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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.

Amino acids






Posterior pituitary

Uterine contraction and milk ejection



Posterior pituitary

Antidiuretic (water-retaining) action in




Endocrine pancreas

Increases blood sugar



Anterior pituitary

Stimulates release of cortisol from adrenal




Stomach lining

Stimulates gastric acid secretion



From precursor in

Regulation of body fluid volume and

the blood


Local agents



In tissues

Dilates blood vessels, stimulates secretions




Constricts blood vessels




Hypothalamus and

Promotes release of pituitary and other

many other brain

hormones, and stimulates sympathetic


nervous activity

Substance P


Nervous system, gut,

Vasodilator; neurotransmitter involved in

inflamed tissue

pain sensation



Duodenal lining;

As hormone, stimulates gall bladder

peripheral nerves and

contraction and pancreatic secretion;

many brain regions

neurotransmitter in brain

Proteins usually fold to form particular three-dimensional shapes (which determine their actions), but polypeptides are not so structurally constrained, so in solution they can adopt many conformations. For example, oxytocin and vasopressin have about a thousand different conformations in solution, all in dynamic equilibrium one with another. How is it therefore that they specifically attach to their receptors, with the requirements for specific shape and charge distribution? The answer is that some part of the polypeptide attaches to the receptor, while adjacent parts turn and rotate until the correct shape is reached. Thus the polypeptides use a ‘zipper’ mechanism to attach to membrane receptors.


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, and Sheila Jennett

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


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A chemical compound consisting of two or more amino acids joined to each other through a bond between the nitrogen atom of one amino acid to an oxygen atom of its neighbor. A more precise term describes the number of amino acid units involved. A dipeptide or tripeptide consists of two or three amino acid units respectively. A few oligopeptides (about ten amino acid units) are of physiological importance. The antibiotics bacitracin, gramicidin S, and tyrocidin A are examples of oligopeptides. The largest polypeptides contain dozens or hundreds of amino acid units and are better known as proteins. The bond between peptide units is especially sensitive to attack by various types of corrosive poisons such as strong acids and bases.


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peptide Any of a group of organic compounds comprising two or more amino acids linked by peptide bonds. These bonds are formed by the reaction between adjacent carboxyl (–COOH) and amino (–NH2) groups with the elimination of water (see illustration). Dipeptides contain two amino acids, tripeptides three, and so on. Polypeptides contain more than ten and usually 100–300. Naturally occurring oligopeptides (of less than ten amino acids) include the tripeptide glutathione and the pituitary hormones antidiuretic hormone and oxytocin, which are octapeptides. Peptides also result from protein breakdown, e.g. during digestion.


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pep·tide / ˈpeptīd/ • n. Biochem. a compound consisting of two or more amino acids linked in a chain, the carboxyl group of each acid being joined to the amino group of the next by a bond of the type −OC−NH−.


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peptide A linear molecule comprising two or more amino acids linked by peptide bonds. The simplest peptide is H2N.CH2.CO.NH.CH2.CO2H (glycylglycine).


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peptide A linear molecule comprising two or more amino acids linked by peptide bonds. The simplest peptide is H2N.CH2.CO.NH.CH2.CO2H (glycylglycine).


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peptide (pep-tyd) n. a molecule consisting of two or more amino acids linked by bonds between the amino group and the carboxyl group. See also polypeptide.


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peptide Compound consisting of two or more linked amino acid molecules. Peptides containing several amino acids are called polypeptides. Proteins consist of polypeptide chains with up to several hundred amino acids cross-linked to each other in various ways.


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peptides 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.


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peptide A linear molecule that consists of 2 or more amino acids linked by peptide bonds.