X-ray structural studies of metal-binding to human serum transferrin: metal-induced conformational changes and biocoordination of metals

Abstract

Human serum transferrin (hTf) is an indispensable serum protein primarily responsible for carrying ferric ions Fe(III) inside the human circulatory system [1]. Metal-induced conformational change of the protein is a momentous process for specific transferrin-receptor (TfR)-transferrin (hTf) recognition, leading to cellular uptake of ferric ions through a ‘receptor-mediated endocytosis’ [1]. It has been reported that hTF is capable of binding various multivalent metals such as Bi(III), Ga(III), In(III), Ru(III), Ti(IV), Yb(III), Dy(III), Pt(II), etc., and may play an critical role in delivery of metallodrugs [1,2]. In addition to the conventional bicarbonate, other anions such as oxalate and malonate, may serve as alternative synergistic anions to stabilize metal coordination [1]. However structural evidence of hTf binding to other metal ions and synergistic anions as well as the resulting conformational changes to hTf are still currently lacking. We had previously reported a diferric-hTf (FeNFeC-hTf) crystal structure (2.8 A ° ) and a bismuth-bound hTf (BiNFeC-hTf) crystal structure (2.4 A ° , which revealed a ‘partially-opened’ conformation in the N-lobe while maintaining a ‘closed’ conformation in the C-lobe [3]. In the current study, we have solved a monoferric-hTf (FeC-hTf) (2.7 A ° ), a mono-ytterbium(III)-hTf (YbC-hTf) (2.5 A ° ) and a monovanadium(IV)-hTf (VN-hTf) (3.15 A ° ) structure. In the former two structures, we observe metal-binding in C-lobe only but not the N-lobe, causing the N-lobe to adopt an opened conformation and C-lobe to a closed conformation. We observed that malonate serves as a synergistic anion in the metal site. In contrast, vanadium binds to the N-lobe only but not the C-lobe in the form of a oxovanadium species, causing the metal-bound N-lobe to adopt a ‘partially-opened’ conformation but ‘opened’ conformation in the C-lobe. No synergistic anion is present for metal binding in this structure. These X-ray structures justify the role of hTf as a metal ion mediator [1] and coordination geometry of metals and conformational changes of hTf can be revealed. Our structures may also provide an insight into the intermediate conformations of hTf during metal binding and release.