Parvalbumin is a small protein of EF-hand family whose main role is considered to be metal buffering. Recent evidences indicate that parvalbumin also fulfills more complicated functions, which may be determined by the diversity in structural changes in response to the binding of different metal cations. In the present work the conformations of alpha and beta isoforms of pike parvalbumin in the Ca2+- and Mg2+-Ioaded state were studied by intrinsic fluorescence, circular dichroism and bis-ANS extrinsic fluorescence. We have determined the structural region causing different spectral response on the binding of Mg2+- and Ca2+ ions in pike beta-parvalbumin. Our data reveal similarity of the metal-bound forms of alpha-parvalbumin. In contrast, those of beta isoform differ significantly in the tyrosine spectral range. We also discuss the possible physiological consequences of the structural rearrangements accompanied Meg(2+)/Ca2+ exchange in pike beta-parvalbumin. (c) 2012 Elsevier Inc. All rights reserved.