Peptides: A tetrapeptide (example Val-Gly-Ser-Ala) with green marked amino end (L-Valine) and blue marked carboxyl end (L-Alanine). Peptides (from Gr. πεπτός, digested, derived from πέσσειν, to digest) are naturally occurring biological molecules. They are short chains of amino acid monomers linked by peptide (amide) bonds. The covalent chemical bonds are formed when the carboxyl group of one amino acid reacts with the amino group of another. The shortest peptides are dipeptides, consisting of 2 amino acids joined by a single peptide bond, followed by tripeptides, tetrapeptides, etc. A polypeptide is a long, continuous, and unbranched peptide chain. Hence, peptides fall under the broad chemical classes of biological oligomers and polymers, alongside nucleic acids, oligo- and polysaccharides, etc. Peptides are distinguished from proteins on the basis of size, and as an arbitrary benchmark can be understood to contain approximately 50 or fewer amino acids.[1] Proteins consist of one or more polypeptides arranged in a biologically functional way, often bound to ligands such as coenzymes and cofactors, or to another protein or other macromolecule (DNA, RNA, etc.), or to complex macromolecular assemblies. Finally, while aspects of the techniques that apply to peptides versus polypeptides and proteins differ (i.e., in the specifics of electrophoresis, chromatography, etc.), the size boundaries that distinguish peptides from polypeptides and proteins are not absolute: long peptides such as amyloid beta have been referred to as proteins, and smaller proteins like insulin have been considered peptides. Amino acids that have been incorporated into peptides are termed residues due to the release of either a hydrogen ion from the amine end or a hydroxyl ion from the carboxyl end, or both, as a water molecule is released during formation of each amide bond.[2] All peptides except cyclic peptides have an N-terminal and C-terminal residue at the end of the peptide (as shown for the tetrapeptide in the image). Peptide classesEdit Peptides are divided into several classes, depending on how they are produced: Milk peptides Two naturally occurring milk peptides are formed from the milk protein casein when digestive enzymes break this down; they can also arise from the proteinases formed by lactobacilli during the fermentation of milk.[3] Ribosomal peptides 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 organisms produce peptides as antibiotics, such as microcins.[4] 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, palmitoylation, glycosylation and disulfide formation. In general, they are linear, although lariat structures have been observed.[5] More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom.[6] Nonribosomal peptides Nonribosomal 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.[7] Other nonribosomal peptides are most common in unicellular organisms, plants, and fungi and are synthesized by modular enzyme complexes called nonribosomal peptide synthetases.[8] 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.[9] 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. The presence of oxazoles or thiazoles often indicates that the compound was synthesized in this fashion.[10] Peptones See also Tryptone Peptones are derived from animal milk or meat digested by proteolysis.[11] In addition to containing small peptides, the resulting spray-dried material [clarification needed] includes fats, metals, salts, vitamins and many other biological compounds. Peptones are used in nutrient media for growing bacteria and fungi.[12] Peptide fragments Peptide fragments refer to fragments of proteins that are used to identify or quantify the source protein.[13] Often these are the products of enzymatic degradation performed in the laboratory on a controlled sample, but can also be forensic or paleontological samples that have been degraded by natural effects.[14][15] Peptide synthesisEdit Main article: Peptide synthesis Table of amino acids Solid-phase peptide synthesis on a rink amide resin using Fmoc-α-amine-protected amino acid Peptides in Molecular BiologyEdit Peptides received prominence in molecular biology for several reasons. The first is that peptides allow the creation of peptide antibodies in animals without the need to purify the protein of interest.[16] This involves synthesizing antigenic peptides of sections of the protein of interest. These will then be used to make antibodies in a rabbit or mouse against the protein. Another reason is that peptides have become instrumental in mass spectrometry, allowing the identification of proteins of interest based on peptide masses and sequence. In this case the peptides are most often generated by in-gel digestion after electrophoretic separation of the proteins. Peptides have recently been used in the study of protein structure and function. For example, synthetic peptides can be used as probes to see where protein-peptide interactions occur- see the page on Protein tags. Inhibitory peptides are also used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases.[17] For example, one of the most promising application is through peptides that target LHRH.[18] These particular peptides act as an agonist, meaning that they bind to a cell in a way that regulates LHRH receptors. The process of inhibiting the cell receptors suggests that peptides could be beneficial in treating prostate cancer. However, additional investigations and experiments are required before the cancer-fighting attributes, exhibited by peptides, can be considered definitive.[19] Well-known peptide familiesEdit The peptide families in this section are ribosomal peptides, usually with hormonal activity. All of these peptides are synthesized by cells as longer propeptides or proproteins and truncated prior to exiting the cell. They are released into the bloodstream where they perform their signaling functions. Tachykinin peptides Main article: Tachykinin peptides Substance P Kassinin Neurokinin A Eledoisin Neurokinin B Vasoactive intestinal peptides Main article: Secretin family VIP (Vasoactive Intestinal Peptide; PHM27) PACAP Pituitary Adenylate Cyclase Activating Peptide Peptide PHI 27 (Peptide Histidine Isoleucine 27) GHRH 1-24 (Growth Hormone Releasing Hormone 1-24) Glucagon Secretin Pancreatic polypeptide-related peptides NPY (NeuroPeptide Y) PYY (Peptide YY) APP (Avian Pancreatic Polypeptide) PPY Pancreatic PolYpeptide Opioid peptides Main article: Opioid peptide Proopiomelanocortin (POMC) peptides Enkephalin pentapeptides Prodynorphin peptides Calcitonin peptides Calcitonin Amylin AGG01 Other peptides B-type Natriuretic Peptide (BNP) - produced in myocardium & useful in medical diagnosis Lactotripeptides - Lactotripeptides might reduce blood pressure,[20][21][22] although the evidence is mixed.[23] Notes on terminology Doping in sports See also References Read in another language Wikipedia ® MobileDesktop Content is available under CC BY-SA 3.0 unless otherwise noted. 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Posted on: Mon, 26 Jan 2015 14:43:06 +0000
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