Why is Zinc Important?
First, zinc can provide a purely structural role because it has a completely filled d-orbital. This role is exhibited by the different zinc finger motifs on the DNA-Protein Interaction page.
Second, Zn++ is a "borderline" acid, which can make different types of interactions depending on the type of ligand it's associating with:
S (Cys) à Soft base
N (His) à Hard base
O (Glu, Asp, H2O) à Hard base