Catalyzes the conversion of oxalomesaconic acid enol (OMAenol) to 4-carboxy-4-hydroxy-2-oxoadipic acid (CHA). Mediates the third step of gallate degradation pathway. 2-hydroxy-4-oxobutane-1,2,4-tricarboxylate = 4-carboxy-2-hydroxy-cis,cis-muconate + H2O Binds 1 zinc ion per subunit. Optimum pH is 7.0. Optimum temperature is 30 degrees Celsius. Lacks 4-oxalmesaconate hydratase activity. Belongs to the MshB deacetylase family.