Allosteric interactions at muscarinic cholinoceptors.

1. An allosteric interaction occurs when the binding of a ligand to its site on a receptor is able to modify the binding of another ligand to a topographically distinct site on the same receptor and vice versa. The muscarinic cholinoceptors represent the best-studied examples of allosteric phenomena among the G-protein-coupled receptor superfamily. 2. The simplest model describing allosteric interactions at muscarinic cholinoceptors is the ternary complex model, which allows for a three-way interaction between the receptor, a classical (orthosteric) ligand and an allosteric modulator. The interaction may be quantified using the dissociation constant of each ligand for its respective binding site on the free receptor and the 'co-operativity factor' alpha. This latter term is the ratio of affinities of a ligand for the occupied versus the unoccupied receptor and is a measure of the magnitude of the cooperativity between two concomitantly bound ligands. 3. Identification of allosteric phenomena requires the utilization of both radioligand binding and functional approaches. Manifestations of allosterism include: (i) a limited ability to influence radioligand binding as the concentration of the latter is increased; (ii) alterations in the dissociation rate of orthosteric ligands; (iii) curvilinear Schild regressions; and (iv) nonadditivity of agonist/orthosteric antagonist/allosteric modulator combination concentration ratios. 4. Allosteric modulators of muscarinic cholinoceptors represent a diverse range of compounds. Some of the most studied agents include gallamine, alcuronium and the bis-ammonium compounds, C7/3'-phth and W84. Alcuronium has proven a most useful pharmacological tool, as it has been shown to display both positive and negative co-operativity, depending on the receptor subtype and orthosteric ligand involved in the interaction. 5. Evidence has accumulated pointing to the existence of a common allosteric binding site on the muscarinic cholinoceptors, located close to the orthosteric site, but at a more extracellular level. However, the possibility of more than one accessory binding site on various receptor subtypes cannot be excluded. 6. Allosteric modulators offer a number of potential therapeutic advantages, including a ceiling level to their effects and the possibility of 'absolute selectivity' of action, based on the degree of co-operativity rather than the affinity of the modulator for any one receptor subtype.

Conformational analysis, molecular shape comparison, and pharmacophore identification of different allosteric modulators of muscarinic receptors.

Structurally dissimilar compounds such as alcuronium and the newly synthesized substances derived from the bisbenzyl ether TMB4 and from hexamethonium stabilize antagonist binding to M2-cholinoceptors which is indicative of an allosteric action. In order to propose a hypothesis for the common pharmacophore and the corresponding active conformations, seven flexible compounds in a data set were individually aligned onto the most active and, additionally, rigid alcuronium molecule using a torsional angle flexible fit. An S-shape conformation was found to be a plausible general active conformation. In a subsequent molecular shape analysis the overlap and the nonoverlap steric volumes, RMS alignment as well as electrostatic field potentials were employed as possible structure--activity correlation descriptors. The corresponding 3D-QSAR formulation exhibits a correlation between allosteric modulation potency and the nonoverlap steric volume as well as the proton and oxygen anion probe electrostatic field potentials. Because of large structural diversity among the small number of compounds studied, the apparent 3D-QSAR is best thought of as a convenient representation of the common spatial pharmacophore hypothesis.