Catalyzes the hydrolysis of the cyclic ribose-phosphate to form alpha-D-ribose 1,5-bisphosphate. alpha-D-ribose 1,2-cyclic phosphate 5-phosphate + H2O = alpha-D-ribose 1,5-bisphosphate + H(+) Binds 2 manganese ions per subunit. Binds 1 zinc ion per subunit. Homodimer. The sequence shown is that of strains K12 and B.