Mechanism of Inverse Agonist Action of Sarpogrelate at the Constitutively Active Mutant of Human 5-HT2A Receptor Revealed by Molecular Modeling

We previously reported that sarpogrelate, a selective 5-HT2A antagonist, showed a potent inverse agonist activity to constitutively active mutant (C322K) of human 5-HT2A receptor (5-HT2AR). However, it remains to be unknown about the actual mechanism of this mutant for its constitutive activation as well as inverse agonist activity of sarpogrelate. Our model shows that mutation (C322K) of 5-HT2AR causes electronic repul-sion between positively charged Arg173(3.50) and Lys322(6.34) residues resulting outward movement of the C-terminus of transmembrane helix (TMH) III. This motion of TMH III leads to a partially active structure of the receptor, which may be a key step in receptor activation. The structural model of the partially active receptor also indicates that the binding of sarpogrelate to the constitutively active receptor causes an inward swing of TMH III to an inactive receptor structure. Therefore, the present study may suggest that the elec-tronic repulsion causing outward movement of the C-terminus of TMH III may be the key step for constitu-tive activation of mutant C322K of 5-HT2AR and the inward movement of TMH III causes the inverse agonist activity of sarpogrelate. Key words 5-HT2A receptor; mutation; constitutive activity; molecular modeling; electronic repulsion It has been established that many G-protein coupled recep-tors (GPCRs) can exist in a constitutively active form in th