Proposed autocatalytic formation of MIO from the tripeptide Ala-Ser-Gly by two water elimination steps. ^[1]

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.^[2] 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.^[1] Numbers are given for the phenylalanine ammonia lyase from Petroselinum crispum (PDB 1W27).

Notes

^ ^a ^b Ritter, Holger; Schulz, Georg E. (2004-12-01). "Structural Basis for the Entrance into the Phenylpropanoid Metabolism Catalyzed by Phenylalanine Ammonia-Lyase". The Plant Cell. 16 (12): 3426–3436. doi:10.1105/tpc.104.025288. ISSN 1040-4651. PMC 535883. PMID 15548745.{{cite journal}}: CS1 maint: PMC format (link)
^ Calabrese, Joseph C.; Jordan, Douglas B.; Boodhoo, Amechand; Sariaslani, Sima; Vannelli, Todd (2004-09). "Crystal Structure of Phenylalanine Ammonia Lyase: Multiple Helix Dipoles Implicated in Catalysis†,‡". Biochemistry. 43 (36): 11403–11416. doi:10.1021/bi049053+. ISSN 0006-2960. {{cite journal}}: Check date values in: |date= (help); no-break space character in |title= at position 50 (help)

[:0-1] Ritter, Holger; Schulz, Georg E. (2004-12-01). "Structural Basis for the Entrance into the Phenylpropanoid Metabolism Catalyzed by Phenylalanine Ammonia-Lyase". The Plant Cell. 16 (12): 3426–3436. doi:10.1105/tpc.104.025288. ISSN 1040-4651. PMC 535883. PMID 15548745.{{cite journal}}: CS1 maint: PMC format (link)

[2] Calabrese, Joseph C.; Jordan, Douglas B.; Boodhoo, Amechand; Sariaslani, Sima; Vannelli, Todd (2004-09). "Crystal Structure of Phenylalanine Ammonia Lyase: Multiple Helix Dipoles Implicated in Catalysis†,‡". Biochemistry. 43 (36): 11403–11416. doi:10.1021/bi049053+. ISSN 0006-2960. {{cite journal}}: Check date values in: |date= (help); no-break space character in |title= at position 50 (help)

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[2]