Catalyzes the hydrolysis of the adenine ring of phosphoribosyl-AMP. 1-(5-phospho-beta-D-ribosyl)-5'-AMP + H2O = 1-(5-phospho-beta-D-ribosyl)-5-[(5-phospho-beta-D-ribosylamino)methylideneamino]imidazole-4-carboxamide Binds 1 Mg(2+) ion per subunit. Binds 1 zinc ion per subunit. kcat is 8 sec(-1). Amino-acid biosynthesis; L-histidine biosynthesis; L-histidine from 5-phospho-alpha-D-ribose 1-diphosphate: step 3/9. Homodimer. Belongs to the PRA-CH family.