The presence of a positive charge appears to be a minimal requirement in monoamine agonist ligands and an aromatic structure is common. The long wavelength absor-bance and intrinsic fluorescence of aromatic amino acids are much appreciated in the study of proteins, and the potent quenching ability of adjacent sulfhydryl and disulfide moieties is a frequent and annoying feature, although it is also evidence of a close affinity between aromatic and cysteine residues. The association of sulfur and p-bonded atoms in forming extended alternating chains was noted many years ago in a survey of protein structures61 — an observation that has been rediscovered many times since then. Aromatic ring stacking is well recognized, but the incorporation of sulfur-containing amino acids into a stack received less attention until recently.62 Cation-p interactions are also widespread in protein structures and extensive associations of all three groups, sulfur, ring and cation, especially in ligand binding, have been reported.63 Of course, these favored interactions do not have to be contributed by different parts of a single molecule. Because sulfur-containing chemicals exhibit an affinity for aromatic rings, then the converse must be true and the aromatic rings usually to be found in ligands would be expected to seek out an association with sulfhydryl and cysteine groups in a receptor. The same interactions would be preferentially selected during ligand-receptor binding and in the generation of bioactive molecules such as hormones and neurotransmitters.7 Certainly, the redox activation of the p-AR implies that amine, aromatic, and sulfhydryl groups are important for agonist activity. Assuming a scenario for ligand-receptor interaction in which important reactions can occur between amine, aromatic, and sulfhydryl groups, how is the redox activity of a monoamine manifest at the receptor and what is the result?
A recent theoretical publication developed a model for a 5-opioid binding site64 in which the docked arrangement of the ligands suggested a reaction mechanism for the cleavage of the disulfide bond (Figure 10.10). Semi-empirical quantum chemical calculations determined that the interaction between the carboxyl side chain of an aspartic acid and the disulfide bond led to the polarization and withdrawal of a proton from the protonated nitrogen of the ligand to one of the sulfur atoms. A mixed sulfenic acid and carboxylic acid anhydrate is formed as an intermediate, as well as a thiol, which results in cleavage of the disulfide bond. The authors concluded that the suggested mechanism may explain the action of agonists and antagonists and it was assumed to be common to many GPCRs.
In a limited experiment, Brandt and colleagues64 showed that agonist-stimulated cells took up more of a cysteine-reactive label than did unstimulated cells. This cannot be interpreted as a convincing demonstration of receptor tagging, especially because the molar excess of reactive thiols as opposed to receptors in a cell is of the order of 1012, defying the generation of a specific signal. A case could be made for a general activation of the cell and an increased turnover in exposed thiol groups, such as those in the heterotrimeric G proteins, which occupy intersubunit interfaces and are exposed on dissociation.65
This chapter started with a sense of vision and it will end with a taste or smell of what is to come in drug discovery with 7TMRs by celebrating the award of the 2004 Nobel Prize for Medicine to Richard Axel and Linda Buck for their discoveries of odorant receptors and the organization of the olfactory system.66
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