Glutamic acid has lower isoelectric point while glutamine `H_(2)N-overset(overs

Glutamic acid has lower isoelectric point while glutamine `H_(2)N-overset(overset(O)(||))(C)-CH_(2)-CH_(2)- underset(underset(NH_(2))(|)) overset(overset(H)(|))(C)-COOH` has higher isoelectric `pH`. Explain ?