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Table One: Discovered cyclotides and
their sequences.
The cyclotides can be divided into two
families based on whether they possess a conceptual twist caused
by a cis-Pro peptide bond in loop 5. Cyclotides possessing a
twist are classified as Moebius and those lacking the cis-Pro
bond are Bracelet cyclotides. The trypsin inhibitor cyclotides
are classfied in their own family based on sequence variation
and natural activity. The X in Violapeptide I was not
determined in the original report but is presumably R based on
sequence homology.
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Table 1 has been split into sub-families, based on the
classification of the backbone as either of Möebius or Bracelet
type (Craik, et al., 1999). This nomenclature arises
because it has been proposed that a cis-Pro peptide bond in loop
5 can be thought of as providing a twist in the conceptual ribbon
of the peptide backbone, leading to the circular backbone being
regarded as a Möebius strip. When this cis-Pro is not present,
all backbone peptide bonds are in the trans arrangement, making
the backbone bracelet-like. Hence, the cyclotide family is
divided based on the presence (or absence) of the putative
cis-Pro peptide bond in loop 5. It is stressed that this is a
convenient conceptual description only and it is not suggested
that the molecules exhibit the topological properties of either
bracelets or Möebius strips.
Figure One also summarises the conserved and variable residues
of the cyclotides: the six conserved cysteine residues are in
yellow, other conserved residue in green and variable residues in
blue (Craik, et al., 1999). |
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Figure One: Schematic of a cyclotide showing
conserved residues (green), cysteine residues (yellow) and
variable residues (blue) along with the three dimensional
structure of cycloviolacin O1. |
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