If you look at the structure of AMP you’ll see that the phosphate group (Po4-) has two Hydrogens attached to two oxygens.
These two hydrogens are acidic because the Oxygens are significantly more electronegative than hydrogens. If the differences in electronegativity is greater than 0.5 (which it is) this is a polar bond. This means that the electrons in the hydrogen-oxygen bond are spending more time near oxygen. Making the hydrogens easier to “leave”
