Catalyzes a salvage reaction resulting in the formation of AMP, that is energically less costly than de novo synthesis. AMP + diphosphate = 5-phospho-alpha-D-ribose 1-diphosphate + adenine Purine metabolism; AMP biosynthesis via salvage pathway; AMP from adenine: step 1/1. Homodimer. Belongs to the purine/pyrimidine phosphoribosyltransferase family. Extended N-terminus.