Conserved Water-mediated Hydrogen Bond Network between TM-I, -II, -VI, and -VII in 7TM Receptor Activation

Abstract

Five highly conserved polar residues connected by a number of structural water molecules together with two rotamer microswitches, TrpVI:13 and TyrVII:20, constitute an extended hydrogen bond network between the intracellular segments of TM-I, -II, -VI, and -VII of 7TM receptors. Molecular dynamics simulations showed that, although the fewer water molecules in rhodopsin were relatively movable, the hydrogen bond network of the ␤2-adrenergic receptor was fully loaded with water molecules that were surprisingly immobilized between the two rotamer switches, both apparently being in their closed conformation. Manipulations of the rotamer state of TyrVII:20 and TrpVI:13 demonstrated that these residues served as gates for the water molecules at the intracellular and extracellular ends of the hydrogen bond network, respectively. TrpVI:13 at the bottom of the main ligand-binding pocket was shown to apparently function as a catching trap for water molecules. Mutational analysis of the ␤2-adrenergic receptor demonstrated that the highly conserved polar residues of the hydrogen bond network were all important for receptor signaling but served different functions, some dampening constitutive activity (AsnI:18, AspII:10, and AsnVII:13), whereas others (AsnVII:12 and Asn-VII:16) located one helical turn apart and sharing a water molecule were shown to be essential for agonist-induced signaling. It is concluded that the conserved water hydrogen bond network of 7TM receptors constitutes an extended allosteric interface between the transmembrane segments being of crucial importance for receptor signaling and that part of the function of the rotamer micro-switches, TyrVII:20 and TrpVI:13, is to gate or trap the water molecules.In recent years several structures of 7TM 2 receptors have been published, making it possible to compare conserved regions in different receptors and study their putative functional importance (1-7). The most extended of these highly conserved regions is the water hydrogen bond network located between the intracellular segments of TM-I, TM-II, TM-VI, and TM-VII reaching from TrpVI:13 of the CWXP motif at the bottom of the main ligand-binding pocket to TyrVII:20 of the NPXXY motif on the intracellular side of the receptor (Fig. 1). A number of conserved polar residues in these transmembrane segments have for many years been suspected to form a hydrogen bond network of likely functional importance. The recent x-ray structures have confirmed this notion but have also revealed that very few hydrogen bonds are in fact found directly between the side chains of these polar residues. Instead, a number of structural water molecules function as integral connecting parts of the hydrogen bond network (Fig. 1). Thus, in the B2AR structure (2, 3) six internal water molecules form hydrogen bonds with the highly conserved polar amino acid residues located between TrpVI:13 and TyrVII:20, whereas in the rhodopsin structures (8 -10) only four water molecules have been identified in this network (Fig. 1). 3Several studies have...

Structure-Activity Analysis of Biased Agonism at the Human Adenosine A3 Receptor

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Critical Hydrogen Bond Formation for Activation of the Angiotensin II Type 1 Receptor

“…However, the role played by this highly conserved N 1.50 in the activation mechanism of the AT 1 receptor appears to be different from its role in the activation mechanism of the ␤ 2 -adrenergic receptor. Indeed, the loss of the polar side chain decreased the constitutive and the agonist-induced activity of the AT 1 receptor, whereas it did the opposite to the ␤ 2 -adrenergic receptor (24). Furthermore, the MD simulations suggested direct side chainside chain interactions between residues D74 2.50 -N295 7.46 and D74 2.50 -N46 1.50 in the AT 1 receptor, whereas the corresponding interactions are water-mediated in the ␤ 2 -adrenergic receptor (24).…”

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