Contributes to the degradation of lignin at the level of the protocatechuate 4,5-cleavage pathway. Catalyzes the hydration of the double bond of (3Z)-2-keto-4-carboxy-3-hexenedioate (KCH) to (4S)-4-carboxy-4-hydroxy-2-oxoadipate (CHA, also named (2S)-2-hydroxy-4-oxobutane-1,2,4-tricarboxylate) (PubMed:29658701, PubMed:30207699). Is involved in the catabolism of both vanillate and syringate (PubMed:11092855). (3Z)-2-oxo-4-carboxy-3-hexenedioate + H2O = (2S)-2-hydroxy-4-oxobutane-1,2,4-tricarboxylate Binds 1 zinc ion per subunit. The metal center contained within the active site of LigJ is not used to activate the water molecule but is utilized to activate the substrate via polarization of the carbonyl oxygen. kcat is 25.6 sec(-1) (at pH 8.0). Secondary metabolite metabolism; lignin degradation. Homodimer. Cells lacking this gene catabolize vanillate and syringate with an accumulation of the enol form of oxalomesaconate (OMA), 2-pyrone-4,6-dicarboxylate (PDC), and a product that is likely to be KCH. Belongs to the metallo-dependent hydrolases superfamily.