Asymmetric reductions of functionalized alkenes provide a rich variety of chiral building blocks. Metal-catalyzed asymmetric hydrogenations of olefins conjugated with aldehydes, ketones, esters or nitro groups are generally problematic. Alkene reductase enzymes offer a useful, new synthetic method for asymmetric olefin reductions. By engineering these enzymes to carry out a subsequent stereoselective nitro aldol (Henry) reaction, their synthetic utility can be increased dramatically. Old yellow enzyme, known to catalyze enone reduction in the presence of NADPH, will be an additional starting point for the development of enzymes as stereoselective catalysts for activated alkene reductions. Thirty-one known and putative alkene reductase genes in the sequence database will be cloned and expressed. The resulting proteins will be characterized with respect to substrate- and stereoselectivities toward a set of functionalized alkenes, providing the first information on the useful range of each protein, particularly with respect to the acceptable alkene activating group (conjugated ketone or aldehyde, ester, acid, nitrile, nitro group). Protein engineering will allow nitroalkene reduction to be followed by a Henry condensation. Finally, alkene reductases (both wild-type and engineered variants) will be applied to the asymmetric synthesis of beta-2-amino acids as well as alpha-hydroxy-beta-amino acids and beta-hydroxy-gamma-amino acids from simple, achiral starting materials.
With this award, the Organic and Macromolecular Chemistry Program is supporting the research of Professor Jon D. Stewart, of the Department of Chemistry at the University of Florida. Professor Stewart and his students are developing methods to adapt and exploit the catalytic capability of enzymes, recruiting their ability to carry out selective and efficient reactions. Enzyme catalysts may be modified to act on non-natural substrates, allowing the addition of new catalytic functions to an existing active site. The project is expected to provide a diverse collection of enzyme catalysts, easily stored and easily employed for organic synthesis, as well as catalysts that facilitate rapid establishment of molecular complexity. Carbon-carbon bond formation is a cornerstone of organic synthesis, and biocatalytic strategies will add appreciably to the arsenal of tools available for the construction of these bonds.
