Purpose: It was shown by several experimental studies that some G protein coupled receptors (GPCR) are sensitive\nto sodium ions. Furthermore, mutagenesis studies or the determination of crystal structures of the adenosine A2A or\n?-opioid receptor revealed an allosteric Na+ binding pocket near to the highly conserved Asp2.50. Within a previous\nstudy, the influence of NaCl concentration onto the steady-state GTPase activity at the human histamine H3 receptor\n(hH3R) in presence of the endogenous histamine or the inverse agonist thioperamide was analyzed. The purpose of the\npresent study was to examine and quantify the Na+-sensitivity of hH3R on a molecular level.\nMethods: To achieve this, we developed a set of equations, describing constitutive activity and the different\nligand-receptor equilibria in absence or presence of sodium ions. Furthermore, in order to gain a better\nunderstanding of the ligand- and Na+-binding to hH3R on molecular level, we performed molecular dynamic\n(MD) simulations.\nResults: The analysis of the previously determined experimental steady-state GTPase data with the set of equations\npresented within this study, reveals that thioperamide binds into the orthosteric binding pocket of the hH3R in absence\nor presence of a Na+ in its allosteric binding site. However, the data suggest that thioperamide binds preferentially into\nthe hH3R in absence of a sodium ion in its allosteric site. These experimental results were supported by MD simulations\nof thioperamide in the binding pocket of the inactive hH3R. Furthermore, the MD simulations revealed two different\nbinding modes for thioperamide in presence or absence of a Na+ in its allosteric site.\nConclusion: The mathematical model presented within this study describes the experimental data regarding the\nNa+-sensitivity of hH3R in an excellent manner. Although the present study is focused onto the Na+-sensitivity\nof the hH3R, the resulting equations, describing Na+- and ligand-binding to a GPCR, can be used for all other\nion-sensitive GPCRs.
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