Lysine

"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 acidfor humans. 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 incatalysis. (The ε-amino group (NH₃⁺) is attached to the fifth carbon beginning from the α-carbon, which is attached to the carboxyl (C=OOH) group.^[2])"

"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. Collagencontains hydroxylysine, which is derived from lysine by lysyl hydroxylase. O-Glycosylation of hydroxylysine residues in the endoplasmic reticulum or Golgi apparatus is used to mark certain proteins for secretion from the cell."