Structural consequences of metal complexation of cyclo[Pro-Phe-Phe-Ala-Xaa](2) decapeptides

The conformational features of both free and Ca2+-complexed cyclo[Pro-Phe-P he-Ala-Xaa](2) (with Xaa = Glu(OtBu), Lys(ClZ), Leu, and Ala) in solution h ave been determined by NMR spectroscopy and extensive distance-geometry cal culations. The decapeptides are conformationally homogeneous in solution an d show common structural features in their free and complexed forms. The st ructures of the free form contain only trans peptide bonds and are topologi cally similar to the structure of gramicidin-S, folded up in two antiparall el extended structures, stabilized by interstrand hydrogen bonds. and close d at both ends by two beta -turns. In contrast, the Ca2+-complexed peptides present two cis peptide bonds and are generally similar to those observed for the metal-complexed forms of antamanide and related analogues, folded i nto a saddle shape with two beta -turns. The Glu(OtBu)-, Leu-, and Lys(ClZ) -containing peptides examined here maintain the biological activity of the cyclolinopeptide A in their ability to competitively inhibit cholate uptake . The natural antamanide and cyclolinopeptide A are both able to inhibit th e uptake of bile salts into hepatocytes, They share the same postulated act ive sequence Pro-Phe-Phe. Based on our structural results, we conclude that the ability to adopt a global conformation, characterized by a clear amphi pathic separation of hydrophobic and hydrophilic surfaces, is an important feature for the functioning of this class of peptides.