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). Testis. Belongs to the ribose-phosphate pyrophosphokinase family. According to PubMed:2168892, this sequence initiates from a non-AUG codon; N-terminal ACG codon did serve as a start codon.