Directed Evolution of Stereoselective Biocatalysts

INTRODUCTION Stereoselective chemical reactions have become extremely important in both academic and industrial settings for the synthesis of useful molecules. Catalytic reactions are of particular importance due to their capacity to effect otherwise impossible transformations using substoichiometric quantities of the catalyst. Traditionally, transition metal catalysts have played a central role in enabling these transformations, and the last century has seen a tremendous amount of work in the research and development of organometallic catalysts based upon a wide variety of metal centers. Another approach is to use nature’s own catalysts, enzymes, to catalyze chemical reactions. This approach has seen less implementation for a number of reasons. The number of enzymes available for use in reactions has, until recently, remained small. Also, the substrate specificity for many enzymes can be high, limiting the generality of such catalysts for use in a laboratory setting, and the enantioselectivity of available enzymes may be insufficient for practical use in a particular system. However, the substrate promiscuity of enzymes may have been underappreciated, and continuing advances in high-throughput combinatorics and molecular biology, combined with the technique of directed evolution, may soon make enzymatic biocatalysis a practical and economical solution to the problem of asymmetric synthesis. Assuming that the issues of low stereoselectivity and substrate scope can be solved, there are a number of advantages associated with biocatalysts over traditional transition-metalmediated catalysis. First, the size and complexity of proteins allows for extraordinarily high diversity in both structure and function. Ironically, this tremendous complexity largely arises from a relatively simple, modular construction of the primary peptide sequence. Harnessing the cellular machinery in bacteria via microbial engineering now allows the generation of large libraries of potential biocatalysts, something that is not true of small-molecule metal catalysts. Furthermore, reactions can be carried out in aqueous media at mild temperatures and pressures, which may facilitate the development of more environmentally friendly “green” chemistry.