Peptide

Source: Wikipedia, the free encyclopedia.
Drosomycin, an example of a peptide

Peptides are short chains of amino acids linked by peptide bonds.[1][2] A polypeptide is a longer, continuous, unbranched peptide chain.[3] Polypeptides that have a molecular mass of 10,000 Da or more are called proteins.[4] Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.

Peptides fall under the broad chemical classes of biological polymers and oligomers, alongside nucleic acids, oligosaccharides, polysaccharides, and others.

Proteins consist of one or more polypeptides arranged in a biologically functional way, often bound to

macromolecular assemblies.[5]

Amino acids that have been incorporated into peptides are termed residues. A water molecule is released during formation of each amide bond.[6] All peptides except cyclic peptides have an N-terminal (amine group) and C-terminal (carboxyl group) residue at the end of the peptide (as shown for the tetrapeptide in the image).

Classification

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/

neurotrophic peptides, and blood–brain peptides.[7]

Some ribosomal peptides are subject to proteolysis. These function, typically in higher organisms, as hormones and signaling molecules. Some microbes produce peptides as antibiotics, such as microcins and bacteriocins.[8]

Peptides frequently have

lariat structures have been observed.[9] More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom.[10]

fungi and are synthesized by modular enzyme complexes called nonribosomal peptide synthetases.[12]

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.

thiazoles often indicates that the compound was synthesized in this fashion.[14]

Peptones are derived from animal milk or meat digested by proteolysis.[15] 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.[16]

Peptide fragments refer to fragments of proteins that are used to identify or quantify the source protein.[17] 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.[18][19]

Chemical synthesis

Fmoc-α-amine-protected amino acid

Protein-peptide interactions

Example of a protein (orange) and peptide (green) interaction. Obtained from Propedia: a peptide-protein interactions database [20].

Peptides can perform interactions with proteins and other macromolecules. They are responsible for numerous important functions in human cells, such as cell signaling, and act as immune modulators.[21] Indeed, studies have reported that 15-40% of all protein-protein interactions in human cells are mediated by peptides.[22] Additionally, it is estimated that at least 10% of the pharmaceutical market is based on peptide products.[21]

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

Tachykinin peptides

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
  • Secretin

Pancreatic polypeptide-related peptides

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

Opioid peptides

Calcitonin peptides

Self-assembling peptides

Other peptides

  • B-type Natriuretic Peptide (BNP)
    - produced in the myocardium and useful in medical diagnosis
  • Lactotripeptides - Lactotripeptides might reduce blood pressure,[31][32][33] although the evidence is mixed.[34]
  • Peptidic components from traditional Chinese medicine Colla Corii Asini in hematopoiesis.[35]
  • Jelleine - produced from royal jelly of honey bees.

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 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

A tripeptide (example Val-Gly-Ala) with
green marked amino end (L-valine) and
blue marked carboxyl end (L-alanine)

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:

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 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

References