Catalyzes the isomerization of D-galactaro-1,5-lactone to D-galactaro-1,4-lactone. 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. D-galactaro-1,5-lactone = D-galactaro-1,4-lactone Does not require a metal cofactor. kcat is 440 sec(-1). Carbohydrate acid metabolism; D-galacturonate degradation via prokaryotic oxidative pathway. Belongs to the metallo-dependent hydrolases superfamily.