Catalyzes the dehydration of galactarate to form 5-dehydro-4-deoxy-D-glucarate (5-KDG). galactarate = 5-dehydro-4-deoxy-D-glucarate + H2O Carbohydrate acid metabolism; galactarate degradation; D-glycerate from galactarate: step 1/3. Belongs to the UxaA family.