Involved in riboflavin biosynthesis. Converts 2,5-diamino-6-(ribosylamino)-4(3H)-pyrimidinone 5'-phosphate into 5-amino-6-(ribosylamino)-2,4(1H,3H)-pyrimidinedione 5'-phosphate (By similarity). 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.