Zinc Compounds - Bio-complexes

Bio-complexes

A very large number of metallo-enzymes contain zinc(II). Also many proteins contain zinc for structural reasons. The zinc ion is invariably 4-coordinate with at least three ligands that are amino-acid side-chains. The imidazole nitrogen of a histidine side-chain is a common ligand. The following are typical examples of the two kinds of zinc-protein complexes.

In the active site of resting Carbonic anhydrase a zinc ion is coordinated by three histidine residues. The fourth position is occupied by a water molecule, which is strongly polarized as in hydrolysis (see above). When carbon dioxide enters the active site, it subject to nucleophilic attack by the oxygen atom which carries a partial negative charge, or indeed a full negative charge if the water molecule is dissociated. The CO₂ is rapidly converted into a bicarbonate ion.

2+ + CO₂ → 2+ + HCO₃- + H+

Some peptidases, such as Glutamate carboxypeptidase II are thought to act in a similar way, with the zinc ion promoting the formation of a nucleophilic reagent.

The zinc finger motif is a rigid substructure in a protein which facilitates the binding of the protein to another molecule such as DNA. In this case all four coordination positions are occupied by the histidine and cysteine residues. The tetrahedral geometry around the zinc ion constrains an α helix fragment and an antiparallel β sheet fragment to a particular orientation with respect to each other.

The magnesium ion, which has a higher concentration in biological fluids, cannot perform these functions as its complexes are much weaker than those of zinc.