Cyclic phosphodiesterase that hydrolyzes the cyclic phosphate of 7,8-dihydroneopterin 2',3'-cyclic phosphate (H2N-cP) and converts it to a mixture of 7,8-dihydroneopterin 2'-phosphate (H2N-2'P) and 7,8-dihydroneopterin 3'-phosphate (H2N-3'P). Is also able to utilize other phosphodiesters as substrates in vitro: hydrolysis of bis-pNPP and pNPPC produces nitrophenyl phosphate, and that of 2',3'-cAMP produces 3'-AMP. ATP, 3',5'-cAMP, GTP, 3',5'-cGMP, and 4',5'-cFMN cannot serve as substrates. 7,8-dihydroneopterin 2',3'-cyclic phosphate + H2O = 7,8-dihydroneopterin 3'-phosphate + H(+) 7,8-dihydroneopterin 2',3'-cyclic phosphate + H2O = 7,8-dihydroneopterin 2'-phosphate + H(+) Binds 1 Fe(2+) ion per subunit. Since MptB requires Fe2(+) for activity, the Fe(2+) ion likely has a catalytic role. The enzyme is also active under high concentrations of Mn(2+) in vitro. Binds 1 Zn(2+) ion per subunit. The Zn(2+) ion is proposed to have a structural role. Optimum pH is 7.5 with bis-pNPP as substrate. Cofactor biosynthesis; 5,6,7,8-tetrahydromethanopterin biosynthesis. Homododecamer.