A Comprehensive Review on Therapeutic Potentials of Natural Cyclic Peptides

Cyclic peptides have emerged as a promising class of organic compounds that possess polypeptide chains with a cyclic ring structure. There is a circular sequence of bonds in which the ring structure is formed via linkage between one end of the peptide bond and the other end with an amide bond or any other chemically stable bonds like ether, thioether, lactone, and disulfide. Generally, the cyclic peptides are isolated from natural resources like invertebrate animals, micro-organisms of marine habitats, and higher plants. These cyclic peptides possess unique structures with diverse pharmacological activities. Nowadays, cyclic peptides possess superior therapeutic value due to several reasons such as greater resistance to enzymatic degradation (in vivo) and higher bio-availability. Some of these cyclic peptides are rich in leucine, proline, while some have other amino acids as their major constituents. Numerous novel cyclic peptides isolated from natural sources are successfully developed as bioactive products. Recently, cyclic peptides derived from natural resources have attracted attention for exploring their numerous beneficial effects. Moreover, it is reported that natural cyclic peptides exhibit various therapeutic activities like an anthelmintic, ACE inhibitor, anti-tumor, microtubule inhibitor, anti-fungal, antimalarial, and platelet aggregation inhibiting activity. In this review, various cyclic peptides are reported with structures and biological activities that are isolated from various natural sources. The natural cyclic peptides possess a wide spectrum of biological activities and can become a drug of the future for replacing the existing drugs which develop resistance.