Cyclopeptide Glossary

One of the two main subfamilies of cyclotides. Bracelet cyclotides lack the cis-proline residue found in Möbius cyclotides, giving their backbone a simple ring shape rather than a twist. They tend to be larger and more structurally diverse than Möbius cyclotides. Examples include cycloviolins from violet plants.

An asparaginyl endopeptidase (AEP) enzyme isolated from butterfly pea (Clitoria ternatea) that catalyses head-to-tail backbone cyclisation of peptides. The fastest peptide ligase known, operating 20,000 times faster than sortase A with >95% yield. Discovery published in Nature Chemical Biology (2014) by Prof. James Tam's group (NTU Singapore). Central to efficient laboratory and industrial production of cyclic peptides.

A class of serine protease inhibitors found in plants, including SFTI-1 from sunflower seeds. They inhibit trypsin and chymotrypsin — digestive enzymes responsible for breaking down proteins in the small intestine. The cyclic Bowman-Birk inhibitor SFTI-1 is the most potent known member of its class and an important drug scaffold.

The defining structural feature of cyclotides. Formed by six conserved cysteine residues that create three interlocking disulfide bonds (S–S bonds), arranged so that one disulfide pair threads through a ring formed by the other two — creating a topological knot. The CCK, combined with the cyclic backbone, makes cyclotides resistant to heat, acid, and all known proteases. It is this motif that gives cyclotides their extraordinary stability.

Any peptide whose amino acid chain forms a closed ring, with no free N- or C-terminus. The cyclic structure eliminates exoprotease attack points and confers enhanced stability compared to linear peptides of the same sequence. Cyclic peptides occur naturally in plants, fungi, and bacteria, and are increasingly produced synthetically for pharmaceutical development.

Synonym for cyclic peptide. Encompasses all head-to-tail cyclic peptide structures found in nature or produced synthetically, including cyclotides, orbitides, and SFTI-1. The term is used broadly to describe the entire class of circular peptide molecules.

A specific subclass of plant cyclopeptides characterised by both a head-to-tail cyclic backbone AND the Cyclic Cystine Knot (CCK) motif. Cyclotides are 28–37 amino acids in length and are found in five plant families: Rubiaceae, Violaceae, Cucurbitaceae, Fabaceae, and Poaceae. They are the most structurally stable naturally occurring proteins known. All cyclotides are cyclopeptides, but not all cyclopeptides are cyclotides.

A cyclopeptide drug derived from a fungal source (Tolypocladium inflatum), approved in 1983 as an immunosuppressant for organ transplant surgery. One of the most commercially significant cyclopeptides in medicine, with annual sales historically exceeding $1 billion. Proof of concept that cyclopeptides can be mainstream pharmaceuticals delivered orally.

The amino acid that enables disulfide bond formation. In cyclotides, six conserved cysteine residues form the three disulfide bonds of the CCK. Written as Cys or C in amino acid notation. Its side chain contains a sulfhydryl group (–SH) that can be oxidised to form a covalent sulfur-sulfur bond with another cysteine.

The compound formed when two cysteine residues are oxidised to form a disulfide bond (S–S). The "cystine" in Cyclic Cystine Knot refers to these disulfide-bonded cysteine pairs. Cystine bonds are covalent and substantially stronger than the hydrogen bonds and hydrophobic interactions that hold most proteins together.

A covalent bond between two sulfur atoms, formed by the oxidation of two cysteine residues. In cyclotides, three disulfide bonds create the cystine knot that locks the 3D structure. Unlike hydrogen bonds or hydrophobic interactions, disulfide bonds are covalent and cannot be broken by heat, acid, or the non-reducing conditions of the digestive tract — making them central to cyclotide stability.

The process of inserting a therapeutically active peptide sequence into a loop of a cyclotide scaffold, so the scaffold provides stability and oral bioavailability while the inserted sequence delivers therapeutic activity. Pioneered by Prof. David Craik's group at UQ. Allows any peptide drug — which would normally require injection — to potentially "borrow" cyclotide stability and be taken orally instead.

An enzyme that cleaves peptide bonds within a protein chain, as opposed to exopeptidases, which cleave from the termini. Butelase-1 is an asparaginyl endopeptidase. Cyclotide CCK structures resist endopeptidase attack because the rigid, compact fold prevents enzyme active sites from accessing the backbone — there is simply nowhere for the enzyme to grip.

An enzyme that cleaves amino acids from the termini (ends) of a peptide chain. Aminopeptidases attack the N-terminus; carboxypeptidases attack the C-terminus. Cyclopeptides have no free termini, making them completely resistant to all exopeptidases. This is the first and simplest reason cyclopeptides survive digestion where linear peptides do not.

See: Drug grafting.

The joining of a peptide chain's N-terminus (amino end) and C-terminus (carboxyl end) by a covalent peptide bond, forming a closed ring. In plants, this reaction is catalysed by asparaginyl endopeptidases including butelase-1. It is the defining feature of all cyclopeptides, and the structural modification that confers resistance to exoproteases.

The first cyclotide to be isolated, named after the kalata-kalata tea plant (Oldenlandia affinis) from which it was extracted by Norwegian physician Dr. Lorents Gran in 1973. The archetypal Möbius cyclotide. 29 amino acids in length. Demonstrates potent uterotonic activity — the property that drew Dr. Gran's attention in Congo. Its cyclic cystine knot structure was not revealed until 1995 using NMR spectroscopy. Forms the structural basis of T20K, the MS drug candidate in human clinical trials.

A broad term for cyclic peptides typically larger than 8–10 amino acids, including cyclotides, engineered synthetic cycles, and stapled peptides. The macrocyclic peptide drug market is projected to grow from $1.22B (2024) to $4.76B (2030), driven by their ability to hit biological targets inaccessible to small molecules and their improving oral bioavailability profiles.

