Cyclic pyranopterin monophosphate synthase (, MOCS1A, MoaA, MoaC, molybdenum cofactor biosynthesis protein 1) is an enzyme with systematic name GTP 8,9-lyase (cyclic pyranopterin monophosphate-forming). This enzyme catalyses the following chemical reaction
This enzyme catalyses an early step in the biosynthesis of molybdopterin.