Peptide

Peptides (from Gr.: 羽汍羽而ός, pept車s "digested"; derived from 羽έ考考汍牝糸, p谷ssein "to digest") are biologically occurring 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 amine 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, oligosaccharides 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. 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.[3] Finally, while aspects of the lab techniques applied to peptides versus polypeptides and proteins differ (e.g., 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. 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 classes

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.

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. 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.More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom.

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. Other nonribosomal peptides are most common in unicellular organisms, plants, and fungi 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 acids and polyketides, hybrid compounds are often found. The presence of oxazoles or thiazoles often indicates that the compound was synthesized in this fashion.

Peptones

See also Tryptone

Peptones are derived from animal milk or meat digested by proteolysis. In addition to containing small peptides, the resulting spray-dried material includes fats, metals, salts, vitamins and many other biological compounds. Peptones are used in nutrient media for growing bacteria and fungi.

Peptide fragments 

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.

Peptides in molecular biology

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 of purifying the protein of interest.[18] 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.

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. For example, one of the most promising application is through peptides that target LHRH. 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.

Well-known peptide 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 func

Notes on terminology

Length:

A polypeptide is a single linear chain of many amino acids, held together by amide bonds.

A protein is one or more polypeptide (more than about 50 amino acids long).

An oligopeptide consists of only a few amino acids (between two and twenty).

A tripeptide (example Val-Gly-Ala) with

green marked amino end (L-Valine) and

blue marked carboxyl end (L-Alanine)

Number of amino acids:

A monopeptide has one amino acid.

A dipeptide has two amino acids.

A tripeptide has three amino acids.

A tetrapeptide has four amino acids.

A pentapeptide has five amino acids.

A hexapeptide has six amino acids.

A heptapeptide has seven amino acids.

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 & angiotensin I).

An undecapeptide (or monodecapeptide) has eleven amino acids, a dodecapeptide (or didecapeptide) has twelve amino acids, a tridecapeptide has thirteen amino acids, and so forth.

An icosapeptide has twenty amino acids, a tricontapeptide has thirty amino acids, a tetracontapeptide has forty amino acids, and so forth.

See also: IUPAC numerical multiplier

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 proteose is a mixture of peptides produced by the hydrolysis of proteins. The term is somewhat archaic.

Doping in sports

The term peptide has been incorrectly or unclearly used to mean illegal secretagogue and peptide hormones in sports doping matters: illegal secretagogue peptides are classified as Schedule 2 (S2) prohibited substances on the World Anti-Doping Agency (WADA) Prohibited List, and are therefore prohibited for use by professional athletes both in and out of competition. Such peptide secretagogues have been on the WADA prohibited substances list since at least 2008. The Australian Crime Commission (incorrectly using the term peptides) cited the alleged misuse of illegal peptide secretagogues used in Australian sport including growth hormone releasing peptides CJC-1295, GHRP-6, and GHSR (gene) hexarelin. There is ongoing controversy on the legality of using peptide secretagogues in sports.

A peptide is a chemical compound containing two or more amino acids (amino acid polymers) that are coupled by a Peptide Bond peptide bond. This bond is a special linkage in which the nitrogen atom of one amino acid binds to the carboxyl carbon atom of another.

Peptides are often classified according to the number of amino acid residues. Oligopeptides have 10 or fewer amino acids. Molecules consisting from 10 to 50 amino acids are called peptides. The term protein describes molecules with more than 50 amino acids.

Molecules with molecular weights ranging from several thousand to several million daltons (D) are called polypeptides. The terms polypeptide and protein are frequently used interchangeably.

In 1901, the first synthetic peptide glycyl-glycine (see picture) was discovered by Emil Fischer, in collaboration with Ernest Fourneau. The first polypeptide (oxytocin - nine amino acid sequence) was synthesized by Vincent du Vigneaud in 1953.

Peptides have a wide range of applications in medicine and biotechnology. They regulate most physiological processes, acting at some sites as endocrine or paracrine signals and at others as neurotransmitters or growth factors.

Insulin was the first therapeutic protein to be introduced to treat insulin-dependent diabetes in the 1920s. There are sixty FDA approved peptide drugs in the market. About 140 peptide drugs are in clinical trials and over 500 are in pre-clinical development.

