GTP cyclohydrolase (GCH) III from Methanocaldococcus jannaschi catalyses the conversion of GTP to 2-amino-5-formylamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate (FAPy). The reaction requires two bound magnesium ions for the catalysis and is activated by monovalent cations such as potassium and ammonium. The enzyme is a tetramer of identical subunits; each monomer is composed of an N- and a C-terminal domain that adopt nearly superimposible structures, suggesting that the protein has arisen by gene duplication [1]. The family is found in archaea and bacteria.

1: A new use for a familiar fold: the X-ray crystal structure of GTP-bound GTP cyclohydrolase III from Methanocaldococcus jannaschii reveals a two metal ion catalytic mechanism. Morrison SD, Roberts SA, Zegeer AM, Montfort WR, Bandarian V; Biochemistry. 2008;47:230-242. PMID:18052207