Dictionary:

peptide

  (pĕp'tīd')

Any of various natural or synthetic compounds containing two or more amino acids linked by the carboxyl group of one amino acid to the amino group of another.

[PEPT(ONE) + –IDE.]

A compound that is made up of two or more amino acids joined by covalent bonds which are formed by the elimination of a molecule of H₂O from the amino group of one amino acid and the carboxyl group of the next amino acid. Peptides larger than about 50 amino acid residues are usually classified as proteins. Glutathione is the most abundant peptide in mammalian tissue. Hormones such as oxytocin (8), vasopressin (8), glucagon (29), and adrenocorticotropic hormone (39) are peptides whose structures have been deduced; in parentheses are the numbers of amino acid residues for each peptide.

For each step in the biological synthesis of a peptide or protein there is a specific enzyme or enzyme complex that catalyzes each reaction in an ordered fashion along the biosynthetic route. However, it is noteworthy that, although the biological synthesis of proteins is directed by messenger RNA on cellular structures called ribosomes, the biological synthesis of peptides does not require either messenger RNA or ribosomes. See also Amino acids; Protein.

Peptides are short chains of amino acids linked together. If there are only two amino acids then the peptide is a dipeptide. Similarly there are tripeptides, tetrapeptides, and so on. If the number of amino acids in the chain reaches around ten or so, such substances are called polypeptides, while large polypeptides are called proteins. There is no particular agreed size at which a large polypeptide becomes a small protein, but generally polypeptides have molecular weights of a few thousand, while proteins have molecular weights of tens of thousands. Depending on which amino acids are involved, between seven and ten amino acids will add about 1000 to the molecular weight.

Protein molecules in the diet are digested by enzymes (which are themselves specialized proteins), that break them down into smaller and smaller lengths, the breakage occurring at the peptide bonds. Peptides and amino acids are thus the final cleavage products of protein digestion. Amino acids are the main protein breakdown product absorbed from the gut, but some di- and tri-peptides are also absorbed, there being specific carrier systems in the cells lining the small intestine to transport these small peptides from the lumen to the blood.

The dipeptide carnosine, formed from the amino acids alanine and histidine, was identified in muscle a century ago, but only recently has research revealed its properties and the likely variety and significance of its functions. It is known to be present also in the brain, where it may act as a neurotransmitter. In muscle it is likely to be important in making the contractile filaments more sensitive to calcium ions and in controlling the internal acidity of these fibres. It has been suggested that it may also be a scavenger of free radicals. Its strong binding with zinc may be important in co-absorption from the gut of this essential trace element; and physiologically significant interactions between carnosine, zinc, and histamine are being discovered.

The tripeptide glutathione (glutamic acid-cysteine-glycine) is an important co-factor for many enzymes, increasing their activity.

Polypeptide hormones

Polypeptides control or trigger a great many bodily functions, acting close to or at a distance from the site at which they are produced and released. The table below gives a few examples, giving the site of production, the number of amino acids, and an indication of the functions that the polypeptides promote.

Neuropeptides

There are many different peptides in neurons, released along with other neurotransmitters. Some peptides that were originally identified as hormones, thought to be produced at one particular site and to act at certain ‘target’ sites, have more recently been found to be made elsewhere also, and to have other functions. The body utilizes the same peptide for different purposes. This is true, for example, of cholecystokinin (CCK), a 33-amino-acid polypeptide that was known for many decades as a hormone that originated in the duodenum and caused emptying of the gall bladder. Since the 1980s it has been revealed to be a modulator of neural activity, produced by many nerve cells, widespread in the nervous system. Likewise, corticotrophin releasing factor (CRF), with 41 amino acids, was originally known to be made and released by a group of neurons in the hypothalamus, passing to the pituitary gland and there stimulating the secretion of ACTH (adrenocorticotrophic hormone). But it too has been found to be a neuromodulator produced by neurons in many parts of the brain.

A family of peptides called opioid peptides or endorphins, found in the brain and elsewhere in the body, are responsible for the modulation of pain sensation. One group of these, the pentapeptide enkephalins, are released as neurotransmitters by nerve cells in certain parts of the brain and spinal cord. They bind to opiate receptors (the membrane receptors on which opiate drugs act) on other nerve cells in the pathways that mediate pain, hence acting as ‘endogenous’ (internally generated) analgesics.

— Alan W. Cuthbert, Sheila Jennett

See also amino acids; hormones; opiates; opioids; pain; proteins.

Compounds formed when amino acids are linked together through the —CO—NH— (peptide) linkage. Two amino acids so linked form a dipeptide, three a tripeptide, etc.; medium-length chains of amino acids (four up to about 50) are known as oligopeptides, longer chains are polypeptides or proteins.

A compound of two or more amino acids in which the α-carboxyl group of one is united with the α-amino group of another, with the elimination of a molecule of water, creating a peptide bond— CO—NH—.

For more information on peptide, visit Britannica.com.

Any of a class of compounds of low molecular weight which yield two or more amino acids on hydrolysis; known as di-, tri-, tetra- etc. peptides, depending on the number of amino acids in the molecule. Peptides form the constituent parts of proteins. See also polypeptide.

• leader p. — a step in the signal hypothesis advanced to explain the mechanisms governing the fate of newly formed polypeptides or secretory proteins.
• p. map — a pattern of peptide fragments, characteristic of a particular protein. Produced by using either proteolytic enzymes such as trypsin or chemicals such as cyanogen bromide to cut proteins at a relatively small number of particular sites, the peptide fragments are then separated by chromatographic or electrophoretic procedures. Called also fingerprint.
• p.-para-aminobenzoic acid test — see bt-paba test.

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