Isoform Mocs1a and isoform Mocs1b probably form a complex that catalyzes the conversion of 5'-GTP to cyclic pyranopterin monophosphate (cPMP). Mocs1a 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. 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 Mocs1a and isoform Mocs1b probably form a heterooligomer. Isoform Mocs1a seems to be translated from a bicistronic transcript while isoform Mocs1b seems to be translated from a monocistronic mRNA that is derived by alternative splicing. In the C-terminal section; belongs to the MoaC family. In the N-terminal section; belongs to the radical SAM superfamily. MoaA family. The C-terminus of Mocs1a was previously believed to be thiocarboxylated, but it is now known not to be the case. Wrong choice of frame.