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Structural basis for sequential displacement of Ca²⁺ by Yb³⁺ in a protozoan EF-hand calcium binding protein

¹ Department of Chemical Sciences, Tata Institute of Fundamental Research, Mumbai-400005 India
² Centro Risonanze Magnetiche, University of Florence, Florence, Italy

Reprint requests to: K.V.R. Chary, Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai-400005, India; e-mail: chary{at}tifr.res.in; fax: 0091 (22) 2152110.

We have studied the displacement of Ca²⁺by the trivalent lanthanide ions (Yb³⁺) in a protozoan (Entamoeba histolytica) Ca²⁺-binding protein (EhCaBP), by NMR and thermodynamics. We have demonstrated, for the first time, how one can use in a combined fashion the utility of NMR and thermodynamics to have an insight to the relative binding specificities/affinity between Ca²⁺ and Yb³⁺. As revealed by the titration experiments, Yb³⁺ displaces Ca²⁺ from the four metal binding sites present in EhCaBP in a sequential manner. The study provides a structural origin for such a sequential Ca²⁺ displacement by Yb³⁺ in EhCaBP.

Keywords: EF-hand protein; calmodulin; EhCaBP; pseudocontact shifts; chemical shifts; ytterbium

Abbreviations: NMR, nuclear magnetic resonance • ITC, isothernmal calorimetry • HSQC, heteronuclear single quantum coherence • EhCaBP, Entamoeba histolytica calcium binding protein • CaM, calmodulin • TnC, Troponin C

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