Metal-specific structural changes in parvalbumin

► Structural difference between Ca2+- and Mg2+-loaded state of parvalbumin was studied. ► Ca2+- and Mg2+-bound forms of pike β-parvalbumin differ significantly. ► Cation-induced conformational changes are identical in the homologous α-isoform. ► Difference of spectral response on Ca2+/Mg2+ binding by β-parvalbumin is explained.