In enzymology, a chloromuconate cycloisomerase (EC 5.5.1.7) is an enzyme that catalyzes the chemical reaction

2-chloro-2,5-dihydro-5-oxofuran-2-acetate ${\displaystyle \rightleftharpoons }$ 3-chloro-cis,cis-muconate

Hence, this enzyme has one substrate, 2-chloro-2,5-dihydro-5-oxofuran-2-acetate, and one product, 3-chloro-cis,cis-muconate.

This enzyme belongs to the family of isomerases, specifically the class of intramolecular lyases. The systematic name of this enzyme class is 2-chloro-2,5-dihydro-5-oxofuran-2-acetate lyase (decyclizing). This enzyme is also called muconate cycloisomerase II. This enzyme participates in gamma-hexachlorocyclohexane degradation and 1,4-dichlorobenzene degradation. It employs one cofactor, manganese.

As of late 2007, 3 structures have been solved for this class of enzymes, with PDB accession codes 1CHR, 1NU5, and 2CHR.