Transforms 2-oxo-hept-4-ene-1,7-dioate (OHED) into 4-hydroxy-2-oxoheptanedioate, a step in the 4-hydroxyphenylacetic acid (4-HPA) degradation pathway. (4Z)-2-oxohept-4-enedioate + H2O = (4S)-4-hydroxy-2-oxoheptanedioate Binds 1 Mg(2+) ion per subunit. Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 6/7. Homodecamer. Belongs to the hydratase/decarboxylase family.