About one-third of the proteins in all organisms require a transition metal ion in order to function. Among them, metalloenzymes catalyze diverse biological reactions, some of which are essential for life (nitrogen fixation, respiration, etc.). Maintaining an appropriate intracellular concentration and homeostasis of essential metals, while avoiding toxicity caused by excess amounts, is therefore critical for cells. If the concentration becomes too low, the cell will suffer from inactivation of essential enzymes. Non-physiological high intracellular concentrations of metals can lead to replacement of the native metals in essential enzymes, causing their inactivation and, for some metals, catalyze the formation of highly toxic reactive oxygen species.