In enzymology, a carboxymethylenebutenolidase (, also known as CMBL and dienelactone hydrolase) is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are 4-carboxymethylenebut-2-en-4-olide and H<sub>2</sub>O, 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 , , , , , , , , , and .