Conserved features of the sequences of dopamine receptors and of homologous G-protein-coupled receptors point to regions, and amino acid residues within these regions, that contribute to their ligand binding sites. Differences in binding specificities among the catecholamine receptors, however, must stem from their nonconserved residues. Using the substituted-cysteine accessibility method, we have identified the residues that form the surface of the water-accessible binding-site crevice in the dopamine D2 receptor. Of approximately 80 membrane-spanning residues that differ between the D2 and D4 receptors, only 20 were found to be accessible, and 6 of these 20 are conservative aliphatic substitutions. In a D2 receptor background, we mutated the 14 accessible, nonconserved residues, individually or in combinations, to the aligned residues in the D4 receptor. We also made the reciprocal mutations in a D4 receptor background. The combined substitution of four to six of these residues was sufficient to switch the affinity of the receptors for several chemically distinct D4-selective antagonists by three orders of magnitude in both directions (D2- to D4-like and D4- to D2-like). The mutated residues are in the second, third, and seventh membrane-spanning segments (M2, M3, M7) and form a cluster in the binding-site crevice. Mutation of a single residue in this cluster in M2 was sufficient to increase the affinity for clozapine to D4-like levels. We can rationalize the data in terms of a set of chemical moieties in the ligands interacting with a divergent aromatic microdomain in M2-M3-M7 of the D2 and D4 receptors.
Wormholes through chemical space: Spirocyclic oxetanes are described as analogues of morpholine and also as topological siblings of their carbonyl counterparts. They are particularly promising in terms of both their physicochemical properties and the ease with which they can be grafted onto molecular structures. The data collected highlight oxetanes as both the hydrophilic sister of a gem‐dimethyl unit and the carbonyl group's lipophilic brother.
Wurmlöcher im chemischen Raum: Spirocyclische Oxetane werden als Analoga von Morpholin und als topologische Gegenstücke entsprechender Carbonylverbindungen beschrieben. Solche Oxetane sind vielversprechend bezüglich ihrer physikochemischen Eigenschaften und der Leichtigkeit, mit der sie in Molekülstrukturen eingebaut werden können. Die gesammelten Daten präsentieren die Oxetan‐Gruppe einerseits als hydrophile Entsprechung einer geminalen Dimethyleinheit und andererseits als lipophile Entsprechung einer Carbonylgruppe.
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