Mediates the conversion of L-galactonate to 2-dehydro-3-deoxy-L-galactonate, the second step in D-galacturonate catabolic process. L-galactonate = 2-dehydro-3-deoxy-L-galactonate + H2O Divalent metal ions. Magnesium seems to be the preferred ion. Carbohydrate acid metabolism. Cells are unable to grow on D-galacturonate. Belongs to the mandelate racemase/muconate lactonizing enzyme family.