Protein-based catalysts

The structure of proteins, as with the structure of carbohydrates, has various levels—primary, secondary, tertiary and quaternary structure. The tertiary and quaternary structures and their subtleties are most important in the biological function of the molecule. Consider an enzyme (a protein-based catalyst)—its structure allows the binding of specific molecules which then react catalytically to give products. Conversely, enzymes are very susceptible to environmental conditions which alter their tertiary structure. 

Before specific examples are discussed, we want to briefly recall the fundamental principles of enzyme catalysis. The astonishing efficiency of protein-based catalysts is the result of precisely positioned functional groups, which constitute a dynamic binding pocket for the substrate(s) and the transition state of the reaction. For a unimolecular reaction (S P), the simplified energy profile is depicted in Fig. 1. 

Although the systems described here have not been used for nanoencapsulated cascade reactions, or of course, for mutually incompatible catalysts, they offer an attractive possibility for the extension of this field, especially given the availability of a wide range of protein-based nanometer-sized cages, such as chaperonins, DNA binding proteins, and the extensive class of viruses [107]. 

Compared with the heme proteins discussed in Section 2.2, the non-heme iron proteins presented here have a much more flexible coordination geometry. Taken together with the differences in electronic properties - heme enzymes contain mostly low-spin iron whereas non-heme enzymes contain mostly a high-spin iron - this is responsible for the more diverse chemistry found for the non-heme iron proteins. The great versatility of these enzymes makes them a treasure trove for the development of iron-based catalysts. Inspired by their biological archetypes, numerous catalytic reactions await to be reproduced by iron catalysts in organic synthesis. 

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