Berks has written a fascinating review of trace elements and their binding to proteins. It has been estimated that a third of all proteins require the help of metal ions to carry out their biological functions’. In most cases, these proteins obtain their metal cofactors directly from their surroundings, and so they need a mechanism to select the correct metal from the pool of metal ions to which they are exposed. A constraint on the selection of di-positive transition-metal ions by proteins is that the strength of the metal-protein interaction which is controlled the type metal ion, rather than by the nature of the binding groups supplied by the protein. This is shown by the Irving- Williams series, in which the affinity of metal ions for binding groups increases in the order manganese, iron, cobalt, zinc, nickel and copper. A protein cannot select one transition-metal ion over another just by changing the metal-binding groups.
Proteins have, therefore, evolved alternative strategies that enable them to bind to specific metals. Binding groups in proteins can be held in fixed positions that match the radius or preferred coordination geometry – the orientation in which a molecule binds a metal – of particular metal ions. Alternatively, specific helper proteins folds, or use cellular energy sources to bias the selectivity of the process. More generally, cells can control the relative availabilities of competing metal ions. For example, a cell can insert the weakly binding manganese ion (Mn2+) into a protein in the face of potential competition from strongly binding zinc (Zn2+) and copper ( Cu2+), because the cell keeps the cytoplasmic concentrations of zinc and copper thousands of times lower than that of manganese.
Berks 2008 Cells enforce an ion curtain Nature vol 455 pp 1043-4

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