SER(165) OF TRANSMEMBRANE HELIX-IV IS NOT INVOLVED IN THE INTERACTIONOF CATECHOLAMINES WITH THE ALPHA-2A-ADRENOCEPTOR

Molecular modeling studies have predicted that the beta-hydroxyl group of the catecholamines interacts with the beta(2)-adrenoceptor at the serine residue at position 165 (Ser(165)) located on transmembrane hel ix IV; however, this has not been confirmed by site-directed mutagenes is. It has been inferred that this site, which is conserved in all of the nine known alpha- and beta-adrenoceptor subtypes, is also involved in the interaction of catecholamines with the az,alpha(2a)-adrenocept or. To test the hypothesis that the beta-hydroxyl group of the catecho lamines interacts with Ser(165) of the alpha(2a)-adrenoceptor, we prep ared a mutant alpha(2a)-adrenoceptor where Ser(165) was mutated to ala nine. Mutation of Ser(165) of the alpha(2a)-adrenoceptor to alanine ha d no effect on the affinity of dopamine (which lacks the beta-hydroxyl group) or either enantiomer of norepinephrine or epinephrine (bath of which possess the beta-hydroxyl group), indicating that Ser(165) is n ot involved in the interaction of the catecholamines with the alpha(2a )-adrenoceptor. We have previously shown that mutation of Ser(90), loc ated in transmembrane helix II, to either alanine or cysteine produces a selective reduction in the affinity of the (-)-enantiomers of the c atecholamines for the alpha(2a)-adrenoceptor, with no effect on the ()-enantiomers or the corresponding beta-desoxy analogs. This is consis tent with the known stereoselectivity involved in the interactions of catecholamines with the alpha(2a)-adrenoceptor. The results of the pre sent investigation indicate that Ser(165) is not involved in the inter action of catecholamines with the alpha(2a)-adrenoceptor. Because all known alpha-adrenoceptor subtypes have a serine residue at a position corresponding to Ser(90) of the alpha(2a)-adrenoceptor, it would appea r that this site represents an important point for attachment of the b eta-hydroxyl group of catecholamines.