sentence: The 3 contact rings consist of , 1 ) a negatively charged narrow ‘ pore ring ’ involving the Tyr111 carbonyl groups that form part of the K selectivity filter , 2 ) a negatively charged ‘ mid level ring ’ formed by Glu90 side chains contributed by all four channel subunits , and 3 ) a negatively charged ‘ upper ring ’ formed by Asp83 side chains in 2 adjacent channel ‘ turrets ’ , and negatively charged carbonyl groups from 2 neighboring residues ( Tyr80 , Pro81 ) in a third turret .