In enzymology, an ATP phosphoribosyltransferase (EC 2.4.2.17) is an enzyme that catalyzes the chemical reaction 1-(5-phospho-D-ribosyl)-ATP + diphosphate ATP + 5-phospho-alpha-D-ribose 1-diphosphate Thus, the two substrates of this enzyme are and diphosphate, whereas its two products are ATP and .