Lysine

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Lysine (abbreviated as Lys or K)[1] is an α-amino acid with the chemical formula HO2CCH(NH2)(CH2)4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG.

Lysine is a base, as are arginine and histidine. The ε-amino group often participates in hydrogen bonding and as a general base in catalysis. Common posttranslational modifications include methylation of the ε-amino group, giving methyl-, dimethyl-, and trimethyllysine. The latter occurs in calmodulin. Other posttranslational modifications at lysine residues include acetylation and ubiquitination. Collagen contains hydroxylysine which is derived from lysine by lysyl hydroxylase. O-Glycosylation of lysine residues in the endoplasmic reticulum or Golgi apparatus is used to mark certain proteins for secretion from the cell.

Contents

Biosynthesis

As an essential amino acid, lysine is not synthesized in animals, hence it must be ingested as lysine or lysine-containing proteins. In plants and bacteria, it is synthesized from aspartic acid (aspartate):[2]

  • L-aspartate is first converted to L-aspartyl-4-phosphate by aspartokinase (or Aspartate kinase). ATP is needed as an energy source for this step.
  • β-aspartate semialdehyde dehydrogenase converts this into β-aspartyl-4-semialdehyde (or β-aspartate-4-semialdehyde). Energy from NADPH is used in this conversion.
  • Dihydrodipicolinate synthase adds a pyruvate group to the β-aspartyl-4-semialdehyde, and two water molecules are removed. This causes cyclization and gives rise to 2,3-dihydrodipicolinate.
  • This product is reduced to 2,3,4,5-tetrahydrodipicolinate (or Δ1-piperidine-2,6-dicarboxylate, in the figure: (S)-2,3,4,5-tetrahydropyridine-2,6-dicarboxylate) by dihydrodipicolinate reductase. This reaction consumes a NADPH molecule.
  • Tetrahydrodipicolinate N-acetyltransferase opens this ring and gives rise to N-succinyl-L-2-amino-6-oxoheptanedionate (or N-acyl-2-amino-6-oxopimelate). Two water molecules and one acyl-CoA (succinyl-CoA) enzyme are used in this reaction.
  • N-succinyl-L-2-amino-6-oxoheptanedionate is converted into N-succinyl-LL-2,6-diaminoheptanedionate (N-acyl-2,6-diaminopimelate). This reaction is catalyzed by the enzyme succinyl diaminopimelate aminotransferase. A glutaric acid molecule is used in this reaction and an oxoacid is produced as a byproduct.
  • N-succinyl-LL-2,6-diaminoheptanedionate (N-acyl-2,6-diaminopimelate)is converted into LL-2,6-diaminoheptanedionate (L,L-2,6-diaminopimelate) by succinyl diaminopimelate desuccinylase (acyldiaminopimelate deacylase). A water molecule is consumed in this reaction and a succinate is produced a byproduct.
  • LL-2,6-diaminoheptanedionate is converted by diaminopimelate epimerase into meso-2,6-diamino-heptanedionate (meso-2,6-diaminopimelate).
  • Finally meso-2,6-diamino-heptanedionate is converted into L-lysine by diaminopimelate decarboxylase.

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