Comparison of the inward‐ and outward‐open homology models and ligand binding of human P‐glycoprotein

An homology model of human P-glycoprotein, based on the X-ray structure of the recently resolved mouse P-glycoprotein, is presented. The model corresponds to the inward-facing conformation competent for drug binding. From the model, the residues involved in the protein-binding cavity are identified and compared with those in the outward-facing conformation of human P-glycoprotein developed previously based on the Sav1866 structure. A detailed analysis of the interactions of the cyclic peptides QZ59-RRR and QZ59-SSS is presented in both the X-ray structures of mouse P-glycoprotein and the human P-glycoprotein model generated by ligand docking. The results confirm the functional role of transmembrane domains TM4, TM6, TM10 and TM12 as entrance gates to the protein cavity, and also imply differences in their functions. The analysis of the cavities in both models suggests that the ligands remain bound to the same residues during the transition from the inward- to the outward-facing conformations. The analysis of the ligand–protein interactions in the X-ray complexes shows differences in the residues involved, as well as in the specific interactions performed by the same ligand within the same protein. This observation is supported by docking of the QZ59 ligands into human P-glycoprotein, thus aiding in the understanding of the complex behavior of P-glycoprotein substrates and inhibitors. The results confirm the possibility for multispecific drug interactions of the protein, and are important for elucidating the P-glycoprotein function and ligand interactions.