Metal Binding

Another method by which the folded state of proteins can be stabilized is metal binding, in which metal ions are coordinated, usually by lone pair donation from oxygen or nitrogen atoms.

Here is a picture of the zinc binding protein, carboxypeptidase A from PDB's collection of GIF images.

Also see examples in Jane Richardson's Protein Tourist kinamage.

Experiments have shown that metal binding can contribute 6 - 9 kcal/mol (Braxton, 1996 and references therein) to stability. However, this is a little misleading as the comparisons made are between the apo- and the holo- enzyme; in the apo-enzyme there is often a destabilising cluster of negative charge from coordinating acidic side chains. Perhaps a fairer estimation of the contribution of metal binding to protein stability comes from an experiment in which a metal chelating site was introduced into an alpha helix of iso-cytochrome c and gave rise to a 1 kcal/mol increase in stability in the presence of saturating Cu(II) (Kellis et al., 1991). Another interesting study was that of Kuroki et al. (1989). They observed that the sequence and tertiary structure of alpha-lactalbumin (Ca²⁺ binding) and c-type lysozymes (non-Ca²⁺ binding) are homologous.

They recruited the binding site from alpha-lactalbumin into human c-type lysozyme (Q86D/A92D). The mutant protein binds one mole of calcium ions and has optimal activity at about 10°C higher than the wild-type. Interestingly, the apo-enzyme is about 5 °C less stable than wild-type.

Here are pictures of the two proteins, showing the structural similarity and the recruited calcium site. Also links to the pdb files.

Aromatic Interactions Disulphide BondsBeginning