Although metal ions are present only in trace amounts in living organisms,^1,2  almost half of the proteome encodes for metal-binding proteins^3,4  and ca. 40% of the structures deposited in the Protein Data Bank contain metals.⁵  Metal ions (and metal-containing cofactors, such as haem groups) play crucial structural and functional roles in biological systems. For instance, divalent metal ions (such as Ca²⁺ and Mg²⁺) are used in signalling cascades, as well as in DNA and RNA processing. Transition metal ions (e.g. Fe, Co, Mn or Cu) are involved in binding and transport of oxygen and other gases, and they also act as electron transfer conduits in respiration and photosynthesis, because of their ability to change oxidation state. Moreover, metal-containing enzymes (hereafter metalloenzymes) are implicated in drug clearance and resistance, as well as protection against oxidative stress. Therefore, understanding the function of metalloproteins and metalloenzymes is not only a fundamental (bio)chemical question, but it can also help to understand the molecular basis of diseases and design new drugs aimed at their treatment, as well as provide hints to design biomimetic compounds to be used in biocatalysis.