ATP + D-ribose 5-phosphate = 5-phospho-alpha-D-ribose 1-diphosphate + AMP + H(+) Belongs to the ribose-phosphate pyrophosphokinase family. This protein has lost two of the potential magnesium binding sites found in 5-phosphoribose 1-diphosphate synthases. It is also divergent from other prokaryotic member of the family. It may therefore have acquired another function.