Transforms 5-carboxymethyl-2-hydroxy-muconic acid (CHM) into 5-oxo-pent-3-ene-1,2,5-tricarboxylic acid (OPET). (2E,4Z)-5-hydroxypenta-2,4-diene-1,2,5-tricarboxylate = (3E,5R)-5-carboxy-2-oxohept-3-enedioate Optimum pH is 7.6. Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 4/7. Homotrimer.