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Peptides (, ) are short chains of
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), m ...
s linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include
dipeptide A dipeptide is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millio ...
s, tripeptides, and tetrapeptides. A polypeptide is a longer, continuous, unbranched peptide chain. Hence, peptides fall under the broad chemical classes of biological polymers and oligomers, alongside
nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nu ...
s, oligosaccharides, polysaccharides, and others. A polypeptide that contains more than approximately 50 amino acids is known as a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
. 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 such as DNA or RNA, or to complex macromolecular assemblies. Amino acids that have been incorporated into peptides are termed residues. A water molecule is released during formation of each amide bond.. All peptides except cyclic peptides have an
N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue ...
(amine group) and
C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid Amino acids are organic compound In chemistry, organic compounds are generally any ch ...
(carboxyl group) residue at the end of the peptide (as shown for the tetrapeptide in the image).


Classes

There are numerous types of peptides that have been classified according to their sources and functions. According to the Handbook of Biologically Active Peptides, some groups of peptides include plant peptides, bacterial/ antibiotic peptides, fungal peptides, invertebrate peptides, amphibian/skin peptides, venom peptides, cancer/anticancer peptides, vaccine peptides, immune/inflammatory peptides, brain peptides, endocrine peptides, ingestive peptides, gastrointestinal peptides, cardiovascular peptides, renal peptides, respiratory peptides, opiate peptides, neurotrophic peptides, and blood–brain peptides. Some ribosomal peptides are subject to
proteolysis Proteolysis is the breakdown of protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing ...
. These function, typically in higher organisms, as
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are requir ...
s and signaling molecules. Some microbes produce peptides as antibiotics, such as microcins and bacteriocins. Peptides frequently have
post-translational modification Post-translational modification (PTM) is the covalent and generally enzymatic modification of protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a ...
s such as
phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, ...
, hydroxylation, sulfonation, palmitoylation, glycosylation, and disulfide formation. In general, peptides are linear, although lariat structures have been observed. More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom. '' Nonribosomal peptides'' are assembled by
enzyme Enzymes () are protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic rea ...
s, not the ribosome. A common non-ribosomal peptide is glutathione, a component of the antioxidant defenses of most aerobic organisms. Other nonribosomal peptides are most common in unicellular organisms, plants, and
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold A mold () or mould () is one of the structures certain fungi can form. The dust-like, colored a ...
and are synthesized by modular enzyme complexes called ''nonribosomal peptide synthetases''. 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. 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 acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms ...
s and polyketides, hybrid compounds are often found. The presence of oxazoles or thiazoles often indicates that the compound was synthesized in this fashion. ' are derived from animal milk or meat digested by
proteolysis Proteolysis is the breakdown of protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing ...
. In addition to containing small peptides, the resulting material includes fats, metals, salts, vitamins, and many other biological compounds. Peptones are used in nutrient media for growing bacteria and fungi. ''Peptide fragments'' refer to fragments of proteins that are used to identify or quantify the source protein. 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.


Chemical synthesis


Example families

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.


Antimicrobial peptides

* Magainin family * Cecropin family * Cathelicidin family * Defensin family


Tachykinin peptides

* Substance P * Kassinin * Neurokinin A * Eledoisin * Neurokinin B


Vasoactive intestinal peptides

* 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 Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It raises concentration of glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbo ...
* Secretin


Pancreatic polypeptide-related peptides

* NPY ''(NeuroPeptide Y) * PYY ''(Peptide YY)'' * APP ''(Avian Pancreatic Polypeptide)'' * PPY ''Pancreatic PolYpeptide''


Opioid peptides

* Proopiomelanocortin (POMC) peptides * Enkephalin pentapeptides * Prodynorphin peptides


Calcitonin peptides

* Calcitonin * Amylin * AGG01


Self-assembling peptides

* Aromatic short peptides * Biomimetic peptides * Peptide amphiphiles *Peptide dendrimers


Other peptides

* B-type Natriuretic Peptide (BNP) - produced in the myocardium and useful in medical diagnosis * Lactotripeptides - Lactotripeptides might reduce blood pressure, although the evidence is mixed. * Peptidic components from traditional Chinese medicine Colla Corii Asini in hematopoiesis.


Terminology


Length

Several terms related to peptides have no strict length definitions, and there is often overlap in their usage: * A ''polypeptide'' is a single linear chain of many amino acids (any length), held together by amide bonds. * A ''
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
'' consists of one or more polypeptides (more than about 50 amino acids long). * An '' oligopeptide'' consists of only a few amino acids (between two and twenty).


Number of amino acids

Peptides and proteins are often described by the number of amino acids in their chain, e.g. a protein with 158 amino acids may be described as a "158 amino-acid-long protein". Peptides of specific shorter lengths are named using IUPAC numerical multiplier prefixes: * A '' monopeptide'' has one amino acid. * A ''
dipeptide A dipeptide is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millio ...
'' has two amino acids. * A '' tripeptide'' has three amino acids. * A '' tetrapeptide'' has four amino acids. * A ''pentapeptide'' has five amino acids. (''e.g.'', enkephalin). * A ''hexapeptide'' has six amino acids. (''e.g.'', angiotensin IV). * A ''heptapeptide'' has seven amino acids. (''e.g.'', spinorphin). * An ''octapeptide'' has eight amino acids (''e.g.'', angiotensin II). * A ''nonapeptide'' has nine amino acids (''e.g.'', oxytocin). * A ''decapeptide'' has ten amino acids (''e.g.'', gonadotropin-releasing hormone and angiotensin I). * A ''undecapeptide'' has eleven amino acids (''e.g.'', substance P). The same words are also used to describe a group of residues in a larger polypeptide (''e.g.'', RGD motif).


Function

* A '' neuropeptide'' is a peptide that is active in association with neural tissue. * A '' lipopeptide'' is a peptide that has a lipid connected to it, and '' pepducins'' are lipopeptides that interact with GPCRs. * A '' peptide hormone'' is a peptide that acts as a
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are requir ...
. * A proteose is a mixture of peptides produced by the hydrolysis of proteins. The term is somewhat archaic. * A peptidergic agent (or drug) is a chemical which functions to directly modulate the peptide systems in the body or brain. An example is opioidergics, which are neuropeptidergics. * A cell-penetrating peptide is a peptide able to penetrate the cell membrane.


See also

* Acetyl hexapeptide-3 * Beefy meaty peptide * Collagen hybridizing peptide, a short peptide that can bind to denatured collagen in tissues * Bis-peptide * CLE peptide * Epidermal growth factor * '' Journal of Peptide Science'' * Lactotripeptides * Micropeptide * Multifunctional peptide * Neuropeptide * Palmitoyl pentapeptide-4 * Pancreatic hormone * peptide spectral library *
Peptide synthesis In organic chemistry, peptide synthesis is the production of peptides, compounds where multiple amino acids are linked via amide bonds, also known as peptide bonds. Peptides are chemically synthesized by the condensation reaction of the car ...
* Peptidomimetics (such as peptoids and β-peptides) to peptides, but with different properties. * Protein tag, describing addition of peptide sequences to enable protein isolation or detection * Replikins * Ribosome * Translation (biology)


References

{{Authority control Peptides