 Hydroxylysine

Hydroxylysine is a hydroxylated derivative of the amino acid lysine. In the important collagen-associated form, a hydroxyl group is attached at carbon 5 of the side chain. Free 5-hydroxylysine has formula ${\mathrm{C}}_6{\mathrm{H}}_{14}{\mathrm{N}}_2{\mathrm{O}}_3$, and its connectivity can be writtenThis formula specifies the arrangement of the groups but does not by itself distinguish the possible stereoisomers. ^[ref]

In collagen, hydroxylysine occurs as a modified amino acid residue within a protein. A concrete example is the sulfilimine bond linking hydroxylysine-211 of one Collagen IV unit to methionine-93 of another. Mass spectrometry and nuclear magnetic resonance identified that sulfur–nitrogen connection.^[ref] The example shows why a modified side chain matters: changing a particular residue changes the chemistry available for joining protein units. Free hydroxylysine and a residue incorporated into collagen must nevertheless be distinguished, because incorporation places its backbone in peptide bonds.