In enzymology, a 3-carboxy-cis,cis-muconate cycloisomerase (EC 5.5.1.2) is an enzyme that catalyzes the chemical reaction

2-carboxy-2,5-dihydro-5-oxofuran-2-acetate ${\displaystyle \rightleftharpoons }$ cis,cis-butadiene-1,2,4-tricarboxylate

Hence, this enzyme has one substrate, 2-carboxy-2,5-dihydro-5-oxofuran-2-acetate, and one product, cis,cis-butadiene-1,2,4-tricarboxylate.

This enzyme belongs to the family of isomerases, specifically intramolecular lyases. The systematic name of this enzyme class is 2-carboxy-2,5-dihydro-5-oxofuran-2-acetate lyase (decyclizing). Other names in common use include beta-carboxymuconate lactonizing enzyme, and 3-carboxymuconolactone hydrolase. This enzyme participates in benzoate degradation via hydroxylation.

As of late 2007, only one structure has been solved for this class of enzymes, with the PDB accession code 1Q5N.