Analysis of Ca2+/Mg2+ selectivity in alpha-lactalbumin and Ca(2+)-binding lysozyme reveals a distinct Mg(2+)-specific site in lysozyme.
The triggering of Ca(2+) signaling pathways relies on Ca(2+)/Mg(2+) specificity of proteins mediating these pathways. Two homologous milk Ca(2+)-binding proteins, bovine alpha-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 Ca(2+)/Mg(2+) by native proteins is entropy-driven. Both proteins exhibit strong temperature dependences of apparent affinities to Ca(2+) and Mg(2+), due to low thermal stabilities of their apo-forms and relatively high unfavorable enthalpies of Mg(2+) association. The ratios of their apparent affinities to Ca(2+) and Mg(2+), 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 Ca(2+) and Mg(2+) binding with experimental data strongly suggests that the association of Ca(2+) and Mg(2+) ions with bLA is a competitive process, whereas the primary Mg(2+) site of EQL is different from its Ca(2+)-binding site. The later conclusion is corroborated by qualitatively different molar ellipticity changes in near-UV region accompanying Mg(2+) and Ca(2+) association. The Ca(2+)/Mg(2+) selectivity of Mg(2+)-site of EQL is below an order of magnitude. EQL exhibits a distinct Mg(2+)-specific site, probably arising as an adaptation to the extracellular environment.
Permyakov
Sergei E
SE
Institute for Biological Instrumentation of the Russian Academy of Sciences, Pushchino, Moscow region 142290, Russia.
Khokhlova
Tatyana I
TI
Uversky
Vladimir N
VN
Permyakov
Eugene A
EA
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D002417:Cattle; D006736:Horses; D006801:Humans; D007477:Ions / Q000737:chemistry; D007768:Lactalbumin / Q000737:chemistry* / Q000378:metabolism; D008274:Magnesium / Q000737:chemistry*; D009113:Muramidase / Q000737:chemistry* / Q000378:metabolism; D011489:Protein Denaturation; D013696:Temperature; D013816:Thermodynamics
D007477:Ions; D007768:Lactalbumin; D009113:Muramidase; D008274:Magnesium; D002118:Calcium