Photochemical tools for studying metal ion signaling and homeostasis.

Metal ions have well-established catalytic and structural roles in proteins. Much of the knowledge acquired about metalloenzymes has been derived using spectroscopic techniques and X-ray crystallography, but these methodologies are less effective for studying metal ions that are not tightly bound to biomacromolecules. In order to prevent deleterious chemistry, cells tightly regulate the uptake, distribution, and intracellular concentrations of metal ions. Investigation into these homeostasis mechanisms has necessitated the development of alternative ways to study metal ions. Photochemical tools such as small molecule and protein-based fluorescent sensors as well as photocaged complexes have provided insight into the homeostasis and signaling mechanisms of Ca(2+), Zn(2+), and Cu(+), but a comprehensive picture of metal ions in biology will require additional development of these techniques, which are reviewed in this Current Topics article.