Catalyzes the ring opening of D-galactaro-1,4-lactone to yield 5-keto-4-deoxy-D-glucarate (KDG) via a beta-elimination reaction. This is a step in the oxidative degradation pathway of D-galacturonate, which allows A.tumefaciens to utilize D-galacturonate as a sole carbon source. To a lesser extent, can also use D-glucaro-1,4-lactone as substrate to produce KDG, but cannot use D-galactaro-1,5-lactone, D-glucaro-6,3-lactone and linear D-glucarate. D-glucaro-1,4-lactone = 5-dehydro-4-deoxy-D-glucarate + H(+) D-galactaro-1,4-lactone = 5-dehydro-4-deoxy-D-glucarate + H(+) Optimum pH is 8.5. Carbohydrate acid metabolism; D-galacturonate degradation via prokaryotic oxidative pathway. Homooctamer. Belongs to the mandelate racemase/muconate lactonizing enzyme family.