|Oldenlandia affinis was used by native women in the Zaire as an aid during childbirth. A tea was made of the leaves and imbibed during labour.|
Cyclotides are plant-derived peptides of approximately 30 amino acids. They have the characteristic structural features of a head-to-tail cyclized backbone and a cystine knot arrangement of their three conserved disulfide bonds. Their unique structural features lead to exceptional stability. This and their amenability to chemical synthesis have made it possible to use cyclotides as templates in protein engineering and drug design applications.
David J Craik, University of Queensland, Brisbane, Australia, whose laboratory is working over 20 years in the field, summarizes the history of cyclotides
more info on cyclotides
This is how it was discovered: a physician working in the Democratic Republic of Congo noticed that laboring women were drinking tea made from Oleanda affinis to induce childbirth. Theactive ingredient was the first cyclotide to be discovered. Since then, cyclotides have been shown to be antibiotic, antiviral and insecticidal.
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 cyclic backbone.|
Cyclotides are small disulfide rich peptides isolated from plants.Typically containing 28-37 amino acids, they are characterized by their head-to-tail cyclised peptide backbone and the interlocking arrangement of their three disulfide bonds. These combined features have been termed the cyclic cystine knot (CCK) motif (Figure 1). To date, over 100 cyclotides have been isolated and characterized from species of the Rubiaceae, Violaceae, and Cucurbitaceae families. Cyclotides have also been identified in agriculturally important families such as the Fabaceae and Poaceae.,
Cyclotides have been reported to have a wide range of biological activities, including anti-HIV, insecticidal, anti-tumour, antifouling, anti-microbial, hemolytic, neurotensinantagonism, trypsin inhibition, and uterotonic activities. An ability to induceuterine contractions was what prompted the initial discovery of kalata B1.
The potent insecticidal activity of cyclotides kalata B1 and kalata B2 has prompted the belief that cyclotides act as plant host-defence agents (Figure 2). The observations that dozens or more cyclotides may be present in a single plant and the cyclotide architecture comprises a conserved core onto which a series of hypervariable loops is displayed suggest that, cyclotides may be able to target many pests/pathogens simultaneously.
The cyclotides have been recognised as a family of novel circular proteins only in the last few years but the discovery of the first member of this family may be traced back to reports of native medicine applications in the early 1970s.
Kalata B1, was discovered because it is an active ingredient in a herbal medicine used by African women to assist childbirth . While on a Red Cross relief effort in the Congo region in the 1960s a Norwegian doctor, Lorents Gran, noted that during labour African women often ingested a tea made from leaves of the plant Oldenlandia affinis because of its uterotonic effects. The active ingredient was determined to be a peptide that was named kalata B1, after the local name for the native medicine. Subsequent in vivo studies in rats confirmed uterotonic activity of the purified peptide but it was not characterised as a macrocyclic peptide until some 20 year later.
The mid-1990�s was a key period in the discovery of macrocyclic peptides, with several independent groups discovering such peptides while screening for various biological activities and our group determining the three dimensional structure of kalata B1 . In the first fortuitous discovery Sch�pke et al., examined Viola arvensis and V. tricolor in a study aimed at the discovery of new saponins. While assaying for the usual hemolytic activity of saponins they discovered a macrocyclic peptide, violapeptide I, with hemolytic activity. At around the same time bio-assay driven screens for anti-HIV and anti-neurotensin activity led to the discovery of the circulins and cyclopsychotride A respectively.
|Viola arvensis a cyclotide containing plant. Member of the violaceae family and found in temperate regions of Australia and Europe.|
With our report of the three dimensional structure of kalata B1 in 1995 and its sequence homology with the circulins and cyclopsychotride A, we became convinced that macrocyclic peptides might be more common than had earlier been thought and we began searching for other examples. Several other macrocyclic peptides were found in the late 1990s and it became clear that the peptides formed part of a family that we subsequently named the cyclotides.
Several novel cyclotide sequences have been discovered in the last few years , with the known sequences now exceeding 45 and many more currently being characterized in our laboratories. A large proportion of the new cyclotides have been discovered based on their structural properties rather than biological activities. The cyclotides are relatively hydrophobic and can be readily identified from crude plant extracts by their characteristically late elution on RP-HPLC.
The cyclotides described above, all come from plants in the Rubiaceae or Violaceae families but the prevalence of macrocyclic peptides has recently been expanded to include the Cucurbitaceae family. This is based on the discovery of the trypsin inhibitors MCoTI-I and MCoTI-II, 34 residue macrocyclic peptides, from Momordica cochinchinensis . They have no sequence homology to the previously characterized cyclotides, with the exception of the six cysteine residues, but are of a similar size and contain a cystine knot motif (Felizmenio-Quimio, 2001). The MCoTI peptides were originally isolated based on their trypsin inhibitory activity and are homologous to linear cystine knot peptides from the squash family of trypsin inhibitors such as EETI-II and CMTI.
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