Very often metal ions are found encapsulated in the interior of a protein (or located on its surface). These metal ions can undergo two different functions:
• The three-dimensional structure (secondary structure) of a protein is controlled by non-covalent interactions between different parts of the peptide strand. Metal ions form very strong ''non-covalent'' coordination bonds and can therefore fix peptide structures very effectively. A prominent example is the zinc finger motif (Figure B.3.1). A random coil peptide binds to zinc(II) ions through two cysteine and two histidine residues inducing formation of a ¿-sheet structure by the cysteines and an a-helix by the histidines (Figure B.3.1). The biological activity of the resulting conformationally strained peptide is ''switched on'' and the peptide can now interact with DNA .
• In addition to structure control, metal ions can act as reactive centers of proteins or enzymes. The metals can not only bind reaction partners, their special reactivity can induce chemical reaction of the substrate. Very often different redox states of the metal ions play a crucial role in the specific chemistry of the metal. Non-redox-active enzymes, e.g. some hy-drolytic enzymes, often react as a result of their Lewis-acid activity . Binding of substrates is, however, important not only for their chemical modification but also for their transport. Oxygen transport by hemoglobin is an important example of this .
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