Kinetics and thermodynamics of metal-loaded transferrins: transferrin receptor 1 interactions

Abstract

Transferrin receptor 1 (R) and human serum transferrin (T) are the two main actors in iron acquisition by the cell. R binds TFe(2) (iron-loaded transferrin), which allows its internalization in the cytoplasm by endocytosis. T also forms complexes with metals other than iron. In order to follow the iron-acquisition pathway, these metals should obey at least two essential rules: (i) formation of a strong complex with T; and (ii) interaction of this complex with R. In the present paper, we propose a general mechanism for the interaction of five metal-loaded Ts [Fe(III), Al(III), Bi(III), Ga(III) and Co(III)] with R and we discuss their potential incorporation by the iron-acquisition pathway. With iron- and cobalt-loaded Ts, the interaction of R takes place in two steps: the first is detected by the T-jump technique and occurs in the 100 micros range, whereas the second is slow and occurs in the hour range. Bi(III)- and Ga(III)-loaded Ts interact with R in a single fast kinetic step, which occurs in the 100-500 micros range. No interaction is detected between R and aluminium-saturated T. The fast steps are ascribed to the interaction of the C-lobe of metal-loaded T with the helical domain of R: dissociation constant, K'(1), of 0.50+/-0.07, 0.82+/-0.25, 4+/-0.4 and 1.10+/-0.12 microM for Fe(III), Co(III), Bi(III) and Ga(III) respectively. The second slow steps are ascribed to changes in the conformation of the protein-protein adducts which increase the stability to achieve, at thermodynamic equilibrium, an overall dissociation constant, K(1), of 2.3 and 25 nM for Fe(III) and Co(III) respectively. This last step occurs over several hours, whereas endocytosis takes place in several minutes. This implies that metal-loaded Ts are internalized with only the C-lobe interacting with R. This suggests that, despite a lower affinity for R when compared with TFe(2), some metal-loaded Ts can compete kinetically with TFe(2) for the interaction with R and thus follow the iron-acquisition pathway.

Aluminum and Alzheimer's Disease: After a Century of Controversy, Is there a Plausible Link?

“…4) [27,68,175]. Most notably, recent research evidence shows that Al-transferrin complexes are not bound by the TfR [198][199][200] because of an incomplete open/closed form which precludes them from forming specific ionic inter-residual interactions, such as those formed by iron-transferrin and the TfR [198]. This implies that Al-transferrin transfer from the blood stream to cytoplasm may not follow the classical iron-TfR acquisition pathway.…”

Iron, hepcidin, and the metal connection

“…Transferrin has also been reported to interact with lead when present in excess (Leelakunakorn et al, 2005). In addition, when saturated by non-iron metals, transferrin could compete with iron-transferrin during its interaction with transferrin receptor 1 (Ha-Duong et al, 2008; El Hage Chahine et al, 2012). Taken together, these data suggest that metals other than iron could, by using transferrin as a Trojan horse, play a role in metal distribution within the body.…”

The Pharmacokinetics and Toxicology of Aluminum in the Brain

“…However, the affinity of TfR 1 for the Al-Tf complex was found to be very weak in vitro, leading Hémadi et al to caution that Al transfer from blood to cell cytoplasm may not follow the TfR-ME pathway [45]. In another report from the same laboratory Ha-Duong et al found no interaction between Al-Tf and the TfR [46]. Based on structural and physicochemical characteristics of apo-Tf, Fe-Tf, Al-Tf and the TfR (diffusion coefficient, hydrodynamic radius, capillary electrophoretic mobility, and zeta potential) and calculation of acidic amino acid residues, Sakajiri et al concluded that the driving force for the formation of a complex of metal and the TfR is the electrostatic interaction between the negative charge of the former and positive charge of the latter.…”