Catalyzes the tautomerization of the 4-oxalomesaconic acid keto (OMAketo) generated by GalA dioxygenase to 4-oxalomesaconic acid enol (OMAenol). Mediates the second step in gallate degradation pathway. (1E)-4-oxobut-1-ene-1,2,4-tricarboxylate = 4-carboxy-2-hydroxy-cis,cis-muconate Does not prevent growth in 5 mM gallate medium, but the mutant strain grows poorly at gallate concentrations lower than 2 mM. Belongs to the PrpF family.