Catalyzes the ketonization of 2-hydroxymuconate stereoselectively to yield 2-oxo-3-hexenedioate. (2Z,4E)-2-hydroxyhexa-2,4-dienedioate = (3E)-2-oxohex-3-enedioate Aromatic compound metabolism; salicylate degradation. Homohexamer. Belongs to the 4-oxalocrotonate tautomerase family.