In enzymology, a 2-chloro-4-carboxymethylenebut-2-en-1,4-olide isomerase () is an enzyme that catalyzes the chemical reaction
Hence, this enzyme has one substrate, cis-2-chloro-4-carboxymethylenebut-2-en-1,4-olide, and one product, trans-2-chloro-4-carboxymethylenebut-2-en-1,4-olide.
This enzyme belongs to the family of isomerases, specifically cis-trans isomerases. The systematic name of this enzyme class is 2-chloro-4-carboxymethylenebut-2-en-1,4-olide cis-trans-isomerase. Other names in common use include 2-chlorocarboxymethylenebutenolide isomerase, and chlorodienelactone isomerase. This enzyme participates in 1,4-dichlorobenzene degradation.