Protein Ca2+-Sites Prone to Sr2+ Substitution: Implications for Strontium Therapy.

Strontium (Sr), an alkali metal with properties similar to calcium, in the form of soluble salts is used to treat osteoporosis. Despite the information accumulated on the role of Sr2+ as a Ca2+ mimetic in biology and medicine, there is no systematic study of how the outcome of the competition between the two dications depends on the physicochemical properties of (i) the metal ions, (ii) the first- and second-shell ligands, and (iii) the protein matrix. Specifically, the key features of a Ca2+-binding protein that enable Sr2+ to displace Ca2+ remain unclear. To address this, we studied the competition between Ca2+ and Sr2+ in protein Ca2+-binding sites using density functional theory combined with the polarizable continuum model. Our findings indicate that Ca2+-sites with multiple strong charge-donating protein ligands, including one or more bidentately bound Asp-/Glu- that are relatively buried and rigid are protected against Sr2+ attack. On the other hand, Ca2+-sites crowded with multiple protein ligands may be prone to Sr2+ displacement if they are solvent-exposed and flexible enough so that an extra backbone ligand from the outer shell can bind to Sr2+. In addition, solvent-exposed Ca2+ sites with only a few weak charge-donating ligands that can rearrange to fit the strontium's coordination requirements are susceptible to Sr2+ displacement. We provide the physical basis of these results and discuss potential novel protein targets of therapeutic Sr2+.