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Unformatted text preview: NATURE CHEMICAL BIOLOGY VOLUME 5 NUMBER 8 AUGUST 2009 567 REVIEW Biocatalysis involves the use of enzymes, either in semipurified or immobilized form, or as whole-cell systems, for the preparation of molecules that are traditionally made using chemical synthesis. The field continues to expand its horizons, particularly as applied to the synthesis of fine chemicals and pharmaceuticals 1–4 . A useful bio- catalyst (in particular, one that is suitable for application in an indus- trial context) is characterized by a number of features, including high catalytic turnover (>500 min −1 ), high selectivity for the particular transformation (for example, enantio-, regio- and chemoselectivity) and high process stability under the conditions that are required for the chemical transformation 5 . Biocatalysis often complements (rather than directly competes with) chemocatalysis in that it is probably best used for preparing relatively small (molecular weight < 500), enantiomerically pure chiral building blocks that are then combined with other chemically derived molecules to produce the final product. Biocatalysts have historically been widely applied in the brewing and food industries but are now finding increased application in the synthesis of fine chemicals, pharmaceuticals, agrochemicals and even bulk chemicals. A recent and important trend is the development of new biocatalyst-based processes for the production of biofuels using renewable starting materials as an alternative to fossil fuels. In this sense biocatalysis should be viewed as an essential ingredient of the industrial biotechnology revolution that is happening all around us and that will alter the landscape of where the products and processes of the future are derived from. This review covers recent developments in the field of directed evolution of enzymes, highlighting advances in screening and library design and also giving examples of where enzyme evolution has had a major impact on the development of new biocatalysts. Irrespective of the target application that is envisaged, there are a number of key challenges that must be addressed in the development of a robust biocatalyst for practical applications ( Fig. 1 ). The initial phase of enzyme discovery involves (i) screening available microbial culture collections or libraries of enzymes from a variety of sources in order to identify an enzyme or group of enzymes with some activ- ity toward the substrate of interest, (ii) construction of synthetic genes based on literature precedents or (iii) use of bioinformatics to identify homologous genes. In some cases the wild-type enzyme may have the required activity and selectivity for further develop- ment work. However, a more likely scenario is that improvements in catalytic activity, selectivity and (importantly) stability under process conditions are required for practical applications. Even with the benefit of a high-resolution crystal structural of an enzyme, rational redesign to produce optimized biocatalysts is complex and...
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- Spring '10
- Enzyme, directed evolution, Reetz