The stabilizing H-bond is present in the phosphorylated ATP-bound (OF) hCFTR structure but is absent in the phosphorylated ATP-bound (OF) zCFTR structure. (A) Cartoon topology of CFTR ion channel and location of hR117/zR118 and hE1124/zD1132 residues (blue/pale blue and brown dot). TMDs, gray; NBD1, blue; NBD2, green; R domain, pale magenta; membrane, yellow. Catalytic site mutation in the NBD2 Walker B motif is represented by a pale green star. (B and C) Ribbon diagrams of the OF-hCFTR (PDB accession no. 6MSM; B) and OF-zCFTR (PDB accession no. 5W81; C) cryo-EM structures, highlighting residues hR117/zR118 (blue/pale blue) and hE1124/zD1132 (brown) in space fill representation. TMDs and NBDs are color coded as in A; R domain is not depicted. (D and E) Close-up views of the extracellular region are highlighted in B and C by dotted purple boxes. Residues hR117/zR118 and hE1124/zD1132 are shown as sticks, and the H-bond in OF-hCFTR is illustrated by red dotted lines with bond distance marked, TM helices (TM1, TM2, TM11, TM12) are assigned.