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. Death between days 1 and 11 after birth, due to a progressive neurological disorder caused by massive cell death. Death is caused by the absence of molybdenum cofactor, resulting in elevated sulfite and diminished sulfate levels throughout the organism. Mice do not possess any sulfite oxidase or xanthine dehydrogenase activity. No organ abnormalities are observed and the synaptic localization of inhibitory receptors appears normal. Long-term rescue results have been obtained in mice lacking Mocs1 thanks to an adeno-associated virus-mediated gene transfer. 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.