Amino acid: Difference between revisions

Line 26:

The [[carbon]] atom next to the [[carboxyl group]] is called the [[alpha carbon|α–carbon]]. In proteinogenic amino acids, it bears the amine and the R group or [[Substituent|side chain]] specific to each amino acid. With four distinct substituents, the α–carbon is [[stereogenic]] in all α-amino acids except glycine. All chiral proteogenic amino acids have the L configuration. They are "left-handed" [[enantiomer]]s, which refers to the [[stereoisomers]] of the alpha carbon.

A few D-amino acids ("right-handed") have been found in nature, e.g., in [[bacterial envelope]]s, as a [[Neuromodulation|neuromodulator]] (D-[[serine]]), and in some [[antibiotic]]s. Rarely, [[D-Amino acid|D-amino acid residues]] are found in proteins, and are converted from the L-amino acid as a [[post-translational modification]].{{efn|The L and D convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of [[glyceraldehyde]] from which that amino acid can, in theory, be synthesized (D-glyceraldehyde is dextrorotatory; L-glyceraldehyde is levorotatory).

A few D-amino acids ("right-handed") have been found in nature, e.g., in [[bacterial envelope]]s, as a [[Neuromodulation|neuromodulator]] (D-[[serine]]), and in some [[antibiotic]]s. Rarely, [[D-Amino acid|D-amino acid residues]] are found in proteins, and are converted from the L-amino acid as a [[post-translational modification]].

An alternative convention is to use the [[Cahn–Ingold–Prelog priority rules|(''S'') and (''R'') designators]] to specify the ''absolute configuration''. Almost all of the amino acids in proteins are (''S'') at the α carbon, with [[cysteine]] being (''R'') and glycine non-[[Chirality (chemistry)|chiral]]. Cysteine has its side chain in the same geometric location as the other amino acids, but the ''R''/''S'' terminology is reversed because [[sulfur]] has higher atomic number compared to the carboxyl oxygen which gives the side chain a higher priority by the [[Cahn–Ingold–Prelog priority rules|Cahn-Ingold-Prelog sequence rules]].}}

===Side chains===

@@ Line 26: / Line 26: @@
 The [[carbon]] atom next to the [[carboxyl group]] is called the [[alpha carbon|α–carbon]]. In proteinogenic amino acids, it bears the amine and the R group or [[Substituent|side chain]] specific to each amino acid. With four distinct substituents, the α–carbon is [[stereogenic]] in all α-amino acids except glycine. All chiral proteogenic amino acids have the <small>L</small> configuration. They are "left-handed" [[enantiomer]]s, which refers to the [[stereoisomers]] of the alpha carbon.
-A few <small>D</small>-amino acids ("right-handed") have been found in nature, e.g., in [[bacterial envelope]]s, as a [[Neuromodulation|neuromodulator]] (<small>D</small>-[[serine]]), and in some [[antibiotic]]s. Rarely, [[D-Amino acid|<small>D</small>-amino acid residues]] are found in proteins, and are converted from the <small>L</small>-amino acid as a [[post-translational modification]].{{efn|The <small>L</small> and <small>D</small> convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of [[glyceraldehyde]] from which that amino acid can, in theory, be synthesized (<small>D</small>-glyceraldehyde is dextrorotatory; <small>L</small>-glyceraldehyde is levorotatory).
+A few <small>D</small>-amino acids ("right-handed") have been found in nature, e.g., in [[bacterial envelope]]s, as a [[Neuromodulation|neuromodulator]] (<small>D</small>-[[serine]]), and in some [[antibiotic]]s. Rarely, [[D-Amino acid|<small>D</small>-amino acid residues]] are found in proteins, and are converted from the <small>L</small>-amino acid as a [[post-translational modification]].
-An alternative convention is to use the [[Cahn–Ingold–Prelog priority rules|(''S'') and (''R'') designators]] to specify the ''absolute configuration''. Almost all of the amino acids in proteins are (''S'') at the α carbon, with [[cysteine]] being (''R'') and glycine non-[[Chirality (chemistry)|chiral]]. Cysteine has its side chain in the same geometric location as the other amino acids, but the ''R''/''S'' terminology is reversed because [[sulfur]] has higher atomic number compared to the carboxyl oxygen which gives the side chain a higher priority by the [[Cahn–Ingold–Prelog priority rules|Cahn-Ingold-Prelog sequence rules]].}}
 ===Side chains===