In enzymology, an imidazolonepropionase (EC 3.5.2.7) is an enzyme that catalyzes the chemical reaction

(S)-3-(5-oxo-4,5-dihydro-3H-imidazol-4-yl)propanoate + H₂O N-formimidoyl-L-glutamate + H⁺

Thus, the two substrates of this enzyme are (S)-3-(5-oxo-4,5-dihydro-3H-imidazol-4-yl)propanoate and H₂O, whereas its two products are N-formimidoyl-L-glutamate and H⁺.

This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amides. The systematic name of this enzyme class is 3-(5-oxo-4,5-dihydro-3H-imidazol-4-yl)propanoate amidohydrolase. Other names in common use include 4(5)-imidazolone-5(4)-propionic acid hydrolase, and imidazolone propionic acid hydrolase. This enzyme participates in histidine metabolism.

As of late 2007, 6 structures have been solved for this class of enzymes, with PDB accession codes 2BB0, 2G3F, 2GOK, 2OOF, 2PUZ, and 2Q09.

• Rao DR; Greenberg DM (1961). "Studies on the enzymic decomposition of urocanic acid. IV Purification and properties of 4(5)-imidazolone-5(4)-propionic acid hydrolase". J. Biol. Chem. 236: 1758–1763. doi:10.1016/S0021-9258(19)63297-7. PMID 13739526.
• Snyder SH, Silva OL, Kies MW (1961). "The mammalian metabolism of L-histidine. IV. Purification and properties of imidazolone propionic acid hydrolase". J. Biol. Chem. 236 (11): 2996–2998. doi:10.1016/S0021-9258(19)76417-5. PMID 13914653.