sentence: When TPN is stably bound at nanomolar affinity to the outer vestibule , the TPN C-terminal Lys21 residue then prevents channel K conductance through a direct interaction with the lower pore ring Tyr111 carbonyl groups that constitute the exposed part of the K selectivity filter.3.6 Site-directed mutagenesis in silico recapitulates in vitro rKir1.1 channel block kinetics .