Catalyzes the synthesis of phosphoribosylpyrophosphate (PRPP) that is essential for nucleotide synthesis. ATP + D-ribose 5-phosphate = 5-phospho-alpha-D-ribose 1-diphosphate + AMP + H(+) Activated by magnesium and inorganic phosphate. Metabolic intermediate biosynthesis; 5-phospho-alpha-D-ribose 1-diphosphate biosynthesis; 5-phospho-alpha-D-ribose 1-diphosphate from D-ribose 5-phosphate (route I): step 1/1. Homodimer. The active form is probably a hexamer composed of 3 homodimers (By similarity). Expressed in both vestibular and cochlea hair cells in early developing and postnatal mice and can also be detected in the spiral ganglion cells in at post natal day 6. Belongs to the ribose-phosphate pyrophosphokinase family.