Probably forms a complex with isoform b that catalyzes the conversion of 5'-GTP to cyclic pyranopterin monophosphate (cPMP) (By similarity). Catalyzes the cyclization of GTP to (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate and mocs1b catalyzes the subsequent conversion of (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate to cPMP (By similarity). Probably forms a complex with isoform a that catalyzes the conversion of 5'-GTP to cyclic pyranopterin monophosphate (cPMP). AH2 + GTP + S-adenosyl-L-methionine = (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate + 5'-deoxyadenosine + A + H(+) + L-methionine (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate = cyclic pyranopterin phosphate + diphosphate 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. Isoform a and isoform b probably form a heterooligomer. In the C-terminal section; belongs to the MoaC family. In the N-terminal section; belongs to the radical SAM superfamily. MoaA family.