2-dehydro-3-deoxy-D-gluconate 5-dehydrogenase 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 reversible reduction of 2,5-diketo-3-deoxygluconate (DKII or 4,6-dihydroxy-2,5-dioxohexanoate) into 2-keto-3-deoxygluconate (KDG or 2-dehydro-3-deoxygluconate) with a concomitant oxidation of NADH (By similarity). 2-dehydro-3-deoxy-D-gluconate + NAD(+) = 3-deoxy-D-glycero-2,5-hexodiulosonate + H(+) + NADH Homotetramer. By ulvan and rhamnose. Belongs to the short-chain dehydrogenases/reductases (SDR) family.