Decarboxylates OPET (5-oxo-pent-3-ene-1,2,5-tricarboxylic acid) into HHDD (2-hydroxy-hept-2,4-diene-1,7-dioate) and isomerizes it to OHED (2-oxo-hept-3-ene-1,7-dioate). (2E,4Z)-5-hydroxypenta-2,4-diene-1,2,5-tricarboxylate = (3E,5R)-5-carboxy-2-oxohept-3-enedioate (3E,5R)-5-carboxy-2-oxohept-3-enedioate + H(+) = (4Z)-2-oxohept-4-enedioate + CO2 Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 4/7. Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 5/7. Monomer. Belongs to the FAH family.