Applications of Cyclic Peptides

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Discovery of Cyclic Peptides

Cyclic peptides are small, disulfide-rich peptides that were originally discovered in plants. Cyclic peptides were initially reported by Seather et al. in 1995, although they had been used in traditional medicine long before that. The first reported cyclic peptide, kalata B1 (kB1), is an active compound in the plant Oldenlandia affinis, used in traditional indigenous medicine in Africa to accelerate childbirth. Since then, cyclic peptides have been reported in plants from Violaceae, Rubiaceae, Cucurbitaceae, Solanaceae, Poaceae, and Fabaceae, with >280 different cyclic peptides reported so far. Moreover, it has been estimated that a single cyclic peptide family, for example from the Violaceae, might have up 150 000 members, and a single plant might contain over 160 different cyclic peptides.

Structure of Cyclic Peptides

Cyclic peptides are about 30 amino acids long and, structurally, have a unique motif. This motif is known as the cyclic cysteine knot (CCK), which corresponds to a head-to-tail circular knotted topology with three disulfide bonds, with one disulfide penetrating ring formed by the two other disulfide bonds. The disulfides Cys^I–Cys^IV and Cys^II–Cys^V form a ring, which the disulfide Cys^III–Cys^VI goes through. Cyclic peptides also have six loops. These loops correspond to the six backbone segments between the cysteine residues. Interestingly, these loops can be modified in length and amino acid content without affecting the stability of the cyclic peptides. By contrast, removing one type of disulfide connectivity can affect its stability and structure.

A) The NMR structure (PDB code 1NB1) and amino acid sequence of kalata B1 with the labeling of the six conserved Cys residues and the connectivity of the disulfide bonds indicated.
B) A schematic representation of the topology of the cyclic cystine knot (CCK) motif characteristic of cyclotides. (David J Craik)

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Application of Cyclic Peptides

Cyclic peptides' extraordinary properties and wide range of biological activities make them capture the interests of scientists and pharmaceutical researchers to apply them in various fields.

Cyclic Peptides in Drug Discovery

Cyclic peptides' unique cyclic structure, stability, and bioactivity make them hold significant therapeutic potential in drug discovery. They are ideal candidates for developing peptide-based drugs because they have the ability to penetrate cell membranes and resist enzymatic degradation. Moreover, cyclic peptides selectively targeting specific cellular pathways and proteins have demonstrated promising activity against a range of conditions, including cancer, HIV, and inflammatory diseases. Their natural origin and minimal toxicity enhance their appeal as therapeutic agents. Various studies are exploring engineered cyclic peptides to create highly selective and effective treatments to potentially lead to the development of drugs with improved efficacy and reduced side effects. Additionally, they also can used in precision medicine by conjugating with other therapeutic molecules because of their versatility. As research progresses, cyclic peptides are poised to play a crucial role in the next generation of innovative drug therapies, offering hope for more effective and safer treatments for various diseases.

Precision Drug Delivery

Cyclic peptides exhibit immense potential in targeted drug delivery and cellular interactions owing to their unique cyclic structure and membrane-penetrating ability. Engineering these peptides can selectively target specific receptors or cellular pathways to enhance the precision of drug delivery and minimize off-target effects. Their stability against enzymatic degradation ensures the effective transport of therapeutic payloads to the intended site of action within cells. Cyclic peptides are valuable tools to help scientists study biological mechanisms and design tailored therapeutic interventions by modulating cellular processes and protein interactions. Leveraging the remarkable properties of cyclic peptides holds promise for developing highly efficient and targeted drug delivery systems, advancing the field of precision medicine.

Natural Insecticides

Cyclic peptides can offer an eco-friendly alternative to synthetic chemicals by serving as natural insecticides in agriculture to reduce farmers' reliance on chemical insecticides. Their potent insecticidal properties allow them to target specific pests without harming beneficial insects, promoting sustainable pest management practices. Cyclic peptides can effectively control pests by disrupting cell membranes and interfering with essential biological processes in insects. Using cyclic peptides in agriculture can ensure biodegradability and minimize environmental impact, reducing pollution and promoting soil health because of their natural origin. Additionally, cyclic peptides' stability under various environmental conditions enhances their effectiveness in diverse agricultural settings. As a result, cyclic peptide-based insecticides contribute to more sustainable and environmentally responsible agricultural practices.

