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(mĕthī`ənēn), 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 the several essential amino acids needed in the diet; the human body cannot synthesize it from simpler metabolites. It is an important source of dietary sulfur. Methionine reacts with adenosine triphosphateadenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
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 to form S-adenosyl methionine, a potent donor of methyl groups (composed of one carbon and three hydrogen atoms); S-adenosyl methionine is the principal methyl donor in the body and contributes to the synthesis of many important substances, including epinephrineepinephrine
, hormone important to the body's metabolism, also known as adrenaline. Epinephrine, a catecholamine, together with norepinephrine, is secreted principally by the medulla of the adrenal gland.
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 and choline (see acetylcholineacetylcholine
, a small organic molecule liberated at nerve endings as a neurotransmitter. It is particularly important in the stimulation of muscle tissue. The transmission of an impulse to the end of the nerve causes it to release neurotransmitter molecules onto the surface of
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; 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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). Since methionine is the only essential amino acid not present in significant amounts of soybeans, it is produced commercially as an additive for soybean meal. Methionine was isolated from casein (milk protein) in 1922, and its structure was proved by laboratory synthesis in 1928.



α-amino-γ-methylmercaptobutyric acid, CH3SCH2CH2CH(NH2)COOH; a sulfur-bearing monocarboxylic amino acid. Methionine exists in D- and L- forms and in a racemic DL- form. L-methionine is a component of most vegetable and animal proteins. It was isolated in 1922 from the products of the acid hydrolysis of casein.

In mammals and man, methionine is a donor of methylene groups in the body. In its S-adenosyl form (active methionine, a product of the reaction of methionine with ATP in the presence of Mg2+ ions), methionine participates in enzymic transmethylation processes, which lead to the formation of choline, adrenalin, and other biologically important substances. It also serves as a source of sulfur in the biosynthesis of cysteine.

The initial substance in the biosynthesis of methionine is aspartic acid, which undergoes a series of conversions to methionine’s immediate precursor, homocysteine. This series of conversions can occur only in certain microorganisms and plants. Homocysteine can also undergo methylation in mammals, enzymically or by direct transfer of a methyl group from donor molecules.

Methionine is an essential amino acid, the daily adult human requirement for which is 2.5–3 g. Methionine deficiency in the diet of animals and man leads to impairment of protein biosyn-thesis, retardation of growth and development, and severe functional disorders. Synthetic methionine, produced industrially from propylene, is used medicinally and to enrich fodders and food. The D- and L- forms of methionine are of equal value, since they are capable of interconversion in the body.


Maister, A. Biokhimiia aminokislot. Moscow, 1961. (Translated from English.)



C5H11O2NS An essential amino acid; furnishes both labile methyl groups and sulfur necessary for normal metabolism.