Cyclotide biosynthesis

Cyclotides are bioactive macrocyclic peptides from plants that are characterized by their exceptional stability and potential applications as protein engineering or drug design frameworks. Their stability arises from their unique cyclic cystine knot structure, which combines a head-to-tail cyclic peptide backbone with three conserved disulfide bonds having a knotted topology. Cyclotides are ribosomally synthesized by plants and expressed in a wide range of tissues, including leaves, flowers, stems and roots. Here we describe recent studies that have examined the biosynthesis of cyclotides and in particular the mechanism associated with post-translational backbone cyclization

Development of novel frog‐skin peptide scaffolds with selectivity towards melanocortin receptor subtypes

Melanocortin receptors are pharmaceutically important receptors that are involved in complex physiological functions. They have been associated with various diseases including obesity, erectile dysfunction, acne, and skin cancer. It has been challenging to transform nonselective endogenous agonist and antagonist ligands into selective and potent ligands. In this study, we investigated naturally occurring peptides derived from frog skin secretions for selectivity and activity toward melanocortin receptors. Three peptides (ORB, ORB2K and ranacyclin‐T) were found to have selectivity towards the melanocortin receptor 5 (MC5R). ORB and ORB2K had partial binding affinity at nanomolar concentrations, whereas ranacyclin‐T had 57% binding efficiency at 1.6 μM. Backbone cyclization of ORB and ORB2K altered the binding efficiency to melanocortin receptors. Our results suggest that these frog‐skin peptides could be modified for developing melanocortin‐specific ligands and potentially future therapeutics

High-affinity cyclic peptide matriptase inhibitors

The type II transmembrane serine protease matriptase is a key activator of multiple signaling pathways associated with cell proliferation and modification of the extracellular matrix. Deregulated matriptase activity correlates with a number of diseases, including cancer and hence highly selective matriptase inhibitors may have therapeutic potential. The plant-derived cyclic peptide, sunflower trypsin inhibitor-1 (SFTI-1), is a promising drug scaffold with potent matriptase inhibitory activity. In the current study we have analyzed the structure-activity relationships of SFTI-1 and Momordica cochinchinensis trypsin inhibitor-II (MCoTI-II), a structurally divergent trypsin inhibitor from Momordica cochinchinensis that also contains a cyclic backbone. We show that MCoTI-II is a significantly more potent matriptase inhibitor than SFTI-1 and that all alanine mutants of both peptides, generated using positional scanning mutagenesis, have decreased trypsin affinity, whereas several mutations either maintain or result in enhanced matriptase inhibitory activity. These intriguing results were used to design one of the most potent matriptase inhibitors known to date with a 290 pM equilibrium dissociation constant, and provide the first indication on how to modulate affinity for matriptase over trypsin in cyclic peptides. This information might be useful for the design of more selective and therapeutically relevant inhibitors of matriptase

Effects of cyclotides against cutaneous infections caused by Staphylococcus aureus

The main bacterium associated with skin infection is Staphylococcus aureus, occurring especially in infections acquired via surgical wounds, commonly leading to lethal hospital-acquired infections, emphasizing the importance of identifying new antimicrobial compounds. Among them, cyclotides have gained interest due to their high stability and multifunctional properties. Here, cycloviolacin 2 (CyO2) and kalata B2 (KB2) were evaluated to determinate their anti-staphylococcal activities using a subcutaneous infection model. Anti-staphylococcal activities of 50 mM for KB2 and 25 mM for CyO2 were detected with no cytotoxic activities against RAW 264.7 monocytes. In the in vivo assays, both cyclotides reduced bacterial load and CyO2 demonstrated an increase in the phagocytosis index, suggesting that the CyO2 in vivo anti-staphylococcal activity may be associated with phagocytic activity, additionally to direct anti-pathogenicactivity. (C) 2014 Elsevier Inc. All rights reserved