Converts GTP to 7,8-dihydro-D-neopterin 2',3'-cyclic phosphate, the first intermediate in the biosynthesis of coenzyme methanopterin. It is also able to utilize a variety of GTP analogs as substrates, including GDP, beta,gamma-methylene-GTP and GTP-[gamma-thio]. GTP + H2O = 7,8-dihydroneopterin 2',3'-cyclic phosphate + diphosphate + formate + H(+) Binds 1 Fe(2+) ion per subunit. Mn(2+) and Zn(2+) can be used to a lesser extent, but not at a relevant physiological concentration. Inhibited by GTP concentrations greater than 0.3 mM and by 2-amino-5-formylamino-6-ribofuranosylamino-4(3H)-pyrimidinone 5'-phosphate (fapyGMP). Partial inhibition is observed when 2 mM GMP, dGTP, or 7-methyl-GTP was included along with 2 mM GTP. Other purine nucleotides including ATP, ITP, dGTP, GMP, and 7-methyl-GTP do not serve as substrates. Optimum pH is 6.5. MptA loses 27% activity when it incubates for 10 min at 80 degrees Celsius, 60% activity at 90 degrees Celsius, and 89% activity at 100 degrees Celsius. Cofactor biosynthesis; 5,6,7,8-tetrahydromethanopterin biosynthesis. Homodimer. MptA is the archetype of a new class of GTP cyclohydrolases that catalyzes a series of reactions most similar to that seen with GTPCHI but unique in that the cyclic phosphate is the product. Belongs to the GTP cyclohydrolase IV family.