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. Each Mg(2+) binds only one residue (His-136 and Asp-175, respectively) of this protein, however the magnesium ions also bind substrates and water molecules to complete their coordination spheres (PubMed:2169413, PubMed:11790837). Can also use Mn(2+) and Cd(2+) ions, but the activity is less than that obtained with Mg(2+) ions (PubMed:2169413, PubMed:11790837). Activated by inorganic phosphate, and to a lesser extent by sulfate ions (PubMed:2169413). In addition to form a complex with ATP, Mg(2+) also acts as a cofactor (PubMed:2169413). Strongly inhibited by ADP through competitive binding at the activation site and at a specific allosteric site (PubMed:2169413, PubMed:16008562). Less strongly inhibited by alpha,beta-methylene ATP (mADP), AMP, GDP, GMP and UTP (PubMed:2169413, PubMed:16008562). Optimum pH is 8-8.5. Activities at pH 7 and pH 9.5 are 35% and 85% of the maximal activity, respectively. 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; trimer of dimers. 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.