The chimeric peptide [Lys(−2)-Arg(−1)]-sarafotoxin-S6b, composed of the endothelin pro-sequence and sarafotoxin, retains the salt-bridge staple between Arg(−1) and Asp8 previously observed in [Lys(−2)-Arg(−1)]-endothelin

The chimeric peptide [Lys(−2)-Arg(−1)]-sarafotoxin-S6b (KR-SRTb) designed from the Lys–2-Arg–1 dipeptide of the endothelin pro-sequence and the sarafotoxin-S6b sequence was synthesized. Its contractile activity was found to be decreased markedly when compared with that of the parent SRTb. In contrast, the extension by the Lys-Arg dipeptide was found to increase the formation of the native disulfide isomer (82/18 versus 96/4) when the reaction was carried out in the presence of redox reagents. The solution structure of KR-SRTb was determined by NMR as a function of pH. In the carboxylic acid state, the structure consists of the cystine-stabilized α-helical motif, with the α-helical part spanning residues 9–15, and of an unstructured C-terminal tail. In the carboxylate state, the structure is characterized by a salt-bridge between Arg(−1) and Asp8, which we identified previously in the [Lys(−2)-Arg(−1)]-endothelin-1 peptide (KR-ET-1). The fact that this salt-bridge is commonly observed in KR-SRTb and KR-ET-1, despite the 33% sequence difference between the corresponding parental peptides, highlights the remarkable adaptability of the Lys-Arg extension for the formation of a special salt-bridge. As a consequence, this salt-bridge, which does not depend on either the 4–7 sequence of the loop or the C-terminal sequence, appears to be particularly well suited to improve the stability of the cystine-stabilized α-helical motif. Therefore, because of its high yield in the native disulfide arrangement and its high permissiveness for sequence mutation in the 4–7 loop, such a stabilized cystine-stabilized α-helical motif could be a valuable scaffold for the presentation of a library of constrained short peptides.