Head-to-Tail Cyclization after Interaction with Trypsin: A Scorpion Venom Peptide that Resembles Plant Cyclotides

Peptidase
inhibitors (PIs) have been broadly studied due to their
wide therapeutic potential for human diseases. A potent trypsin inhibitor
from Tityus obscurus scorpion venom
was characterized and named ToPI1, with 33 amino acid residues and
three disulfide bonds. The X-ray structure of the ToPI1:trypsin complex,
in association with the mass spectrometry data, indicate a sequential
set of events: the complex formation with the inhibitor Lys32 in the trypsin S1 pocket, the inhibitor C-terminal residue Ser33 cleavage, and the cyclization of ToPI1 via a peptide bond
between residues Ile1 and Lys32. Kinetic and
thermodynamic characterization of the complex was obtained. ToPI1
shares no sequence similarity with other PIs characterized to date
and is the first PI with CS-α/β motif described from animal
venoms. In its cyclic form, it shares structural similarities with
plant cyclotides that also inhibit trypsin. These results bring new
insights for studies with venom compounds, PIs, and drug design.