In enzymology, a carboxymethylenebutenolidase (EC 3.1.1.45, also known as CMBL and dienelactone hydrolase) is an enzyme that catalyzes the chemical reaction

4-carboxymethylenebut-2-en-4-olide + H₂O ${\displaystyle \rightleftharpoons }$ 4-oxohex-2-enedioate

Thus, the two substrates of this enzyme are 4-carboxymethylenebut-2-en-4-olide and H₂O, whereas its product is 4-oxohex-2-enedioate.

This enzyme belongs to the family of hydrolases, specifically those acting on carboxylic ester bonds. The systematic name of this enzyme class is 4-carboxymethylenebut-2-en-4-olide lactonohydrolase. Other names in common use include maleylacetate enol-lactonase, dienelactone hydrolase, and carboxymethylene butenolide hydrolase. This enzyme participates in gamma-hexachlorocyclohexane degradation and 1,4-dichlorobenzene degradation.

As of late 2007, 10 structures have been solved for this class of enzymes, with PDB accession codes 1DIN, 1GGV, 1ZI6, 1ZI8, 1ZI9, 1ZIC, 1ZIX, 1ZIY, 1ZJ4, and 1ZJ5.

• Schmidt E, Knackmuss HJ (1980). "Chemical structure and biodegradability of halogenated aromatic compounds. Conversion of chlorinated muconic acids into maleoylacetic acid". Biochem. J. 192 (1): 339–47. doi:10.1042/bj1920339. PMC 1162339. PMID 7305906.