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

The early steps in the biosynthesis of 7,8-didemethyl-8-hydroxy-5-deazariboflavin (Fo) and riboflavin in the archaea differ from the established eukaryotic and bacterial pathways. The archaeal pathway has been proposed to begin with an archaeal-specific GTP cyclohydrolase III that hydrolyzes the imidazole ring of GTP but does not remove the resulting formyl group from the formamide [Graham, D. E., Xu, H., and White, R. H. (2002) Biochemistry 41, 15074-15084 ]. This enzyme is different than the bacterial GTP cyclohydrolase II which catalyzes both reactions. Here we describe the identification and characterization of the formamide hydrolase that catalyzes the second step in the archaeal Fo and riboflavin biosynthetic pathway. The Methanocaldococcus jannaschii MJ0116 gene was cloned and heterologously expressed, and the resulting enzyme was shown to catalyze the formation of 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate (APy) and formate from 2-amino-5-formylamino-6-ribosylamino-4(3H)-pyrimidinone 5'-monophosphate (FAPy). The MJ0116-derived protein has been named ArfB to indicate that it catalyzes the second step in archaeal riboflavin and Fo biosynthesis. ArfB was found to require ferrous iron for activity although metal analysis by ICP indicated the presence of zinc as well as iron in the purified protein. The identification of this enzyme confirms the involvement of GTP cyclohydrolase III (ArfA) in archaeal riboflavin and Fo biosynthesis.