The hydrogen bond network in the backyard of the SF. (A–D) Upper: The horizontal view from the center of the ion pathway of the protein structures around Met181 and adjacent subunit’s Glu177 and Gln172 of the SF of NavAb N49K, S178T^NK, S178A^NK, and S178G^NK mutants, respectively. Background (BG) indicates N49K mutant without the Ser178 mutation. These figures correspond to the enlarged view of the subunit at the back center in Fig. 6. Lower: The vertical view from the extracellular side of the protein structures around Met181, indicating Arg185 interaction was lost in S178G^NK.The residues of the left subunit are numbered with a prime symbol.