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.