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 - Implications of this salt-bridge in the contractile activity and the oxidative folding reaction

The chimeric peptide [Lys(-2)-Arg(-1)]-sarafotoxin-S6b (KR-SRTb) designed f rom the Lys-2-Arg-1 dipeptide of the endothelin pro-sequence and the sarafo toxin-S6b sequence was synthesized. Its contractile activity was found to b e decreased markedly when compared with that of the parent SRTb. In contras t, the extension by the Lys-Arg dipeptide was found to increase the formati on 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 K R-SRTb was determined by NMR as a function of pH. In the carboxylic acid st ate, the structure consists of the cystine-stabilized a-helical motif, with the or-helical part spanning residues 9-15, and of an unstructured C-termi nal tail. In the carboxylate state, the structure is characterized by a sal t-bridge between Arg(-1) and Asp8, which we identified previously in the [L ys(-2)-Arg(-1)]-endothelin-1 peptide (KR-ET-1). The fact that this salt-bri dge is commonly observed in KR-SRTb and KR-ET-1, despite the 33% sequence d ifference between the corresponding parental peptides, highlights the remar kable 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 e ither the 4-7 sequence of the loop or the C-terminal sequence, appears to b e particularly well suited to improve the stability of the cystine-stabiliz ed alpha-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 alpha-helical motif coul d be a valuable scaffold for the presentation of a library of constrained s hort peptides.