Does the ternary complex act as a secondary proton pump and a GTP synthase?

It has been suggested that G protein-coupled receptors can act as proton transporters, with the activated G protein-coupled receptor transporting H+ across the membrane from the extracellular side to the cytoplasm. In this article, Paul Nederkoorn, Henk Timmerman and Gabriëlle Donné-Op den Kelder summarize the various H+ translocation mechanisms and how these compare with activated G protein-coupled receptors. The G protein, being part of the ternary complex, is proposed to use translocated protons to synthesize GTP from GDP and Pi, thus functioning in a similar manner to ATP synthase. The importance of these events in physiological effects such as signal amplification is discussed.

Development of a three-dimensional CysLT1 (LTD4) antagonist model with an incorporated amino acid residue from the receptor.

This paper describes the molecular modeling of leukotriene CysLT1 (or LTD4) receptor antagonists. Several different structural classes of CysLT1 antagonists were superimposed onto the new and highly rigid CysLT1 antagonist 8-carboxy-3'-[2-(2-quinolinyl)ethenyl]flavone (1, VUF 5017) to generate a common pharmacophoric arrangement. On the basis of known structure-activity relationships of CysLT1 antagonists, the quinoline nitrogen (or a bioisosteric equivalent thereof) and an acidic function were taken as the matching points. In order to optimize the fitting of acidic moieties of all antagonists, an arginine residue from the receptor was proposed as the interaction site for the acidic moieties. Incorporation of this amino acid residue into the model revealed additional interactions between the guanidine group and the nitrogen atoms of quinoline-containing CysLT1 antagonists. In some cases, the arginine may even interact with pi-clouds of phenyl residues of CysLT1 antagonists. The alignment of Montelukast (MK-476) suggests the presence of an additional pocket in the binding site for CysLT1 antagonists. The derived model should be useful for a better understanding of the molecular recognition of the leukotriene CysLT1 receptor.

Characterization of the binding site of the histamine H(3) receptor. 2. Synthesis, in vitro pharmacology, and QSAR of a series of monosubstituted benzyl analogues of thioperamide.

A series of monosubstituted benzyl analogues of the histamine H(3) receptor antagonist thioperamide were synthesized and evaluated for their histamine H(3) receptor activity on the guinea pig jejunum. Incorporation of Cl, Br, and I at the ortho position of the benzyl moiety led to an increase of the pA(2) value, whereas the same substituents at the para position led to a decrease. However, a fluorine substituent gave a strong decrease in pA(2), regardless of the position. Molecular modeling revealed a QSAR with a correlation (r = 0.93) between the pA(2) and the dihedral angle between the thiourea and the benzyl moiety and the calculated electron density on the substituted carbon atom. To verify whether this QSAR model had a predictive value, the ortho tert-butyl and methyl analogues were synthesized and evaluated. Indeed it was shown that the predicted pA(2) values of these two compounds were in accordance with the measured pA(2) values.

Development of a pharmacophore model for histamine H3 receptor antagonists, using the newly developed molecular modeling program SLATE.

New molecular modeling tools were developed to construct a qualitative pharmacophore model for histamine H3 receptor antagonists. The program SLATE superposes ligands assuming optimum hydrogen bond geometry. One or two ligands are allowed to flex in the procedure, thereby enabling the determination of the bioactive conformation of flexible H3 antagonists. In the derived model, four hydrogen-bonding site points and two hydrophobic pockets available for binding antagonists are revealed. The model results in a better understanding of the structure-activity relationships of H3 antagonists. To validate the model, a series of new antagonists was synthesized. The compounds were designed to interact with all four hydrogen-bonding site points and the two hydrophobic pockets simultaneously. These ligands have high H3 receptor affinity, thereby illustrating how the model can be used in the design of new classes of H3 antagonists.

A qualitative model for the histamine H3 receptor explaining agonistic and antagonistic activity simultaneously.

