferences of the 21-residue p eptide hormone endothelin-1 (ET-1). an extensive conformational search was carried out ill vacuo using a combination of high temperature mol ecular dynamics/annealing and a Monte Carlo/minimization search in tor sion angle space. Fully minimized conformations from the search were g rouped into families rising a clustering technique based on rms fittin g over the Cartesian coordinates of the atoms of the peptide backbone of the ring region. A wide range of local energy minima were identifie d even through two disulfide bridges (Cys(1)-Cys(15) and Cys(3)-Cys(11 )) constrain the structure of the peptide. Low energy, conformers of E T-1 as a nonionized species in vacuo are stabilized by intramolecular inter action of the ring region (residues 1-15) with the tail (residue s 16-21). Strained conformations for individual residues are observed. Conformational similarity to protein loops is established by marching to protein crystal structures. In older to assess the influence of aq ueous environment on conformational preference, the electrostatic cont ribution to the solvation energy, was calculated for ET-1 as a fully, ionized species (Asp(8), Lys(9), Glu(10), Asp(18), N- and C-terminus) using a continuum electrostatics model (DelPhi) for each of the confor mers generated in vacuo, and the total solvation free energy was estim ated by adding a hydrophobic contribution proportional to solvent acce ssible surface area. Solvation dramatically alters the relative energe tics of ET-1 conformers front that calculated in vacuo. Conformers of ET-I favored by the electrostatic solvation energy in water include co nformers with helical secondary structure in the region of residues 9- 15. Perhaps of most importance, it was demonstrated that the contribut ion to solvation by an individual charge depends not only on its solve nt 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 charge s systematically turned ''on'' and ''off.'' The cooperative effect of multiple charges on solvation demonstrated in this manner calls into q uestion models that relate solvation energy simply to solvent accessib ility by atom or residue alone. (C) 1995 John Wiley and Sons, Inc.