Function 
Cyclohydrolase is an enzyme which catalyzes the reversible cyclization or the ring opening of a purine ring. 
•	GTP cyclohydrolase I catalyzes the conversion of GTP to 7,8-dihydronepterin triphosphate.  GCH I is part of the folate and biopterin biosynthesis[1] 
•	GTP cyclohydrolase II catalyzes the conversion of GTP to formate, diphosphate and diamino-hydroxy-phosphoribosylamino pyrimidine. GCH II is part of riboflavin metabolism.[2]   
•	IMP cyclohydrolase catalyzes the conversion of IMP to formido-phosphosibosyl imidazole carboxamide.  ICH is part of the purine biosynthesis[3] See also Bifunctional purine biosynthesis protein PURH.
•	Methenyltetrahydromethanopterin cyclohydrolase catalyzes the conversion of methenyltetrahydromethanopterin to formyl-tetrahydromethanopterin.  MCH is part of the folate biosynthesis[4] 
- Methylenetetrahydrofolate dehydrogenase/cyclohydrolase catalyzes the interconversion of 5,10-methylenetetrahydrofolate and 10-formyltetrahydrofolate in mitochondria of mammalian cells.[5] 
- Phosphoribosyl AMP-cyclohydrolase catalyzes the third step of His biosynthesis.[6] 
  Disease 
Mutations in GTP cyclohydrolase I are associated with malignant phenylketonuria, hyperphenylalaninemia and DOPA-responsive dystonia.
  Structural highlights 
 from Escherichia coli (1n3r) is pentamer.  at the intersubunit interface.[7] .
 3D structures of cyclohydrolase
Cyclohydrolase 3D structures