A pharmacophore model for histamine H3 ligands is derived that reveals the putative interaction of both H3 agonists and antagonists with an aspartate residue of the receptor. This interaction is determined by applying the density functional theory implemented in a program package adapted for parallel computers. The model reveals a molecular determinant explaining efficacy as the conformation of the aspartic acid residue differs according to whether it is binding to agonists or antagonists. The differences in structure-activity relationships (SAR) observed for the lipophilic tails of different classes of H3 antagonists are now explained, since the model reveals two distinct lipophilic pockets available for antagonist binding.

Trimeric G alpha beta gamma protein coupling to a 3D model of G protein-coupled receptors confirming a new molecular mechanism for activation.

The present-day model for G protein activation and the associated theory on how a G protein-coupled receptor may activate the G protein are summarized. Experimental data are outlined which seem not to be in accordance with this present-day model. An alternative molecular mechanism for ternary complex activation is presented together with a three-dimensional model for a receptor coupled to the appropriate trimeric G protein. This 3D structure confirms our new molecular mechanism of activation.

The ligand-receptor-G-protein ternary complex as a GTP-synthase. steady-state proton pumping and dose-response relationships for beta -adrenoceptors.

Steady-state solutions are developed for the rate of G alpha.GTP production in a synthase model of the ligand-receptor-G-protein ternary complex activated by a ligand-receptor proton pumping mechanism. The effective rate, k(31), defining the proton transfer, phosphorylation and G alpha.GTP release is a controlling rate of the synthase in the presence of a ligand with an efficient mode of signal activation, the ligand-receptor interaction taking place under effectively equilibrium conditions. The composite rate, however, becomes an amplifying factor in any dose-response relationship. The amplification is a triple product of the rate, k(31), the equilibrium constant associated with the activation of the proton signal, K(act)and the fraction of agonist conformer transmitting the signal, f(*). Where the rate of activation of the proton signal becomes critically inefficient, the rate of activation, k(act 1)replaces k(31)K(act). A correlation between beta(1)-adrenergic receptor-stimulated GDP release and adenylate cyclase activation shows that this correlation is not unique to an exchange reaction. Within the initiating Tyr-Arg-Tyr receptor proton shuttle mechanism, the position of Arg(r156) paralleldictates the high-(R(p)) and low-(R(u)) ligand-binding affinities. These states are close to R(*)and R(0)of the equilibrium model (De Lean et al., 1980, J. Biol. Chem.255, 7108-7117). An increased rate of hydrogen ion diffusion into a receptor mutant can give rise to constitutive activity while increased rates of G-protein release and changes in receptor state balance can contribute to the resultant level of action. Constitutive action will arise from a faster rate of G-protein release alone if proton diffusion in the wild-type receptor contributes to a basal level of G-protein activation. Competitive ligand-receptor occupancy for constitutive mutants shows that, where the rate of G-protein activation from the proportion of ligand-occupied receptors is less than the equivalent rate that would be generated from this fraction by proton diffusion, inverse agonism will occur. Rate-dependent dose-responses developed for the proposed synthase mechanism give explicit definition to the operational model for partial agonism (Black & Leff, 1983, Proc. Roy. Soc. Lond. B220, 141-162). When comparable ligands have effectively identical conformational states at the transition state for signal activation, the antagonist component of the binding "in vitro" can be derived by multiplying the apparent binding constant by (1-e) where e is the maximum stimulatory response. This component should be consistent throughout the tissues.

The agonistic binding site at the histamine H2 receptor. II. Theoretical investigations of histamine binding to receptor models of the seven alpha-helical transmembrane domain.

In the first part (pp. 461-478 in this issue) of this study regarding the histamine H2 receptor agonistic binding site, the best possible interactions of histamine with an alpha-helical oligopeptide, mimicking a part of the fifth transmembrane alpha-helical domain (TM5) of the histamine H2 receptor, were considered. It was established that histamine can only bind via two H-bonds with a pure alpha-helical TM5, when the binding site consists of Tyr182/Asp186 and not of the Asp186/Thr190 couple. In this second part, two particular three-dimensional models of G-protein-coupled receptors previously reported in the literature are compared in relation to agonist binding at the histamine H2 receptor. The differences between these two receptor models are discussed in relation to the general benefits and limitations of such receptor models. Also the pros and cons of simplifying receptor models to a relatively easy-to-deal-with oligopeptide for mimicking agonistic binding to an agonistic binding site are addressed. Within complete receptor models, the simultaneous interaction of histamine with both TM3 and TM5 can be analysed. The earlier suggested three-point interaction of histamine with the histamine H2 receptor can be explored. Our results demonstrate that a three-point interaction cannot be established for the Asp98/ Asp186/Thr190 binding site in either of the investigated receptor models, whereas histamine can form three H-bonds in case the agonistic binding site is constituted by the Asp98/Tyr182/Asp186 triplet. Furthermore, this latter triplet is seen to be able to accommodate a series of substituted histamine analogues with known histamine H2 agonistic activity as well.

