Three-binding sites hypothesis

Fig. 15.4-4 Three ligand binding sites model for monoamine-related CPCRs illustrated by a rhodopsin-based 3D model of the 5-HTia receptor (left extracellular view right side view). We recently proposed a three binding site hypothesis for the molecular recognition of ligands at monoamine CPCRs by combining ...

M. Spedding, A three binding site hypothesis for the interaction of ligands with monoamine G-protein coupled receptors implications for combinatorial ligand design, Quant. Struct.-Act. Relat. 1999, 18, 561-572. 

It was originally hypothesized that one role of the Zn center is to help form and stabilize the pterin binding site (81). As shown in Fig. 4, the conserved Serl04, which is only three residues from one Zn ligand, directly H-bonds to the pterin. A disruption of the Zn site should, therefore, perturb interactions at the pterin site. A direct comparison between the Zn-bound and disulfide forms supports this hypothesis. In order to form the S—S bond, the entire section of polypeptide involving Serl04 must slide down and away from the pterin site, which... 

Figure 19. A model for the anion- and iron-binding sites of transferrin depicted assuming an interlocking-site hypothesis. The protein furnishes five ligands to the metal in the iron binding site three tyrosines and two histidines. The carbonate ion binds to an arginine in the anion-binding site and functions as a sixth ligand to the metal center. The carbonate forms a bridge between the metal- and the anion-binding sites in the active center (36).

Based on these resiilts, we have recently tested the hypothesis that GPCRs are assembled from many, rather than only two, autonomous folding units. To address this question, the rat m3 mAChR was split in all three intracellular (il-i3) and all three extracellular loops (o2-o4), thus generating six polypeptide pairs (Nil+Cil, No2+Co2, etc. Fig. 2 ref. 20). As shown in Fig. 3, coexpression of three of the six polypeptide pairs (Ni2+Ci2, No3+Co3, and Ni3-t-Ci3, respectively) led to the appearance of a significant number of specific high-affinity [3H]NMS binding sites. These data clearly indicated that covalent connections between TM III and IV, TM IV and V, and TM V and VI, respectively, are not essential... 

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