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Metabolite Card

Note

5-Hydroxylysine (Hyl), also known as hydroxylysine or 5-Hydroxy-L-lysine, belongs to the class of organic compounds known as L-alpha-amino acids. These are alpha amino acids which have the L-configuration of the alpha-carbon atom. Amino acids are organic compounds that contain amino (–NH2) and carboxyl (–COOH) functional groups, along with a side chain (R group) specific to each amino acid. 5-Hydroxylysine is a hydroxylated derivative of the amino acid lysine that is present in certain collagens, the chief structural protein of mammalian skin and connective tissue. 5-Hydroxylysine arises from a post-translational hydroxy modification of lysine and is biosynthesized from lysine via oxidation by lysyl hydroxylase enzymes. 5-Hydroxylysine can then undergo further modification by glycosylation, giving rise to galactosyl hydroxylysine (GH) and glucosylgalactosyl hydroxylysine (GGH). These glycosylated forms of hydroxylysine contribute to collagen’s unusual toughness and resiliency. The monoglycosylated, galactosyl-hydroxylysine is enriched in bone compared with the disaccharide form, glucosyl-galactosyl-hydroxylysine, which is the major form in skin. 5-Hydroxylysine exists in all eukaryotes, ranging from yeast to humans. It was first discovered in 1921 by Donald Van Slyke. Free forms of hydroxylysine arise through proteolytic degradation of collagen. Urinary excretion of 5-Hydroxylysine and its glycosides can be used as an index of collagen degradation, with high levels being indicative of more rapid or extensive collagen degradation (often seen in patients with thermal burns, Paget's disease of bone or hyperphosphatasia) (PMID: 404321).