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 h istamine (Hist), 2-methylhistamine (2-MeHist) and 2-phenylhistamine (2 -PhHist) at two postulated agonistic binding sites on transmembrane do main 5 (TM5) of the histamine H-1-receptor. For this purpose a conform ational analysis study was performed on three particular residues of T M5, i.e., Lys(200), Th-203 and Asn(207), for which a functional role i n binding has been proposed. The most favourable results were obtained for the interaction between Hist and the Lys(200)/Asn(207) pair. Ther efore, Lys(200) was subsequently mutated and converted to an alanine, resulting in a 50-fold decrease of H-1-receptor stimulation by histami ne. Altogether, the data suggest that the Lys(200)/Asn(207) pair is im portant for activation of the H-1-receptor by histamine. In contrast, analogues of 2-PhHist seem to belong to a distinct subclass of histami ne agonists and an alternative mode of binding is proposed in which th e 2-phenyl ring binds to the same receptor location as one of the arom atic rings of classical histamine H-1-antagonists. Subsequently, the b inding modes of the agonists Hist, 2-MeHist and 2-PhHist and the H-1-a ntagonist cyproheptadine were evaluated in three different seven-alpha -helical models of the H-1-receptor built in homology with bacteriorho dopsin, but using three different alignments. Our findings suggest tha t the position of the carboxylate group of Asp(116) (TM3) within the r eceptor pocket depends on whether an agonist or an antagonist binds to the protein; a conformational change of this aspartate residue upon a gonist binding is expected to play an essential role in receptor stimu lation.