Catalyzes the reversible retro-aldol cleavage of both 5-keto-4-deoxy-D-glucarate and 2-keto-3-deoxy-D-glucarate to pyruvate and tartronic semialdehyde. 5-dehydro-4-deoxy-D-glucarate = 2-hydroxy-3-oxopropanoate + pyruvate 2-dehydro-3-deoxy-D-glucarate = 2-hydroxy-3-oxopropanoate + pyruvate Binds 1 Mg(2+) ion per subunit. Carbohydrate acid metabolism; galactarate degradation; D-glycerate from galactarate: step 2/3. Homohexamer; trimer of dimers. Belongs to the HpcH/HpaI aldolase family. KDGluc aldolase subfamily.