 Biotin

Biotin is a small sulfur-containing cofactor used by enzymes that transfer carboxyl groups. Covalent attachment to a lysine side chain lets protein-bound biotin carry a carboxyl group between catalytic sites.^[ref]

Biotin is also used as a covalent tag in protein-labelling methods. Strong recognition of the tag enables enrichment before mass spectrometry, connecting a natural biochemical recognition system with an analytical separation.