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.
An iron(II) dependent formamide hydrolase catalyzes the second step in the archaeal biosynthetic pathway to riboflavin and 7,8-didemethyl-8-hydroxy-5-deazariboflavin.