THE AGONISTIC BINDING-SITE AT THE HISTAMINE H-2-RECEPTOR .1. THEORETICAL INVESTIGATIONS OF HISTAMINE BINDING TO AN OLIGOPEPTIDE MIMICKING APART OF THE 5TH TRANSMEMBRANE ALPHA-HELIX

Mutation studies on the histamine H-2 receptor were reported by Gantz et al. [J. Biol. Chem., 267 (1992) 20840], which indicate that both th e mutation of the fifth transmembrane Asp(186) (to Ala(186)) alone or in combination with Thr(190) (to Ala(190)) maintained, albeit partiall y, the cAMP response to histamine. Recently, we have shown that histam ine binds to the histamine H-2 receptor as a monocation in its proxima l tautomeric form, and, moreover, we suggested that a proton is donate d from the receptor towards the tele-position of the agonist, thereby triggering the biological effect [Nederkoorn et al., J. Mel. Graph., 1 2 (1994) 242; Eriks et al., Mel. Pharmacol., 44 (1993) 886]. These fin dings result in a close resemblance with the catalytic triad (consisti ng of Ser, His and Asp) found in serine proteases. Thr(190) resembles a triad's serine residue closely, and could also act as a proton donor . However, the mutation of Thr(190) to Ala(190) - the latter is unable to function as a proton donor - does not completely abolish the agoni stic cAMP response. At the fifth transmembrane alpha-helix of the hist amine H-2 receptor near the extracellular surface, another amino acid is present, i.e. Tyr(182), which could act as a proton donor. Furtherm ore, Tyr(182) lies within the proximity of Asp(186), so an alternative couple of amino acids, Tyr(182) and Asp(186), could constitute the hi stamine binding site at the fifth alpha-helix instead of the (mutated) couple Asp(186) and Thr(190). In the first part of our present study. this hypothesis is investigated with the aid of an oligopeptide with an alpha-helical backbone, which represents a part of the fifth transm embrane helix. Both molecular mechanics and ab initio data lead to the conclusion that the Tyr(182)/Asp(186) couple is most likely to act as the binding site for the imidazole ring present in histamine.