Progress in Stereoselective Construction of C-C Bonds Enabled by Aldolases and Hydroxynitrile Lyases

The creation of C-C bonds is an effective strategy for constructing complex compounds from simple synthetic blocks. Although many methods have been developed for C-C bond construction, the stereoselective creation of new C-C bonds remains a challenge. The selectivities (enantioselectivity, regioselectivity, and chemoselectivity) of biocatalysts are higher than those of chemical catalysts, therefore biocatalysts are excellent candidates for use in stereoselective C-C bond formation. Here, we summarize progress made in the past 10 years in stereoselective C-C bond formation enabled by two classic types of enzyme, aldolases and hydroxynitrile lyases. The information in this review will enable the development of new routes to the stereoselective construction of C-C bonds.

Engineering aldolases for asymmetric synthesis

Formation of carbon-carbon bonds is central to synthetic chemistry. The aldol reaction provides the chemistry to fuse a nucleophilic enolate with an electrophilic aldehyde to form a new CC bond between two newly formed asymmetric centers. A major challenge in the reaction is steering the stereochemistry of the product. Aldolases are lyases that catalyze aldol reactions as well as the retro-aldol cleavage, and are abundant in cellular metabolism. Due to the often exquisite stereoselectivity in aldolase catalyzed carboligation reactions, these enzymes are gaining increased interest as potentially important tools in asymmetric synthesis of new useful compounds. Fructose 6-phosphate aldolase from Escherichia coli (FSA) is of special interest because of its very unusual independence of phosphorylated reactant substrates. The current text describes the protein engineering of FSA, applying principles of directed evolution, for the generation, production and characterization of new aldolase variants. A range of new enantiopure polyhydroxylated compounds were produced applying isolated FSA variants.

The engineering of decameric d-fructose-6-phosphate aldolase A by combinatorial modulation of inter- and intra-subunit interactions

The combinatorial modulation of inter- and intra-subunit interactions of decameric d-fructose-6-phosphate aldolase A (FSAA) generated a triple-site variant I31T/Q59T/I195Q FSAA with 27- to 278-fold improvement in activity towards target heteroaromatic aldehydes. X-ray crystallographic data and molecular dynamics simulations ascribed the enhanced activity to the pronounced flexibility of the interface region between subunits, the expanded substrate entrance and binding pocket, and enhanced proton transfer, unambiguously demonstrating the efficiency of this strategy for engineering multimeric enzymes.