Catalyzes the reversible phosphorolytic breakdown of the N-glycosidic bond in the beta-(deoxy)ribonucleoside molecules, with the formation of the corresponding free purine bases and pentose-1-phosphate. Cleavage of adenosine and its derivatives. a purine D-ribonucleoside + phosphate = a purine nucleobase + alpha-D-ribose 1-phosphate a purine 2'-deoxy-D-ribonucleoside + phosphate = 2-deoxy-alpha-D-ribose 1-phosphate + a purine nucleobase Homohexamer; trimer of homodimers. Belongs to the PNP/UDP phosphorylase family.