MODELING AND MUTATION STUDIES ON THE HISTAMINE H-1-RECEPTOR AGONIST BINDING-SITE REVEAL DIFFERENT BINDING MODES FOR H-1-AGONISTS - ASP(116) (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 H-1-receptor. For this purpose a conformational analysis study was performed on three particular residues of TM5, i.e., Lys(200), Th-203 and Asn(207), for which a functional role in binding has been proposed. The most favourable results were obtained for the interaction between Hist and the Lys(200)/Asn(207) pair. Therefore, Lys(200) was subsequently mutated and converted to an alanine, resulting in a 50-fold decrease of H-1-receptor stimulation by histamine. Altogether, the data suggest that the Lys(200)/Asn(207) pair is important for activation of the H-1-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 H-1-antagonists. Subsequently, the binding modes of the agonists Hist, 2-MeHist and 2-PhHist and the H-1-antagonist cyproheptadine were evaluated in three different seven-alpha-helical models of the H-1-receptor built in homology with bacteriorhodopsin, but using three different alignments. Our findings suggest that the position of the carboxylate group of Asp(116) (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.