Catalyzes the conversion of (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate to cyclic pyranopterin monophosphate (cPMP). (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate = cyclic pyranopterin phosphate + diphosphate Cofactor biosynthesis; molybdopterin biosynthesis. Homohexamer; trimer of dimers. Seems to be specific for the biogenesis of DMSO reductase. Belongs to the MoaC family.