One of the two main subfamilies of cyclotides. Möbius cyclotides contain a cis-proline residue in loop 5 that introduces a twist into the backbone, analogous to the twist in a Möbius strip. Kalata B1 is the archetypal Möbius cyclotide. The classification into Möbius and bracelet subfamilies was established by Prof. David Craik's group.

The end of a peptide chain containing a free amino group (–NH&sub2;). In linear peptides, this is an attack point for aminopeptidase enzymes — the first step in many digestive degradation pathways. In cyclopeptides, there is no free N-terminus, eliminating this attack point entirely.

An analytical technique used to determine the three-dimensional structure of molecules in solution. NMR was used to reveal the cyclic cystine knot structure of kalata B1 in 1995, solving the 20-year mystery of its extraordinary stability. NMR remains the primary tool for determining cyclotide structures. Prof. Norelle Daly at James Cook University was central to establishing the structural framework of the CCK by NMR.

The plant from which kalata B1 — the first cyclotide — was isolated. A member of the Rubiaceae (coffee) family, native to sub-Saharan Africa. Known locally as "kalata-kalata." Its leaves were traditionally brewed into a tea used to accelerate labour and delivery — the practice that drew Dr. Lorents Gran's attention during his time in the Congo in the 1960s.

The fraction of an administered dose of a drug that reaches the systemic circulation unchanged. Most peptide drugs have <1% oral bioavailability because they are degraded in the gut by digestive enzymes. Cyclotides have demonstrated meaningful oral bioavailability due to their resistance to digestive enzymes and their amphipathic character, which allows them to permeate gut epithelial cells. This is the property that makes cyclotide drug grafts so pharmaceutically valuable.

A class of plant cyclopeptides that are head-to-tail cyclic but lack disulfide bonds. Smaller and less stable than cyclotides. Found in flax (Linum usitatissimum), Annonaceae, Caryophyllaceae, and other plant families. Being investigated for cytotoxic and anti-inflammatory properties. Orbitides represent a simpler form of natural cyclic peptide — cyclic backbone without the additional CCK architecture.

A digestive protease active in the stomach at pH ~2. Cleaves peptide bonds between hydrophobic amino acids. Rapidly degrades linear peptide drugs taken orally. Cyclotides are resistant to pepsin degradation — one of the multiple barriers to oral peptide delivery that cyclotides overcome.

The use of plants or animals as living bioreactors to produce pharmaceutical compounds. Plant pharming of cyclotides involves engineering crop plants (such as potato or soybean) to express therapeutic cyclopeptide sequences, which accumulate in plant tissue and are harvested for use as medicines or functional foods. The approach is being developed by Prof. David Craik's group at UQ and commercialised by companies including Phyllome (Sydney).

Any enzyme that cleaves peptide bonds. Proteases are the primary reason linear peptide drugs cannot be taken orally — they are simply digested like food. Cyclopeptides are resistant to all known proteases due to their cyclic backbone (no termini for exoproteases to grip) and, in cyclotides, the CCK structure (compact fold prevents endoproteases accessing the backbone).

The ability of a molecule to withstand cleavage by protease enzymes. A key property of cyclopeptides, arising from their cyclic backbone (no termini for exoproteases) and CCK structure (compact, rigid fold resists endoproteases). This resistance extends to boiling, stomach acid (pH 2), pancreatic enzymes, and every digestive enzyme tested.

In drug design, a structural framework into which active sequences can be inserted. Cyclotides are near-ideal drug scaffolds because their surface-exposed loops tolerate sequence variation while the CCK core maintains overall stability. The six loops of the cyclotide backbone can each accept different inserted sequences, giving substantial combinatorial freedom to drug designers.

A 14-amino-acid cyclic peptide from sunflower seeds (Helianthus annuus). The most potent known Bowman-Birk trypsin inhibitor and the only naturally occurring cyclic Bowman-Birk inhibitor. Though smaller than cyclotides and lacking a cystine knot, SFTI-1 is an important drug scaffold for serine protease inhibition — particularly in cancer (where protease activity drives invasion) and inflammatory disease.

A single-point mutant of kalata B1 — threonine at position 20 replaced with lysine — engineered by Prof. Christian Gruber's group at MedUni Vienna. T20K shows immunomodulatory activity and halted MS progression in animal models. It is in Phase I/II human clinical trials (via Cyxone AB, Sweden) for the treatment of multiple sclerosis, taken orally. T20K is the first cyclotide to enter clinical testing in humans and represents the leading edge of the entire cyclopeptide pharmaceutical field.

A pancreatic serine protease that cleaves peptide bonds after arginine and lysine residues. A major digestive enzyme responsible for breaking down proteins in the small intestine. Cyclotides are resistant to trypsin digestion — one of the central demonstrations of their stability, since trypsin resistance is a standard test of protease stability in the drug development process.

The step in cyclotide biosynthesis that occurs in the plant vacuole. After the cyclotide precursor protein is synthesised on the ribosome and transported to the vacuole, asparaginyl endopeptidases cleave the prodomains and catalyse the head-to-tail cyclisation that creates the mature, ring-closed cyclotide. Understanding this process was critical to engineering plants to produce therapeutic cyclotide sequences.

The violet plant family — one of five plant families known to produce cyclotides. European and tropical violet species are rich sources of cyclotide diversity and are extensively studied by Prof. Ulf Göransson's group at Uppsala University, Sweden. Violaceae cyclotides include both Möbius and bracelet subfamily members and have provided many of the structures used in antimicrobial peptide research.