GTP cyclohydrolase (GCH) III from Methanocaldococcus jannaschi catalyzes 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. The family is found in archaea and bacteria.
Jiyao W et al.(2023). "The conserved domain database in 2023.",