A type-II β-turn, proline-containing, cyclic pentapeptide as a building block for the construction of models of the cleavage site of pro-oxytocin

Previous studies have indicated that proteolytic activation of pro-hormones and pro-proteins occurs most frequently at the level of basic amino acids arranged in doublets and that the dibasic sites are situated in or next to beta -turns. investigations utilizing synthetic peptides reproducing the N-terminal processing domain of pro-oxytocin-neurophysin have suggested a close relationship between the secondary structure of the cleavage locus and enzyme recognition, the minimal recognized sequence being the -Pro-Leu-Gly-Gly-Lys-Arg-Ala-Val-Leu- segment of the native precursor. NMR investigations and energy minimization studies have demonstrated that this sequence is organized in two type-II beta -turns involving the -Pro-Leu-Gly-Gly- and -Lys-Arg-Ala-Val- sequences. To further strengthen the above reported hypothesis and to study the role of turn subtypes, a new proline containing cyclic substrate of the processing enzyme, in which the N-terminal side that comes before the Lys-Arg pair is constrained to adopt a type-II beta -turn, has been synthesized. The presence of a type-II g-turn structure in this cyclic peptide model has been demonstrated by a combined NMR, CD and FT-IR absorption investigation. A preliminary study shows that PC1 is able to recognize and process our constrained substrate. Copyright (C) 2001 European Peptide Society and John Wiley & Sons, Ltd.