A new model for the agonistic binding site on the histamine H2-receptor: the catalytic triad in serine proteases as a model for the binding site of histamine H2-receptor agonists.

The historical model for the agonistic binding site on the histamine H2-receptor is based on a postulated activation mechanism: it has been suggested that the histamine monocation binds to the histamine H2-receptor via the formation of three hydrogen bonds. The cationic ammonium group in the side chain and the -NH- group in the tau-position of the imidazole act as proton donors, whereas the =N- atom in the pi-position of the imidazole acts as a proton acceptor. Participation of the ammonium group in H-bonding with a presumed negative charge on the receptor leads to a decrease in positive charge, which is thought to induce a tautomeric change in the imidazole ring system from N tau-H to N pi-H. A consequence of this tautomeric shift is the donation of a proton from the receptor to the agonist on one side, while on the other side a proton is donated from the agonist to the receptor. The propose tautomeric shift has been suggested to trigger the H2-stimulating effect. However, this model for the constitution of the agonistic binding site and the accessory activation mechanism cannot explain the weak histamine H2-activity of beta-histine and the activity of several other recently synthesized H2-agonists. Based on a thorough literature study and with the aid of molecular electrostatic potentials (MEPs) we demonstrate that the sulphur atom present in histamine H2-agonists as dimaprit and 2-amino-5-(2-aminoethyl)thiazole does not function as a proton acceptor, which implicitly means that a tautomeric shift is not a prerequisite for H2-stimulation. As a consequence, the model for the agonistic binding site is adjusted, resulting in a strong resemblance to the nature and orientation of the amino acids constituting the catalytic triad in serine proteases. Within this concept, the N pi-H tautomer of histamine is the biologically active form, in contrast with the existing model in which the N tau-H tautomer is the active form.

The agonistic binding site at the histamine H2 receptor. I. Theoretical investigations of histamine binding to an oligopeptide mimicking a part of the fifth transmembrane alpha-helix.

Mutation studies on the histamine H2 receptor were reported by Gantz et al. [J. Biol. Chem., 267 (1992) 20840], which indicate that both the mutation of the fifth transmembrane Asp186 (to Ala186) alone or in combination with Thr190 (to Ala190) maintained, albeit partially, the cAMP response to histamine. Recently, we have shown that histamine binds to the histamine H2 receptor as a monocation in its proximal tautomeric form, and, moreover, we suggested that a proton is donated from the receptor towards the tele-position of the agonist, thereby triggering the biological effect [Nederkoorn et al., J. Mol. Graph., 12 (1994) 242; Eriks et al., Mol. Pharmacol., 44 (1993) 886]. These findings result in a close resemblance with the catalytic triad (consisting of Ser, His and Asp) found in serine proteases. Thr190 resembles a triad's serine residue closely, and could also act as a proton donor. However, the mutation of Thr190 to Ala190-the latter is unable to function as a proton donor-does not completely abolish the agonistic cAMP response. At the fifth transmembrane alpha-helix of the histamine H2 receptor near the extracellular surface, another amino acid is present, i.e. Tyr182, which could act as a proton donor. Furthermore, Tyr182 lies within the proximity of Asp186, so an alternative couple of amino acids, Tyr182 and Asp186, could constitute the histamine binding site at the fifth alpha-helix instead of the (mutated) couple Asp186 and Thr190. In the first part of our present study, this hypothesis is investigated with the aid of an oligopeptide with an alpha-helical backbone, which represents a part of the fifth transmembrane helix. Both molecular mechanics and ab initio data lead to the conclusion that the Tyr182/Asp186 couple is most likely to act as the binding site for the imidazole ring present in histamine.

