Detailed molecular dynamics simulations of human transferrin provide insights into iron release dynamics at serum and endosomal pH

Human serum transferrin (hTf) transports ferric ions in the blood stream and delivers them to cells via receptor-mediated endocytosis. hTf is folded into two homologous lobes; we utilize three of the available crystal structures delineating large conformational changes involved in iron binding/dissociation. We address the problems of whether the release process follows the same trend at serum (7.4) and endosomal (5.6) pH, and if there is communication between the lobes. In the absence of the transferrin receptor, we study the dynamics of the full structure as well as the separate lobes in different closed, partially open, and open conformations under neutral and endosomal pH conditions. Results corroborate those experimental observations underscoring the distinguishing effect of pH on the dynamics of hTf. Furthermore, in a total of 2 mu s molecular dynamics simulations, residue fluctuations elucidate the cross talk between the lobes correlated by the peptide linker bridging them at serum pH, while their correlations are lost under endosomal conditions. At serum pH, interplay between relative mobility of the lobes is correlated with iron release rates, rendering the initial conformational change an important contributor to the dynamics under these conditions. Interestingly, C-lobe opening lags behind that of the N-lobe as long as there is at least one iron bound, making the more stable C-lobe an attractive target for recognition by receptors. At endosomal pH, both lobes readily open, making irons available for delivery.