Directed evolution of stereoselective enzymes based on genetic selection as opposed to screening systems

Directed evolution of stereoselective enzymes provides a means to generate useful biocatalysts for asymmetric transformations in organic chemistry and biotechnology. Almost all of the numerous examples reported in the literature utilize high-throughput screening systems based on suitable analytical techniques. Since the screening step is the bottleneck of the overall procedure, researchers have considered the use of genetic selection systems as an alternative to screening. In principle, selection would be the most elegant and efficient approach because it is based on growth advantage of host cells harboring stereoselective mutants, but devising such selection systems is very challenging. They must be designed so that the host organism profits from the presence of an enantioselective variant. Progress in this intriguing research area is summarized in this review, which also includes some examples of display systems designed for enantioselectivity as assayed by fluorescence-activated cell sorting (FACS). Although the combination of display systems and FACS is a powerful approach, we also envision innovative ideas combining metabolic engineering and genetic selection systems with protein directed evolution for the development of highly selective and efficient biocatalysts.