Peptide classes
Here are the major classes of peptides, according to how they are produced:
- Ribosomal peptides
- Are synthesized by translation of mRNA. They are often subjected to proteolysis to generate the mature form. These function, typically in higher organisms, as hormones and signaling molecules. Some lower organisms produce peptides as antibiotics, such as microcins.[1] Since they are translated, the amino acid residues involved are restricted to those utilized by the ribosome. However, these peptides frequently have posttranslational modifications, such as phosphorylation, hydroxylation, sulfonation, palmitylation, glycosylation and disulfide formation. In general, they are linear, although lariat structures have been observed.<ref.Pons M, Feliz M, Antònia Molins M, Giralt E (1991). "Conformational analysis of bacitracin A, a naturally occurring lariat". Biopolymers 31 (6): 605–12. PMID 1932561.</ref> More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom.[2]
- Nonribosomal peptides
- These peptides are assembled by enzymes that are specific to each peptide, rather than by the ribosome. The most common non-ribosomal peptide is glutathione, which is a component of the antioxidant defenses of most aerobic organisms.[3] Other nonribosomal peptides are most common in unicellular organisms, plants, and fungi and are synthesized by modular enzyme complexes called nonribosomal peptide synthetases.[4] These complexes are often laid out in a similar fashion, and they can contain many different modules to perform a diverse set of chemical manipulations on the developing product.[5] These peptides are often cyclic and can have highly-complex cyclic structures, although linear nonribosomal peptides are also common. Since the system is closely related to the machinery for building fatty acids and polyketides, hybrid compounds are often found. Oxazoles, thiazoles often indicate that the compound was synthesized in this fashion.[6]
- Peptones
- Are derived from animal meat digested by proteolases. The resulting material is used as a source of proteins in nutrient media for growing bacteria and fungi.[7]
- Peptide Fragments
- Refer to fragments of proteins which used to identify or quantify the source protein.[8] Often these are the products of enzymatic degradation performed in the laboratory on a controlled sample, but can also be forensic or paleontological samples which have been degraded by natural effects.[9][10]
- Info:http://www.wikidoc.org/index.php/Peptide
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