A number of peptides are used for diagnostic purposes, for example C-peptide is used to monitor insulin production and to help determine the cause of low blood sugar (hypoglycemia).

Peptides are synthesized by coupling the carboxyl group or C-terminus of one amino acid to the amino group or N-terminus of another. There are two strategies for peptide synthesis: liquid-phase peptide synthesis and solid-phase peptide synthesis (SPPS).

The goal of this project is to collect all information about peptides and peptide synthesis in one place.


Peptide news
Use of peptide synthesis to probe viral antigens for epitopes to a r...2016-07-28

A procedure is described for rapid concurrent synthesis on solid supports of hundreds of peptides, of sufficient purity to react in an enzyme-linked immunosorbent assay. Interaction of synthesized peptides with antibodies is then easily detected without removing them from the support. In this manner an immunogenic epitope of the immunologic

Human C-peptide in normal and diabetic subjects2016-07-28

Concentrations of C-, IRI (immunoreactive ) and were determined during oral tolerance test (1.75 g /kg ideal body weight) in 14 normal persons (N), 9 maturity-onset diabetics (DI) and 10 -requiring diabetics () never treated with and in 3 formerly treated diabetics. The mean fasting levels of C-and: N: 0.37 +/- 0.02 nM and 0.048 +/- 0.009

Determinants of the Impaired Secretion of Glucagon-Like Peptide-1 in...2016-07-28

To elucidate the causes of the diminished incretin effect in type 2 diabetes mellitus we investigated the secretion of the incretin hormones, glucagon-like peptide-1 and glucose- dependent insulinotropic polypeptide and measured nonesterified fatty acids, and plasma concentrations of insulin, C peptide, pancreatic polypeptide, and glucose

Functional mimicry of a protein hormone by a peptide agonist: the EP...2016-07-28

The functional mimicry of a protein by an unrelated small molecule has been a formidable challenge. Now, however, the biological activity of a 166-residue hematopoietic growth hormone, erythropoietin (EPO), with its class 1 cytokine receptor has been mimicked by a 20-residue cyclic peptide unrelated in sequence to the natural ligand. The cr

Neurodegeneration induced by beta-amyloid peptides in vitro: the rol...2016-07-28

The progressive neurodegeneration of Alzheimer's disease has been hypothesized to be mediated, at least in part, by beta-amyloid protein. A relationship between the aggregation state of beta-amyloid protein and its ability to promote degeneration in vitro has been previously suggested. To evaluate this hypothesis and to define a struc

LEAP-1, a novel highly disulfide-bonded human peptide, exhibits anti...2016-07-28

We report the isolation and characterization of a novel human peptide with antimicrobial activity, termed LEAP-1 (liver-expressed antimicrobial peptide). Using a mass spectrometric assay detecting cysteine-rich peptides, a 25-residue peptide containing four disulfide bonds was identified in human blood ultrafiltrate. LEAP-1 expression was p

Antibacterial agents based on the cyclic D,L-alpha-peptide architect...2016-07-28

The rapid emergence of bacterial infections that are resistant to many drugs underscores the need for new therapeutic agents. Here we report that six- and eight-residue cyclic d,l-alpha-peptides act preferentially on Gram-positive and/or Gram-negative bacterial membranes compared to mammalian cells, increase membrane permeability, collapse tr

Peptides are distinguished from proteins on the basis of size2016-07-28

Peptides  derived from 羽έ考考汍牝糸, p谷ssein "to digest") are biologically occurring 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 amine group of another. The shortest peptides are dipeptides, consisting of 2 amino acids joine

Transducible TAT-HA fusogenic peptide enhances escape of TAT-fusion ...2016-07-27

The TAT protein transduction domain (PTD) has been used to deliver a wide variety of biologically active cargo for the treatment of multiple preclinical disease models, including cancer and stroke. However, the mechanism of transduction remains unknown. Because of the TAT PTD's strong cell-surface binding, early assumptions regarding ce

Peptide-binding specificity of the molecular chaperone BiP2016-07-27

Members of the heat-shock protein family (hsp70s) can distinguish folded from unfolded proteins. This property is crucial to the role of hsp70s as molecular chaperones and is attributable to the amino-acid specificity of the peptide-binding site. The specificity for peptide ligands is investigated using a set of peptides of random sequence