## 1 Introduction
These bioinformatics and structural details are particularly important especially if we consider their impact in guaranteeing an accurate structural description of extra-cellular domains, for which it has already been described as having a key role in defining both the ligand orthosteric binding site and the ligand meta-binding sites [12; 13; 14]. Very recently, crystallographic structures of the hA\({}_{1}\) AR have been solved, representing both its active [15] and inactive conformation [16]. These structures were co-crystallized with the endogenous agonist adenosine (PDB code: 6D9H), as with the antagonist 4-([3-(8-cyclohexyl-2,6-dioxo-1-propyl-1,2,6,7-tetrahydro-3H-purin-3-yl)propyl]carbamoyl]benzene-1-sulfonyl fluoride (PDB code: 5 UEN), which actually constitutes the most representative example of an interaction scheme for the system of interest. As expected, several remarkable differences to the previously solved hA\({}_{2A}\) AR structures have been underlined, in particular, the hA\({}_{1}\) AR revealed a peculiar conformation of the second extracellular loop and a wider ligand binding site (Figure 1). Intriguingly, from a phylogenetic and functional point of view, the hA\({}_{3}\) AR is much closer to the hA\({}_{1}\) AR than to the hA\({}_{2A}\) AR. Indeed, hA\({}_{3}\) AR shows a higher sequence identity with the hA\({}_{1}\) AR (54%) than the hA\({}_{2A}\) AR (49%), and both A\({}_{3}\) AR and A\({}_{1}\) AR, coupled to G\({}_{\text{i/o}}\) proteins share, from a functional point of view, the crucial biochemical pathway based on the inactivation of the adenyl cyclase activity. Structurally, another important element of similarity between the hA\({}_{3}\) AR and the hA\({}_{1}\) AR is related to both receptors having only a single cysteine residue on the second extracellular loop (EL2) which establishes a disulfide bridge, largely conserved among the Rhodopsin-family of GPCRs, along with a second cysteine located on the transmembrane helix 3 (TM3) [15; 16]. Moreover, the lack of the additional disulfide bridges at the EL2 level, as present in the A\({}_{2A}\) AR subtype, involves a significant reorganization of the transmembrane helices 1, 2, 3 and 7, leading to a different shape of the orthosteric binding cavity. An additional disulfide, C260-C263, present in both A\({}_{1}\) AR and A\({}_{2A}\) AR, staples the conformation of the EL3. Taking all the above arguments into consideration, from a computational point of view the question arises spontaneously: \"could the crystallographic structure of the human A\({}_{1}\) AR be a better template than the human A\({}_{2A}\) AR for the construction of a homology model of the A\({}_{3}\) subtype?\", the question which provided the scientific motivation in taking up the present research work. To appreciate the qualitative and the quantitative differences obtained by using the two alternative templates, we decided to retrospectively compare the posing and scoring performance of molecular docking for a selection of well-known potent selective agonists and antagonists, summarized in Figures 2 and 3. Figure 1: Structural superposition of hA\({}_{1}\) (dark blue) and hA\({}_{2a}\) (gold) ARs crystal structures in their active state (PDB code: 6D9H and 2 YDO, respectively). Panel (**A**) and (**B**) representations differ only for camera orientation. Additional A\({}_{2A}\) disulfide bridges are highlighted in red. Figure 2: Chemical structures of agonists selected for docking studies. Experimental Ki values (in nM or activity assay percentages) for each receptor subtype are also reported for comparison.