Dehydroalanine

Dehydroalanine (Dha) is a nonstandard amino acid that features an α,β-unsaturated system. In its common form the side chain is a hydrogen, so the backbone is NH2-CH=CH-COOH. The α,β-unsaturation makes Dha a reactive Michael acceptor, and it often occurs as a post-translational modification in peptides rather than as a free amino acid in proteins.

Biological formation: Dha is generated by dehydration of serine residues in ribosomally synthesized peptides, most notably in lantibiotics. In these pathways, serine (and sometimes threonine) residues are eliminated of water by dedicated dehydratase enzymes (e.g., LanB), yielding Dha and, for threonine, the related dehydrobutyrine (Dhb). Subsequent attack by cysteine thiols creates crosslinks such as lanthionine.

Reactivity and applications: Dha readily undergoes conjugate addition with nucleophiles, especially thiols, amines, and water. This reactivity underpins the formation of thioether crosslinks in lantibiotics and makes Dha a useful handle for covalent bioconjugation and protein engineering. In proteins, Dha can arise under oxidative or enzymatic conditions and alter function.

Occurrence and significance: Dha residues are best known from lantibiotics such as nisin, epidermin, and subtilin, where dehydration and crosslinking generate mature, bioactive peptides. Dha also serves as a reactive intermediate in other natural product biosynthesis and can be used as a diagnostic marker for dehydration events in polypeptides.