GTP cyclohydrolase II (RibA). GTP cyclohydrolase II catalyzes the conversion of GTP to 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5' phosphate, formate, pyrophosphate (APy), and GMP in the biosynthetic pathway of riboflavin. Riboflavin is the precursor molecule for the synthesis of the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) which are essential to cell metabolism. The enzyme is present in plants and numerous pathogenic bacteria, especially gram negative organisms, who are dependent on endogenous synthesis of the vitamin because they lack an appropriate uptake system. For animals and humans, which lack this biosynthetic pathway, riboflavin is the essential vitamin B2. GTP cyclohydrolase II requires magnesium ions for activity and has a bound catalytic zinc. The functionally active form is thought to be a homodimer. A paralogous protein is encoded in the genome of Streptomyces coelicolor, which converts GTP to 2-amino-5-formylamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate (FAPy), an activity that has otherwise been reported for unrelated GTP cyclohydrolases III.
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