Chemistry:Imino acid

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Short description: Organic compound
1-Pyrroline-5-carboxylic acid, an N-substituted imino acid

In organic chemistry, an imino acid is any molecule that contains both imine (>C=NH) and carboxyl (-C(=O)-OH) functional groups.[1]

Imino group attached to carbon
The two functional groups that together define an imino acid

Imino acids are structurally related to amino acids, which have amino group instead of imine—a difference of single vs double-bond between nitrogen and carbon. The simplest example is dehydroglycine.

D-Amino acid oxidase is an enzyme that is able to convert amino acids into imino acids. Also the direct biosynthetic precursor to the amino acid proline is the imino acid (S)-Δ1-pyrroline-5-carboxylate (P5C).

Related terminology


Secondary amino acids, amino acids containing a secondary amine group are sometimes named imino acids,[2][3] though this usage is obsolescent.[1] The only proteinogenic amino acid of this type is proline, although the related non-proteinogenic amino acids hydroxyproline[4][5][6] and pipecolic acid[7] have often been included in studies of this class of compounds.

The term imino acid is also the obsolete term for imidic acids, structures containing the -C(=NH)-OH group, and should not be used for them.[1]


  1. 1.0 1.1 1.2 IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Imino acids". doi:10.1351/goldbook.I02959
  2. Proline at the US National Library of Medicine Medical Subject Headings (MeSH)
  3. "III. Amino Acids". 
  4. Myhill, D.; Jackson, D.S. (1963). "Separation of proline and hydroxyproline using thin-layer chromatography". Analytical Biochemistry 6 (2): 193–198. doi:10.1016/0003-2697(63)90110-6. PMID 14061301. 
  5. Stout, E.R.; Fritz, G.J. (1966). "Role of Oxygen Fixation in Hydroxyproline Biosynthesis by Etiolated Seedlings". Plant Physiology 41 (2): 197–202. doi:10.1104/pp.41.2.197. PMID 16656240. 
  6. Blumenkrantz, N.; Asboe-Hansen, G. (1975). "An assay for hydroxyproline and proline on one sample and a simplified method for hydroxyproline". Analytical Biochemistry 63 (2): 331–340. doi:10.1016/0003-2697(75)90354-1. PMID 1122021. 
  7. Maruyama, S; Miyoshi, S; Nomura, G; Suzuki, M; Tanaka, H; Maeda, H (1993). "Specificity for various imino-acid-residues of a proline-specific dipeptidylcarboxypeptidase from a Streptomyces species". Biochim Biophys Acta 1162 (1–2): 72–76. doi:10.1016/0167-4838(93)90129-f. PMID 8448197. 

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