3.

Thermoring structures of phosphorylated hCFTR/E1371Q with VX-661 bound in the activated state at 4 °C. (a) The grid-like noncovalently interacting mesh network based on the cryo-EM structure of hCFTR/E1371Q in the presence of Mg/ATP/PKA and VX-661 at 4 °C (PDB ID, 7SV7, 3.8 Å). Salt bridges, H-bonds, and π interactions are colored purple, orange, and green, respectively. The constrained grid sizes required to control the least-stable noncovalent interactions in the grids are labeled with black numbers. The least-stable D529–R555 H-bond in the biggest Grid₉ is highlighted. The total grid sizes and the total grid size-controlled noncovalent interactions along the single peptide chain of NBD1 from E384 to Q637 are shown in cyan and black circles, respectively. (b) Noncovalent interactions at the NBD1/ICL4 interface. (c) The structure of the biggest Grid₉ with a 9-residue size to control the least-stable D529–R555 H-bond. The grid size and the equivalent basic H-bonds for the least-stable noncovalent interaction are shown in and near a red circle. (d) The sequence of the biggest Grid₉ to control the least-stable D529–R555 H-bond in the blue box.