These interactions are no longer maintained in the zero-ligand model, except for the salt bridge between GLU119 (Loop A) and ARG147 (Loop E) and to a lesser extent that between LYS49 and ASP96. The GLU119-ARG147 salt bridge appears essential for stabilizing the ECD structure and has been suggested as a key determinant of agonist efficacy, with its influence being agonist-dependent; 57 its location in the protein is shown in Figure in the Supporting Information. Additional notable interactions include those between loop C and residues on the complementary subunit (residue pairs: ASN219-ASN58 and THR220-ARG135), commonly observed in bound pockets and due to the attraction between glycine and loop C.