In this study we have shown that sunflower trypsin lnhibitor (SFTI-1) analogues are readily available through either synthetic or biological means, from simple linear precursors, by N_S acyl transfer followed by spontaneous cyclisation through native chemical ligation. The cyclic synthetic and bacterially derived products are identical. 1H NMR analysis also confirmed that, in contrast to the I10G analogue, SFTI-I10H adopts a single preferred conformation, perhaps emphasising the importance of the steric bulk associated with the side chain of this residue. While chemical synthesis offers advantages of simple incorporation of various amino acid analogues, the biological approach is easily scaled, less wasteful, ideal for library production through mutagenesis, and only requires water as solvent. Perhaps unsurprisingly (fig 1) both I10G and I10H analogues were found to be KLK5 inhibitors in vitro. I10H was determined to have an IC50 of 0.14 uM and the increased activity of SFTI-I10H over I10G was confirmed in human keratinocytes. The results serve as a useful starting point for the development of more potent and selective KLK5 inhibitory peptides, based on the SFTI scaffold