The enzyme catalyses the second step in archaeal riboflavin and 7,8-didemethyl-8-hydroxy-5-deazariboflavin biosynthesis. The first step is catalysed by EC?3.5.4.29?(GTP cyclohydrolase IIa). The bacterial enzyme, EC?3.5.4.25?(GTP cyclohydrolase II) catalyses both reactions.