Lysine (abbreviated as Lys or K) is an essential α-amino acid with the chemical formula HO2CCH(NH2)(CH2)4NH2. Lysine's 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. (The ε-amino group (NH3+) is attached to the fifth carbon from the α-carbon, which is attached to the carboxyl (C=OOH) group.〔(Lysine. ) The Biology Project, Department of Biochemistry and Molecular Biophysics, University of Arizona.〕)
Common posttranslational modifications include methylation of the ε-amino group, giving methyl-, dimethyl-, and trimethyllysine (the latter occurring in calmodulin); also acetylation, sumoylation, ubiquitination, and hydroxylation - producing the hydroxylysine in collagen and other proteins. ''O''-Glycosylation of hydroxylysine residues in the endoplasmic reticulum or Golgi apparatus is used to mark certain proteins for secretion from the cell. In opsins like rhodopsin and the visual opsins (encoded by the genes OPN1SW, OPN1MW, and OPN1LW), retinaldehyde forms a Schiff base with a conserved lysine residue, and interaction of light with the retinylidene group causes signal transduction in color vision (See visual cycle for details). Deficiencies may cause blindness, as well as many other problems due to its ubiquitous presence in proteins.
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 most bacteria, it is synthesized from aspartic acid (aspartate):
* 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.
* 4-hydroxy-tetrahydrodipicolinate synthase adds a pyruvate group to the β-aspartyl-4-semialdehyde, and a water molecule is removed. This causes cyclization and gives rise to (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate.
* 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 4-hydroxy-tetrahydrodipicolinate reductase. This reaction consumes an NADPH molecule and releases a second water 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 glutamic 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.
Enzymes involved in this biosynthesis include:〔
# Aspartate-semialdehyde dehydrogenase
# 4-hydroxy-tetrahydrodipicolinate synthase
# 4-hydroxy-tetrahydrodipicolinate reductase
# 2,3,4,5-tetrahydropyridine-2,6-dicarboxylate N-succinyltransferase
# Succinyldiaminopimelate transaminase
# Succinyl-diaminopimelate desuccinylase
# Diaminopimelate epimerase
# Diaminopimelate decarboxylase.
It is worth noting, however, that in fungi, euglenoids and some prokaryotes lysine is synthesized via the alpha-aminoadipate pathway.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』