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: