A threonine in the core peptide of interest preceded by a cleavable leader peptide (Lp) is dehydrated by a LanB dehydratase forming a dehydrobutyrine (Dhb) after which a LanC cyclase couples the formed dehydrobutyrine to a cysteine forming a d,l methyllanthionine (dAbu-S-Ala). Directly flanking amino acids (light green) of the Thr and of the Cys (dark green) may affect the extent of dehydration and cyclization [32,34,35]. The enzymes LanB and LanC can also introduce a lanthionine via LanB-catalyzed dehydration of a serine yielding dehydroalanine and subsequent LanC-catalyzed coupling of the dehydroalanine to a cysteine. However, dehydroalanines are much more reactive than dehydrobutyrines and can spontaneously react with cysteines without stereospecificity.