Isoform MOCS1A and isoform MOCS1B probably form a complex that catalyzes the conversion of 5'-GTP to cyclic pyranopterin monophosphate (cPMP) (PubMed:11891227, PubMed:23627491, PubMed:29368224, PubMed:31996372). 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 (PubMed:11891227, PubMed:23627491, PubMed:29368224, PubMed:31996372). {ECO:0000269|PubMed:11891227, ECO:0000269|PubMed:23627491, ECO:0000269|PubMed:29368224, ECO:0000269|PubMed:31996372}. [Isoform 9]: Has very weak catalytic activity. . This is the function of MOCS1 (molybdenum cofactor synthesis 1, Ensembl gene identifier ENSG00000124615).