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Related to pyrrolidine-2-carboxylate: L-proline


(prō`lēn), 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 not essential to the human diet, since it can be synthesized in the body from glutamic acidglutamic acid
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer occurs in mammalian proteins.
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. The amino group through which it can link to other amino acids (see peptidepeptide,
organic compound composed of amino acids linked together chemically by peptide bonds. The peptide bond always involves a single covalent link between the α-carboxyl (oxygen-bearing carbon) of one amino acid and the amino nitrogen of a second amino acid.
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, proteinprotein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
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) is part of a circlelike array of atoms—unique to proline. This is significant because when the amino acid is incorporated into protein, its peculiar structure leads to sharp bends, or kinks, in the peptide chain, thus figuring prominently in the determination of the protein's shape. Proline and its derivate hydroxyproline, make up some 21% of the amino-acid residues found in collagencollagen
, any of a group of proteins found in skin, ligaments, tendons, bone and cartilage, and other connective tissue. Cells called fibroblasts form the various fibers in connective tissue in the body.
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, the fibrous protein of connective tissue. Its chemical synthesis was accomplished in 1900; in 1901 proline was isolated from casein, the milk protein, and its structure was shown to be the same as that of the synthetic compound.



(2-pyrrolidinecarboxylic acid), a heterocyclic amino acid, more precisely, an imino acid; it exists in the optically active D- and L-forms and in the racemic DL-form. The secondary amine in proline determines its unusual ninhydrin reaction (yellow stain instead of blue-purple). Its structure is

L-proline is present in all natural proteins, especially in plant proteins, such as prolamins, proteins of connective tissue (10–15 percent in collagen), and β-casein. It is a component of such biologically important peptides as insulin, adrenocorticotropic hormone, and gramicidin S. D-proline forms a constituent part of certain alkaloids. The enzyme prolinase catalyzes the hydrolysis of peptide bonds formed by the CO group of L-proline, which is a constituent of peptides, and the NH group of an adjacent amino acid, while the enzyme prolidase hydrolyzes peptide bonds, in which the NH group of L-proline is present.

Proline is a nonessential amino acid; its biosynthesis in living organisms is effected either by the gamma-half-aldehyde of glutamic acid or by ornithine. Proline is converted into hydroxy-proline upon oxidation in the presence of ascorbic acid. DL-proline was synthesized in 1900 by R. Willstätter and was isolated, together with L-proline, from casein hydrolysate in 1901 by E. Fischer.


C5H9O2 A heterocyclic amino acid occurring in essentially all proteins, and as a major constituent in collagen proteins.