2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine + H(+) + H2O = 5-amino-6-(5-phospho-D-ribosylamino)uracil + NH4(+) Binds 1 zinc ion. Cofactor biosynthesis; riboflavin biosynthesis; 5-amino-6-(D-ribitylamino)uracil from GTP: step 2/4. Belongs to the cytidine and deoxycytidylate deaminase family.