GTP synthases. Proton pumping and phosphorylation in ligand-receptor-G alpha-protein complexes.

A structural model for a ligand-receptor-Gs alpha-protein complex to function as a GTP synthase is presented. The mechanism which is dependent on the movement and rotation of the G alpha-protein alpha 2-helix is seen to involve the delivery of, at least, one proton to the phosphorylation site in the rotation of this helix. The cycle is driven by a ligand-mediated proton pump through the alpha-helices of the receptor, attachment of the conserved Tyr-Arg-Tyr receptor proton shuttle being made to an aspartate group on the Gs alpha-protein terminal sidechain, which is itself linked to the Asn-Gln interaction known to control movement and rotation of the alpha 2-helix between .GDP and .GTP structures. The energetics of proton transfer through the shuttle mechanism and delivery of a proton to the aspartate group are shown to be sufficient to rupture this controlling interaction and its associated backbone bond. The complex leads to full spatial and energetic definition of the receptor proton shuttle mechanism, while there is a striking association of further Tyrosine and Arginine residues in the vicinity of the Gs alpha-protein Asn-Gln interaction. Calculations at the HF 6-31G** level confirm that a critical balance between ion pair and neutral forms of Tyr-Arg interactions under multiply hydrogen bonded conditions in a hydrophobic environment controls proton transfer and recovery mechanisms. The intrinsic preference of the neutral Tyr-Arg form over the ion-pair is 14.0 kcal/mol. Activation of the Tyrosine oxygen atom in the neutral form by single-NH or -OH groups reduces this difference by some 6.4-8.6 kcal/mol but the dominance of the neutral form is maintained. The expected slight overestimates are consistent with the maximum activation enthalpy of 11.0-12.0 kcal/ mol required to initiate proton transfer through the shuttle. The extended form of the shuttle with the Arginine acting competitively between the two Tyrosine residues allows interpretation of observed enthalpic differences in ligand binding with and without the presence of GTP. The uniqueness of Gs proteins among the G alpha-proteins is seen as their inability to transfer a proton directly through the alpha 2-helix switch Asn-Gln residues. A possible proton pathway to the mid-point of the Gs alpha-protein alpha 2 helix is outlined.

Modelling and mutation studies on the histamine H1-receptor agonist binding site reveal different binding modes for H1-agonists: Asp116 (TM3) has a constitutive role in receptor stimulation.

A modelling study has been carried out, investigating the binding of histamine (Hist), 2-methylhistamine (2-MeHist) and 2-phenylhistamine (2-PhHist) at two postulated agonistic binding sites on transmembrane domain 5 (TM5) of the histamine H1-receptor. For this purpose a conformational analysis study was performed on three particular residues of TM5, i.e., Lys200, Thr203 and Asn207, for which a functional role in binding has been proposed. The most favourable results were obtained for the interaction between Hist and the Lys200/Asn207 pair. Therefore, Lys200 was subsequently mutated and converted to an alanine, resulting in a 50-fold decrease of H1-receptor stimulation by histamine. Altogether, the data suggest that the Lys200/Asn207 pair is important for activation of the H1-receptor by histamine. In contrast, analogues of 2-PhHist seem to belong to a distinct subclass of histamine agonists and an alternative mode of binding is proposed in which the 2-phenyl ring binds to the same receptor location as one of the aromatic rings of classical histamine H1-antagonists. Subsequently, the binding modes of the agonists Hist, 2-MeHist and 2-PhHist and the H1-antagonist cyproheptadine were evaluated in three different seven-alpha-helical models of the H1-receptor built in homology with bacteriorhodopsin, but using three different alignments. Our findings suggest that the position of the carboxylate group of Asp116 (TM3) within the receptor pocket depends on whether an agonist or an antagonist binds to the protein; a conformational change of this aspartate residue upon agonist binding is expected to play an essential role in receptor stimulation.