Contributes to the degradation of lignin at the level of the protocatechuate 4,5-cleavage pathway. Catalyzes the isomerization of the double bond between C4 and C5 in (4E)-oxalomesaconate (OMA) to (3Z)-2-keto-4-carboxy-3-hexenedioate (KCH), where the double bond has migrated between C3 and C4 via a 1,3-allylic isomerization. (1E)-4-oxobut-1-ene-1,2,4-tricarboxylate = (3Z)-2-oxo-4-carboxy-3-hexenedioate kcat is 1300 sec(-1) (at pH 8.0). Secondary metabolite metabolism; lignin degradation. Belongs to the PrpF family.