Catalyzes an anti cycloisomerization. 2-(carboxymethyl)-5-oxo-2,5-dihydro-2-furoate = 3-carboxy-cis,cis-muconate + H(+) Aromatic compound metabolism; beta-ketoadipate pathway; 5-oxo-4,5-dihydro-2-furylacetate from 3-carboxy-cis,cis-muconate: step 1/2. Homotetramer. Belongs to the class-II fumarase/aspartase family.