Catalyzes a syn cycloisomerization. Also possesses mle activity. 3-carboxy-2,5-dihydro-5-oxofuran-2-acetate = 3-carboxy-cis,cis-muconate Aromatic compound metabolism; beta-ketoadipate pathway; 3-carboxy-cis,cis-muconate from 3-carboxy-2,5-dihydro-5-oxofuran-2-acetate: step 1/1. Homotetramer. Belongs to the cycloisomerase 2 family.