Cyclic peptides are polypeptide chains which contain a circular sequence of bonds. This can be through a connection between the amino and carboxyl ends of the peptide, for example in cyclosporin; a connection between the amino end and a side chain, for example in

bacitracin Bacitracin is a polypeptide antibiotic. It is a mixture of related cyclic peptides produced by '' Bacillus licheniformis'' bacteria, that was first isolated from the variety "Tracy I" ( ATCC 10716) in 1945. These peptides disrupt Gram-positive ...

; the carboxyl end and a side chain, for example in colistin; or two side chains or more complicated arrangements, for example in amanitin. Many cyclic peptides have been discovered in nature and many others have been synthesized in the laboratory. Their length ranges from just two amino acid residues to hundreds. In nature they are frequently antimicrobial or toxic; in medicine they have various applications, for example as antibiotics and immunosuppressive agents. Thin-Layer Chromatography (TLC) is a convenient method to detect cyclic peptides in crude extract from bio-mass.

Classification

Cyclic peptides can be classified according to the types of bonds that comprise the ring. *Homodetic cyclic peptides, such as cyclosporine A, are those in which the ring is composed exclusively of normal peptide bonds (i.e. between the alpha carboxyl of one residue to the alpha amine of another). The smallest such species are 2,5-diketopiperazines, being derived from the cyclisation of a dipeptide. *Cyclic isopeptides contain at least one non-alpha amide linkage, such as a linkage between the side chain of one residue to the alpha carboxyl group of another residue, as in microcystin and bacitracin. *Cyclic depsipeptides, such as aureobasidin A and HUN-7293, have at least one lactone (ester) linkage in place of one of the amides. Some cyclic depsipeptides are cyclized between the C-terminal carboxyl and the side chain of a Thr or Ser residue in the chain, such as kahalalide F, theonellapeptolide, and

didemnin Didemnins are cyclic depsipeptide compounds isolated from a tunicate (ascidian, or sea-squirt) of the genus '' Trididemnum'' (family of Didemnidæ) that were collected in the Caribbean Sea. They were first isolated in 1978 at the University of ...

B. * Bicyclics such as the

amanitins Amanitin may refer to several related amatoxins: * α-Amanitin * β-Amanitin * γ-Amanitin * ε-Amanitin See also * Amatoxin, a class of toxic compounds that include the amanitins * Amanin Amanin is a cyclic peptide. It is one of the amatox ...

and the phalloidins contain a bridging group, generally between two of the side chains. In the amatoxins, this is formed as a sulfoxide bridge between the Trp and Cys residues. Other bicyclic peptides include echinomycin, triostin A, and Celogentin C. * There are a number of bi and monocyclic peptides which are cyclized through a

disulfide bond In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups ...

between two

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, s ...

oxytocin Oxytocin (Oxt or OT) is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. It plays a role in social bonding, reproduction, childbirth, and the period after childbirth. Oxytoci ...

being a notable example.

Biosynthesis

Cyclic peptides in plants are synthesized via a two-step process; the

translation Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...

of a linear peptide chain, and its subsequent formation into a cyclic structure through activities of a protease-like enzyme or other ways.

Properties and applications

Cyclic peptides tend to be extremely resistant to the process of digestion, making them of interest to scientists working on novel oral medications. Examples include: *

Amanitins Amanitin may refer to several related amatoxins: * α-Amanitin * β-Amanitin * γ-Amanitin * ε-Amanitin See also * Amatoxin, a class of toxic compounds that include the amanitins * Amanin Amanin is a cyclic peptide. It is one of the amatox ...

Bacitracin Bacitracin is a polypeptide antibiotic. It is a mixture of related cyclic peptides produced by '' Bacillus licheniformis'' bacteria, that was first isolated from the variety "Tracy I" ( ATCC 10716) in 1945. These peptides disrupt Gram-positive ...

* Colistin * Ciclosporin * Dactinomycin * Daptomycin * Gramicidin S *

Hymenistatin Hymenistatin is a cyclic peptide isolated from the sea sponge ''Phakellia fusca ''Phakellia'' is a genus of sponges belonging to the family Bubaridae. The genus has a cosmopolitan distribution In biogeography, cosmopolitan distribution i ...

* Microcystins * Nisin * Nodularin * Octreotide * Polymyxin B * Pristinamycin * Valinomycin (technically a depsipeptide) * Vancomycin *

Viscumamide Viscumamide is a cyclic peptide isolated from endophytic fungi of mangrove A mangrove is a shrub or tree that grows in coastal saline water, saline or brackish water. The term is also used for tropical coastal vegetation consisting of such s ...

Yunnanin A Yunnanin A is a biologically active cyclic peptide Cyclic peptides are polypeptide chains which contain a circular sequence of bonds. This can be through a connection between the amino and carboxyl ends of the peptide, for example in cyclospor ...

References

External links

Cybase
* {{MeshName, Cyclic+Peptides Peripheral membrane proteins