Binding of Selected Ligands to Human Protein Disulfide Isomerase and Microsomal Triglyceride Transfer Protein Complex...

Human protein disulfide isomerase (PDI) is a multifunctional protein, and also serves as the β subunit of the human microsomal triglyceride transfer protein (MTP) complex, a lipid transfer machinery. Dysfunction of the MTP complex is associated with certain disease conditions such as abetalipoproteinemia and cardiovascular diseases. It is known that the functions of PDI or the MTP complex can be regulated by the binding of a small-molecule ligand to either of these two proteins. In the present study, the conformational changes of the MTP complex upon the binding of three selected small-molecule ligands (17β-estradiol, lomitapide and a phospholipid) are investigated based on the available biochemical and structural information by using the protein-ligand docking method and molecular dynamics (MD) simulation. The ligand-binding sites, the binding poses and binding strengths, the key binding site residues, and the ligand binding-induced conformational changes in the MTP complex are analyzed based on the MD trajectories. The open-to-closed or closed-to-open transitions of PDI is found to occur in both reduced and oxidized states of PDI and also independent of the presence or absence of small-molecule ligands. It is predicted that lomitapide and 1,2-diacyl-sn-glycero-3-phosphocholine (a phospholipid) can bind inside the lipid-binding pocket in the MTP complex with high affinities, whereas 17β-estradiol interacts with the lipid-binding pocket in addition to its binding to the interface region of the MTP complex. Additionally, lomitapide can bind to the b' domain of PDI as reported earlier for E₂. Key residues for the ligand-binding interactions are identified in this study. It will be of interest to further explore whether the binding of small molecules can facilitate the conformational transitions of PDI in the future. The molecular and structural insights gained from the present work are of value for understanding some of the important biological functions of PDI and the MTP complex.