Abstract
Purpose It was shown by several experimental studies that some G protein coupled receptors (GPCR) are sensitive to sodium ions. Furthermore, mutagenesis studies or the determination of crystal structures of the adenosine A 2A or δ-opioid receptor revealed an allosteric Na + binding pocket near to the highly conserved Asp 2.50 . Within a previous study, the influence of NaCl concentration onto the steady-state GTPase activity at the human histamine H 3 receptor (hH 3 R) in presence of the endogenous histamine or the inverse agonist thioperamide was analyzed. The purpose of the present study was to examine and quantify the Na + -sensitivity of hH 3 R on a molecular level. Methods To achieve this, we developed a set of equations, describing constitutive activity and the different ligand-receptor equilibria in absence or presence of sodium ions. Furthermore, in order to gain a better understanding of the ligand- and Na + -binding to hH 3 R on molecular level, we performed molecular dynamic (MD) simulations. Results The analysis of the previously determined experimental steady-state GTPase data with the set of equations presented within this study, reveals that thioperamide binds into the orthosteric binding pocket of the hH 3 R in absence or presence of a Na + in its allosteric binding site. However, the data suggest that thioperamide binds preferentially into the hH 3 R in absence of a sodium ion in its allosteric site. These experimental results were supported by MD simulations of thioperamide in the binding pocket of the inactive hH 3 R. Furthermore, the MD simulations revealed two different binding modes for thioperamide in presence or absence of a Na + in its allosteric site. Conclusion The mathematical model presented within this study describes the experimental data regarding the Na + -sensitivity of hH 3 R in an excellent manner. Although the present study is focused onto the Na + -sensitivity of the hH 3 R, the resulting equations, describing Na + - and ligand-binding to a GPCR, can be used for all other ion-sensitive GPCRs.