Cyclic peptides are a diverse class of compounds with biological activities ranging from antibiotics to toxins. They are resistant to proteases and can bind proteins in cells. They have an unusual circular structure and can be synthesized with specialized techniques. They are of interest as potential drug sources and as vectors for drug delivery. Natural cyclic peptide biosynthesis involves non-ribosomal peptide synthetases and can be genetically engineered to form new compounds.
Cyclic peptides are an unusual class of compounds that were first discovered by microorganisms due to their biological activities. They range from antibiotics, such as bacitracin and polymyxnB, to cyclosporine, an immunosuppressant drug. They can also be toxins. Amanitin alpha, the poison of the death cap mushroom, is also a cyclic peptide.
These compounds are under very active investigation as potential new sources of drugs and antibiotics. They are much more resistant to proteases, enzymes that break down proteins, than a linear peptide chain. This resistance to proteolysis means they tend to survive the human digestive process. They can also bind proteins in the cell where traditional drugs cannot.
The unusual properties of cyclic peptides are due to both their circular structure and their unusual mode of biosynthesis, often incorporating uncommon compounds. In addition to the 20 amino acids normally used in proteins, 300 different natural compounds can be used in the synthesis of cyclic peptides. For example, they may contain D amino acids.
Like all peptides, they consist of chains of amino acids linked by a peptide bond. Most peptides are linear, having an N-terminus with an amino group at one end and a C-terminus with a carboxyl group at the other. In cyclic peptides, the N and C ends are linked together, forming a cyclic polypeptide chain. When there are fewer than 50 amino acids, the compound is known as a cyclic peptide. A larger compound is known as a cyclic protein.
There is a lot of research aiming at the synthesis of new cyclic peptides. They can be synthesized with specialized techniques, known as peptide synthesis. There are biotech companies that specialize in manufacturing custom peptides for research studies. Cyclic peptides have shown promise in the treatment of Alzheimer’s and Huntington’s diseases. As more information about their interactions with cellular proteins becomes available, it may be possible to develop even more drugs from this diverse class of compounds.
Apart from the biological activity of cyclic peptides, they are of interest as vectors. This means they can be designed to carry drugs around the body. Particularly if the drug is a peptide, it can be synthesized as part of the cyclic peptide and taken orally.
Natural cyclic peptide biosynthesis is of interest to biologists, as it often involves non-ribosomal peptide synthetases. Most peptides are made up of ribosomes, which use messenger RNA (mRNA) as a template and then assemble amino acids to form a peptide or protein. Some microorganisms have large enzyme complexes composed of modules that they use to assemble cyclic peptides. They do not use ribosomes or mRNA. In some cases it is possible to alter the modules and genetically engineer the cyclic peptide, possibly forming new compounds with biological activity.
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