27Background: Up-regulation of kallikreins (KLK) including KLK5 has been reported in atopic 28

a b s t r a c tSunflower Trypsin Inhibitor (SFTI-1) analogues have been prepared from simple linear precursors produced either by chemical synthesis or following purification from Escherichia coli. We have shown, for the first time that these linear SFTI-1 derived peptide sequences can be converted to circular peptides via selective consecutive acyl transfer reactions, and that the products derived from synthetic and bacterial origin are identical. Preliminary analysis of the semi-synthetic SFTI-1 analogues confirmed SFTI-I10H as an inhibitor of Kallikrein-5 (KLK5) protease that could also mediate its action on human keratinocytes. The preliminary results obtained serve as a useful starting point for the biological production of SFTI-1 based, selective KLK5 inhibitors for the treatment of atopic dermatitis.

Understanding the factors that influence N → S acyl transfer in native peptide sequences, and discovery of new reagents that facilitate it, will be key to expanding its scope and applicability. Here, through a study of short model peptides in thioester formation and cyclisation reactions, we demonstrate that a wider variety of Xaa-Cys motifs than originally envisaged are capable of undergoing efficient N → S acyl transfer. We present data for the relative rates of thioester formation and cyclisation for a representative set of amino acids, and show how this expanded scope can be applied to the production of the natural protease inhibitor Sunflower Trypsin Inhibitor-1 (SFTI-1).