tryptophan

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tryptophan

(trĭp`təfăn), organic compound, one of the 20 amino acidsamino acid
, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins.
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 commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet; human beings cannot synthesize it from simpler metabolites. Young adults require about 7 mg of this amino acid per day per kg (3 mg per lb) of body weight. Nicotinic acid (niacin), a vitaminvitamin,
group of organic substances that are required in the diet of humans and animals for normal growth, maintenance of life, and normal reproduction. Vitamins act as catalysts; very often either the vitamins themselves are coenzymes, or they form integral parts of coenzymes.
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 of the B complex, can be made from tryptophan in the body, but evidently the rate of transformation is insufficient for the demands of normal growth and maintenance, and hence nicotinic acid must be supplied in the diet. Deficiency of tryptophan in the diet enhances the progress of the vitamin-deficiency disease pellagrapellagra
, deficiency disease due to a lack of niacin (nicotinic acid), one of the components of the B complex vitamins in the diet. Niacin is plentiful in yeast, organ meats, peanuts, and wheat germ.
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, which is treated by restoring nicotinic acid to the diet, usually supplemented with tryptophan. Bacteria in the intestine break tryptophan down to compounds such as skatole and indole, which to a great extent are responsible for the unpleasant odor of feces. Tryptophan contributes to the structure of proteins into which it has been incorporated by the tendency of its side chain to participate in hydrophobic interactions (see isoleucineisoleucine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein.
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). The amino acid was isolated from casein (milk protein) in 1901, and its structure was established in 1907.

Tryptophan

 

β-(β-indolyl)-α-aminopropionic acid, one of the most important natural amino acids. It exists in the form of optically active L- and D- forms and a racemic DL-form. L-tryptophan is a minor constituent of gamma globulins, fibrinogen, casein, and other proteins.

L-tryptophan is an essential amino acid. The daily requirements for adults and children up to seven years of age are 0.25 g and about 1 g, respectively. Tryptophan is synthesized in microorganisms and plants by condensation of the amino acid serine with indole, which is catalyzed by the enzyme tryptophan synthetase. (The biosynthesis of tryptophan in E. coli was used to demonstrate the collinearity of a gene and the polypeptide chain coded by that gene, in which the position of each amino acid in the polypeptide chain is determined by a special portion of the gene.) L-tryptophan undergoes complex transformations in animals, forming a number of essential compounds: its decomposition products are the basis for the formation of nicotinic acid and serotonin in humans and other mammals, eye pigments (ommo-chromes) in insects, and heteroauxins, indigo, and several alkaloids in plants. Skatole and indole are formed from tryptophan during putrefactive processes in the intestine. During the normal breakdown in the body, six of the 11 carbon atoms of tryptophan are included in the Krebs cycle through acetyl and acetoacetyl coenzyme A, and the other five are converted to CO2.

In humans, the congenital absence of tryptophan pyrrolase, the enzyme that oxidizes tryptophan, results in feeblemindedness. Disruption of tryptophan metabolism in humans may be indicative of several serious diseases, including tuberculosis, cancer, and diabetes. Functional and organic disorders may also be caused by an insufficiency of tryptophan in the human diet and animal feed, resulting from the low content of tryptophan in many natural proteins. The food value of many proteins can be increased through the addition of synthetic tryptophan, obtained by chemical synthesis from acrylonitrile, ammonia, hydrogen cyanide, or phenylhydrazine. Techniques for enzymic synthesis of tryptophan from indole, pyruvic acid, and ammonia are under development.

REFERENCES

Braunshtein, A. E. Biokhimiia aminokislotnogo obmena. Moscow, 1949.
Lehninger, A. Biokhimiia. Moscow, 1974. (Translated from English.)
Meister, A. Biokhimiia aminokislot. Moscow, 1961. (Translated from English.)
Safonova, E. N., and V. M. Belikov. “Uspekhi v oblasti sinteza i proizvodstva a-aminokislot.” Uspekhi khimii, 1974, vol. 43, no. 9.

E. N. SAFONOVA

tryptophan

[′trip·tə‚fan]
(biochemistry)
C11H12O2N2 An amino acid obtained from casein, fibrin, and certain other proteins; it is a precursor of indoleacetic acid, serotonin, and nicotinic acid.
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