Catalyzes the isomerization of 5-dehydro-4-deoxy-D-glucuronate to 3-deoxy-D-glycero-2,5-hexodiulosonate. 5-dehydro-4-deoxy-D-glucuronate = 3-deoxy-D-glycero-2,5-hexodiulosonate Binds 1 zinc ion per subunit. Glycan metabolism; pectin degradation; 2-dehydro-3-deoxy-D-gluconate from pectin: step 4/5. Belongs to the KduI family.