Substitution of Arg121 with lysine in zfGPCR and the absence of loop 3 should not affect the hydrogen bonding and the salt bridge network found in the crystal structures of the NECD of hGLP-1R in complex with exendin(9–39) or GLP-1 The hydrogen bond interaction between Arg121 in hGLP-1R with the main chain oxygen of position 27 in hGLP-1 (3.0Å) and exendin(9–39) (2.8Å, not shown) can be maintained by the conserved substitution of arginine with lysine in zfGPCR (Fig 3).

<p>Light blue ribbon diagram represents the structure of the hGLP-1R NECD bound to GLP-1 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0167718#pone.0167718.ref036" target="_blank">36</a>] (PDB entry 3IOL) and green the predicted structure of zfGPCR. Lys121 in zfGPCR was modeled from the structure of the hGLP-1R NECD in complex with hGLP-1 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0167718#pone.0167718.ref036" target="_blank">36</a>] (PDB entry 3IOL) by removing the missing residues and closing the gap with PyMOL’s sculpting module.