Introduction to Zinc Sensing

In aqueous solutions of metal ions, the distinction is often made between "free" and "bound" metal ions.  This is an important concept in understanding the chemistry and biology of metal ions in solution. The term "free" is something of a misnomer, since essentially any ion will be solvated nearly all the time in aqueous solution, and likely to be at least weakly liganded by other ions and other molecular species that may also be present as well; others have proposed the use of alternative terms such as "mobile" or "labile"or "exchangeable" for such weakly bound metal ions.  

The important distinction is between weakly liganded metal ions and those more tightly bound, for instance bound as coenzymes to enzymes.   The free ion, bound by water, chloride, or other weak ligands, can rapidly exchange them for other, stronger ligands, and thus readily bind to other ligands and binding sites, which may bind more tightly and/or exchange more slowly(Stumm and Morgan 1996).  For example, a zinc ion in dilute aqueous solution will have water and anions such as chloride weakly bound to it, which adsorb and desorb rapidly (time frame of seconds, or faster).  When that zinc ion binds to apocarbonic anhydrase II in the active site (KD ≈ 4 pM), the dissociation rate constant is very slow (t1/2 ≈ months at room temperature without a catalyst), so the zinc-protein complex is very stable(Henkens and Sturtevant 1968).  Thus the free zinc is available to bind to other binding sites, while the protein-bound zinc is sequestered and essentially unavailable.  Thus an ordinary eukaryotic cell might have 100 μM total zinc ion, but less than one nM free zinc present in its cytoplasm, with the balance bound to proteins, amino acids, and other ligands present in the cell.  It is also important to note that there is a continuum of ligand affinities and kinetics in the cell, with both varying over a wide range(Bozym, Hurst et al. 2008).  Thus the zinc bound to (abundant) glutamate in the cell is not nearly as tightly bound at that to some proteins, and may exchange, albeit slowly.  

References:

Bozym, R., T. K. Hurst, et al. (2008). Determination of zinc using carbonic anhydrase-based fluorescence biosensors. Fluorescence Spectroscopy. L. Brand and M. Johnson. San Diego, Academic Press. 450: 287-309.

Henkens, R. W. and J. M. Sturtevant (1968). "The kinetics of the binding of Zn(II) by apocarbonic anhydrase." Journal of the American Chemical Society 90: 2669 - 2676.

Stumm, W. and J. J. Morgan (1996). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. New York, Wiley-Interscience.