Computational structure models of apo and diferric transferrin-transferrin receptor complexes

Complexation of transferrin (Tf) and its receptor (TfR) is an essential event for iron uptake by the cell. Much data has been accumulated regarding Tf-TfR complexation, such as results from mutagenesis. We created 3D structural models of apo-human Tf-TfR (apoTf-TfR) and Fe(III)(2)Tf-TfR (Fe(2)Tf-TfR) complexes by computational rigid body refinement. The models are consistent with published mutagenesis experiments. In our models, the C-lobes of apoTf and Fe(2)Tf bind to the helical domain of TfR, and the N-lobes are sandwiched between the ectodomain of TfR and the cell membrane as previously reported. Further, the molecules of apoTf and Fe(2)Tf are not forced to undergo large conformational changes upon complexation. The creation of the models led a new and important finding that a residue of TfR, R651, which is called a hot spot for Tf-TfR binding, interacts with Tf E385 when either apoTf or Fe(2)Tf bind to TfR. The models rationally interpret the iron release from Fe(2)Tf-TfR upon acidification, dissociation of apoTf from TfR at slightly alkaline pH, and metal specific recognition of TfR.