Comparative Modeling ofthe Human Monoamine Transporters:Similarities in Substrate Binding

摘要

The amino acid compositions of the substrate binding pockets of the three human monoamine transporters are compared as is the orientation of the endogenous substrates, serotonin, dopamine, and norepinephrine, bound in these. Through a combination of homology modeling, induced fit dockings, molecular dynamics simulations, and uptake experiments in mutant transporters, we propose a common binding mode for the three substrates. The longitudinal axis of the substrates is similarly oriented with these, forming an ionic interaction between the ammonium group and a highly conserved aspartate, Asp98 (serotonin transporter, hSERT), Asp79 (dopamine transporter, hDAT), and Asp75 (norepinephrine transporter, hNET). The 6-position of serotonin and the para-hydroxyl groups of dopamine and norepinephrine were found to face Ala173 in hSERT, Gly153 in hDAT, and Gly149 in hNET. Three rotations of the substrates around the longitudinal axis were identified. In each mode, an aromatic hydroxyl group of the substrates occupied equivalent volumes of the three binding pockets, where small changes in amino acid compositionexplains the differences in selectivity. Uptake experiments supportthat the 5-hydroxyl group of serotonin and the meta-hydroxyl group norepinephrine and dopamine are placed in the hydrophilicpocket around Ala173, Ser438, and Thr439 in hSERT corresponding toGly149, Ser419, Ser420 in hNET and Gly153 Ser422 and Ala423 in hDAT.Furthermore, hDAT was found to possess an additional hydrophilic pocketaround Ser149 to accommodate the para-hydroxyl group.Understanding these subtle differences between the binding site compositionsof the three transporters is imperative for understanding the substrateselectivity, which could eventually aid in developing future selectivemedicines.