Isomerase involved in ulvan degradation (Probable). Ulvan is the main polysaccharide component of the Ulvales (green seaweed) cell wall. It is composed of disaccharide building blocks comprising 3-sulfated rhamnose (Rha3S) linked to D-glucuronic acid (GlcA), L-iduronic acid (IduA), or D-xylose (Xyl) (Probable). Catalyzes the isomerization of 5-dehydro-4-deoxy-D-glucuronate to 3-deoxy-D-glycero-2,5-hexodiulosonate (By similarity). 5-dehydro-4-deoxy-D-glucuronate = 3-deoxy-D-glycero-2,5-hexodiulosonate Binds 1 zinc ion per subunit. By ulvan and rhamnose. Belongs to the KduI family.