Cyclotides are small disulfide-rich proteins that have the
unusual feature of a cyclic backbone (hence the name
cyclo - peptides). They contain
six conserved cystine residues that are arranged in a cystine knot
topology in which two disulfide bonds and their connecting
backbone segments form an embedded ring in the structure that is
penetrated by a third disulfide bond, as shown below.
Cyclotides have a range of interesting biological activities
including anti-HIV and neurotensin inhibition, anti-microbial
activity and insecticidal activity. They are found in a variety
of tropical plants from the Rubiaceae and Violaceae families.
The structure of kalata B1 showing the distorted
beta-sheet topology and the loop nomenclature enabled by the