Metalloproteins and Metalloenzymes

The general family of functional proteins containing metal ions is the metalloproteins, of which one important branch is the metalloenzymes. The task of a metalloenzyme put simpl.

Table 8.1 Division of metal-containing biomolecules sub-classes and examples.

is usually to bind a specific organic molecule (the substrate) and mediate the performance of some catalysed chemical reaction on that substrate to convert it to a different product(s). The site of this reaction is called the active site, and features one or more metal ion(s) in a defined coordination environment that exists within a wider environment defined by the shape of the surrounding biopolymer. The role of the metal ion in metalloenzymes may involve one or more of the following:

• coordination and activation of a substrate;
• organization' of the environment for facilitating reactions involving the substrate:
• providing a coordinated (and highly active) nucleophile;
•  causing major pk changes of molecules upon binding to promote reaction;
• using the redox properties of the metal ion.

The chemistry taking place at the active site of metalloenzymes is not unusual - it is fairly standard coordination chemistry often metal-directed organic reactions with the substrate as a coordinated ligand. The main difference over small-molecule analogues is that the three- dimensional structure of the parent protein polymer (in effect also the 'ligand") has a big role to play in the chemistry such as providing a specially-shaped space for the substrate to occupy and in which the chemistry occurs. However, in reality, even this is not unexpected. as the chemistry of even simple metal complexes is influenced by the non-participating or 'spectator' ligands around the metal. In effect, metalloenzymes are just 'supersized coordination complexes. They are usually very large molecules, with the active site being a very small part of the overall structure. This active site can be modelled by the synthesis of small metal complexes. Simple models perform one of two tasks - they mimic either the shape of the biomolecule's active site, or they perform the same chemistry-sometimes even both. It is arguably not surprising that the structure at active sites is often very easily mirrored in the laboratory -- after all the enzyme must form under very mild conditions in nature.

We will examine a number of examples of Metallo biomolecules including enzymes, from a viewpoint of the structure of the metal 'core' and the chemistry that occurs there. This is peculiarly a coordination chemist's view of biosystems but this is most appropriate in the context of this textbook. Our target here is to identify and exemplify the metal coordination chemistry met abundantly in biomolecules.

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