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. kcat is 0.092 min(-1) for (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate. Cofactor biosynthesis; molybdopterin biosynthesis. Isoform MOCS1A and isoform MOCS1B probably form a heterooligomer. So far, the different types of MOCS1A and MOCS1B isoforms have been investigated independently and several combinations might be possible. Isoform MOCS1A and isoform 2 are widely expressed. The disease is caused by variants affecting the gene represented in this entry. The MOCS1 locus has initially been reported to produce MOCS1A and MOCS1B from non-overlapping reading frames within a bicistronic transcript. However, only isoform MOCS1A seems to be translated from the bicistronic transcript. Isoform MOCS1B seems to be translated from a monocistronic mRNA that is derived by alternative splicing. Multidomain protein with inactive MOCS1A and active MOCS1B. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Multidomain protein with inactive MOCS1A and active MOCS1B. 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. Alternative splicing in the MOCS1A-MOCS1B joining region. Alternative splicing in the MOCS1A-MOCS1B joining region.