Cyclic Peptides-Based Biomaterials

Cyclic peptides attracted various researchers' imaginations to apply them in biotechnology and biomaterials science. That's because they own stability and diverse biological activities. Moreover, cyclic peptide-based biomaterials offer the advantages of biocompatibility, biodegradability, and tunable properties, making them versatile platforms for various biomedical applications. For example, numerous studies are exploring their applications in protein engineering, where cyclic peptides can serve as scaffolds for designing novel proteins with enhanced stability and functionality. Moreover, they can self-assemble into nanostructures making them promising candidates to help researchers develop biomaterials for drug delivery, tissue engineering, and regenerative medicine applications.

Anti-Aging and Hydration

Cyclic peptides have emerged as groundbreaking ingredients in the cosmetic industry, offering a multitude of benefits for enhancing skin health and beauty. Their unique structural stability and bioactivity make them exceptionally suited for anti-aging formulations, where they promote collagen synthesis, enhance skin elasticity, and visibly reduce the appearance of fine lines and wrinkles. Cyclic peptides also exhibit powerful moisturizing properties, bolstering the skin's natural barrier to prevent moisture loss and ensure optimal hydration, resulting in supple, resilient skin.

Additionally, they are the ideal candidate to soothe irritated skin and support the healing of minor wounds and conditions, which is particularly beneficial for sensitive and problem-prone skin types due to their anti-inflammatory and wound-healing capabilities. Cyclic peptides inhibit the activity of melanin-producing enzymes to reduce hyperpigmentation and dark spots and achieve a brighter, more even complexion. As plant-derived peptides, cyclic peptides align perfectly with the clean beauty trend, catering to the growing consumer demand for natural, sustainable, and effective skincare solutions. Their multifunctional properties enable the development of innovative and versatile cosmetic products that address a variety of skin concerns, providing comprehensive skincare benefits in a single formulation.

Cyclic Peptides-Based Supramolecular Nanotubes

Cyclic peptides are making significant strides in nanotechnology through their ability to self-assemble into supramolecular nanotubes. These stable, biocompatible structures are ideal for drug delivery, encapsulating therapeutic agents to protect them from degradation and enable targeted delivery, thus enhancing treatment efficacy and reducing side effects. Additionally, cyclic peptide-based nanotubes are being explored for tissue engineering, where their robust properties support tissue growth and regeneration. In materials science, these nanotubes provide a versatile platform for developing advanced nanomaterials suitable for electronics, sensors, and catalysis. The integration of cyclic peptides into nanotubes opens new avenues for nanoscale device design, leveraging their precise molecular architecture and functional diversity.

Insights into Evolution and Biodiversity

Studying cyclic peptides, unraveling the biosynthetic pathways of cyclic peptides, and understanding cyclic peptides' ecological functions can help researchers gain valuable insights and a deeper understanding of plant defense mechanisms and natural product biosynthesis into the evolution and biodiversity of plant species. The distribution and diversity of cyclic peptides across different plant families shed light on the evolutionary relationships between plants and their ecological roles. This knowledge not only informs drug discovery efforts but also can help people understand ecosystem dynamics and biodiversity conservation.

Conclusion

To summarize, cyclic peptides hold huge potential for applications across various fields. From drug discovery and targeted drug delivery to agriculture, biotechnology, and beyond, cyclic peptides offer innovative solutions to some of the most pressing challenges facing humanity. Although the full extent of cyclic peptides' applications and their impact on science and society are yet to be fully realized, the research in this field continues to advance.

Reference

1. Craik D J. Joseph Rudinger memorial lecture: discovery and applications of cyclotides[J]. Journal of Peptide Science, 2013, 19(7): 393-407.