User:Kateryna Kukil (UU)/sandbox



= 3,5-dihydro-5-methyldiene-4H-imidazol-4-one = 3,5-dihydro-5-methyldiene-4H-imidazol-4-one (MIO) is a cofactor of histidine ammonia lyase (HAL) and phenylalanine ammonia lyase (PAL) which is formed by cyclization and dehydration of conserved Ala-Ser-Gly tripeptide. MIO is a more electrophilic version of dehydroalanine that was previously thought to serve as cofactor in PAL and HAL.

Mechanism
The first step is a cyclization-elimination by an intramolecular nucleophilic attack of the nitrogen of Gly204 at the carbonyl group of Ala202. A subsequent water elimination from the side chain of Ser203 completes the system of crossconjugated double bonds. Numbers are given for the phenylalanine ammonia lyase from Petroselinum crispum (PDB 1W27).