Analysis of Ca2+/Mg2+ Selectivity in α-lactalbumin and Ca2+-binding Lysozyme Reveals a Distinct Mg2+-specific Site in Lysozyme

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A-lactalbumin, Lysozyme, Thermo-dynamics, Phase Transition, Phase Diagram, Statediagram, Thermal Stability, Ligand Binding, Metalbinding, Metal Selectivity

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The triggering of Ca2+ signaling pathways relies on Ca2+/Mg2+ specificity of proteins mediating these pathways. Two homologous milk Ca2+-binding proteins, bovine α-lactalbumin (bLA) and equine lysozyme (EQL), were analyzed using the simplest “four-state” scheme of metal- and temperature-induced structural changes in a protein. The association of Ca2+/Mg2+ by native proteins is entropy-driven. Both proteins exhibit strong temperature dependences of apparent affinities to Ca2+ and Mg2+, due to low thermal stabilities of their apo-forms and relatively high unfavorable enthalpies of Mg2+ association. The ratios of their apparent affinities to Ca2+ and Mg2+, being unusually high at low temperatures (5.3–6.5 orders of magnitude), reach the values inherent to classical EF-hand motifs at physiological temperatures. The comparison of phase diagrams predicted within the model of competitive Ca2+ and Mg2+ binding with experimental data strongly suggests that the association of Ca2+ and Mg2+ ions with bLA is a competitive process, whereas the primary Mg2+ site of EQL is different from its Ca2+-binding site. The later conclusion is corroborated by qualitatively different molar ellipticity changes in near-UV region accompanying Mg2+ and Ca2+ association. The Ca2+/Mg2+ selectivity of Mg2+-site of EQL is below an order of magnitude. EQL exhibits a distinct Mg2+-specific site, probably arising as an adaptation to the extracellular environment. Proteins 2010. © 2010 Wiley-Liss, Inc.

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Proteins: Structure, Function, and Bioinformatics, v. 78, issue 12, p. 2571-2723