The thermodynamic and binding properties of the transferrins as studied by isothermal titration calorimetry

• Published in: Biochimica et biophysica acta Vol. 1820; no. 3; pp. 318 - 325
• Main Authors: Bou-Abdallah, Fadi, Terpstra, Tyson R.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.03.2012
• Subjects: Animals; binding properties; blood; calorimetry; Calorimetry - methods; cytoplasm; Endocytosis; homeostasis; Humans; iron; Iron - metabolism; iron absorption; mammals; metal ions; Protein Binding; Protein Structure, Tertiary; Receptors, Transferrin - chemistry; Receptors, Transferrin - metabolism; stoichiometry; Thermodynamics; titration; transferrin; Transferrins - chemistry; Transferrins - metabolism
• ISSN: 0304-4165; 0006-3002; 1872-8006
• DOI: 10.1016/j.bbagen.2011.07.013

In mammals, serum-transferrins transport iron from the neutral environment of the blood to the cytoplasm by receptor-mediated endocytosis. Extensive in-vitro studies have focused on the thermodynamics and kinetics of Fe3+ binding to a number of transferrins. However, little attention has been given to the thermodynamic characterization of the interaction of transferrin with its receptor. Iron-loaded transferrin (Tf) binds with high affinity to the specific transferrin receptor (TfR) on the cell surface. The Tf–TfR complex is then internalized via receptor mediated endocytosis into an endosome where iron is released. Here, we provide an overview of recent studies that have used ITC to quantify the interaction of various metal ions with transferrin and highlight our current understanding of the thermodynamics of the transferrin–transferrin receptor system at physiological pH. The interaction of the iron-loaded transferrin with the transferrin receptor is a key cellular process that occurs during the normal course of iron metabolism. Understanding the thermodynamics of this interaction is important for iron homeostasis since the physiological requirement of iron must be appropriately maintained to avoid iron-related diseases. The thermodynamic data revealed stoichiometric binding of all tested metal ions to transferrin with very high affinities ranging between 1017 and 1022M−1. Iron-loaded transferrin (monoferric or diferric) is shown to bind avidly (K~107–108M−1) to the receptor at neutral pH with a stoichiometry of one Tf molecule per TfR monomer. Significantly, both the N- and the C-lobe contribute to the binding interaction which is shown to be both enthalpically and entropically driven. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders. ► ITC is a valuable tool to characterize the thermodynamics of binding interactions. ► A large number of metal ions exhibit very high binding affinity to transferrin. ► Holo-hTf displays high affinity to TfR and favorable enthalpy and entropy changes. ► Both lobes of hTf appear to contribute to the thermodynamics of hTf-TfR interaction.