Involved in the biosynthesis of the central metabolite phospho-alpha-D-ribosyl-1-pyrophosphate (PRPP) via the transfer of pyrophosphoryl group from ATP to 1-hydroxyl of ribose-5-phosphate (Rib-5-P). ATP + D-ribose 5-phosphate = 5-phospho-alpha-D-ribose 1-diphosphate + AMP + H(+) Binds 2 Mg(2+) ions per subunit (Potential). Can also use Mn(2+) ions (PubMed:4306285). Activated by inorganic phosphate (PubMed:4306285, PubMed:4324215). In addition to form a complex with ATP, Mg(2+) also acts as a cofactor (PubMed:4306285, PubMed:4324215). Strongly inhibited by ADP (or Mg-ADP) through competitive binding at the activation site and at a specific allosteric site (PubMed:4324215, PubMed:4346344, PubMed:6277896). Competitively inhibited by Ca(2+) (PubMed:4324215, PubMed:4346344). Optimum pH is 8.1-8.6. 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. Homohexamer. This enzyme uses a steady state ordered mechanism, where Mg(2+) binds first, followed by Mg-ATP and lastly, ribose 5-phosphate. Belongs to the ribose-phosphate pyrophosphokinase family. Class I subfamily.