GTP cyclohydrolase

GTP cyclohydrolases are enzymes that catalyze imidazole ring opening of guanosine triphosphate (GTP).^[1] This reaction is the committed step in the biosynthesis of multiple coenzymes (such as riboflavin and folate), tRNA bases, and the phytotoxin toxoflavin.^[1] Several GTP cyclohydrolases exist, which sometimes synthesize different products for different purposes:

• GTP cyclohydrolase I, part of the tetrahydrobiopterin, tetrahydrofolate, queuosine and other biosynthetic pathways^[1]
• GTP cyclohydrolase Ia,
• GTP cyclohydrolase Ib, part of the tetrahydrobiopterin, tetrahydrofolate, queueosine and other biosynthetic pathways^[1]
• GTP cyclohydrolase II, part of the riboflavin and toxoflavin biosynthetic pathways^[1]
• GTP cyclohydrolase IIa^[2] (or GTP cyclohydrolase III^[3]), part of the riboflavin and deazaflavin cofactor biosynthetic pathways^[1]
• GTP cyclohydrolase IV
• GTP cyclohydrolase MptA, GTP cyclohydrolase Ib paralog^[1]

These enzymes require divalent cations for catalysis.^[1]