Catalyzes, together with MoaC, the conversion of 5'-GTP to cyclic pyranopterin monophosphate (cPMP or molybdopterin precursor Z). Catalyzes the cyclization of GTP to (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate. AH2 + GTP + S-adenosyl-L-methionine = (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate + 5'-deoxyadenosine + A + H(+) + L-methionine Binds 2 [4Fe-4S] clusters. Binds 1 [4Fe-4S] cluster coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine and 1 [4Fe-4S] cluster coordinated with 3 cysteines and the GTP-derived substrate. Cofactor biosynthesis; molybdopterin biosynthesis. Monomer. PubMed:8706892 shows that MoaA seems to contain a 3Fe-4S cluster, but it may actually be two 4Fe-4S clusters as was shown in the crystallographic study of the protein homolog from Staphylococcus aureus. Belongs to the radical SAM superfamily. MoaA family.