The EF-hand protein S100A2 is a cell cycle regulator involved in tumorigenesis, acting through regulation of the p53 activation state. Metal ion-free S100A2 is homodimeric and contains two Ca2+-binding sites and two Zn2+-binding sites per subunit, whereby the Zn2+ ion binding to one of the sites is coordinated by residues from two homodimers. The effect of selective binding of these metal ions was investigated using site-specific mutants which lacked one or both zinc sites. CD analysis of secondary structure changes on metallation showed that Zn2+ binding was associated with a decrease in the secondary structure content, whereas Ca2+ had the opposite effect in two of the three S100A2 mutants studied. The energy of unfolding (Delta G(U)) of the apo wild-type S100A2 was determined to be 89.9 kJ.mol(-1), and the apparent midpoint transition temperature (T-m(app)) was 58.4 degrees C. In addition, a detailed study of the urea and thermal unfolding of the S100A2 mutants in different metallation states (apo, Zn2+ and Ca2+) was performed. Thermal denaturation experiments showed that Zn2+ acts as a destabilizer and Ca2+ as a stabilizer of the protein conformation. This suggests a synergistic effect between metal binding, protein stability and S100A2 biological activity, according to which Ca2+ activates and stabilizes the protein, the opposite being observed on Zn2+ binding.