Structure-Guided Engineering of D-Fructose-6-Phosphate Aldolase for Improved Acceptor Tolerance in Biocatalytic Aldol Additions

A structure‐guided redesign of D‐fructose‐6‐phosphate aldolase from Escherichia coli (FSA) was devised for improving the acceptor tolerance towards α‐substituted and conformationally constrained aldehydes. FSA A129S/R134X/A165G/S166G and L107Y/A129G/R134X/A165G/S166G variants, where X was R, V, P, or S, were the most suited biocatalysts for dihydroxyacetone, hydroxyacetone and glycolaldehyde additions to 20 α‐substituted N‐Cbz‐aminoaldehydes (Cbz=benzyloxycarbonyl) including pyrrolidine and piperidine derivatives. Full kinetic stereocontrol for si‐si face addition of the aldolase‐bound nucleophile to the N‐Cbz‐aminoaldehyde carbonyl was observed, furnishing the corresponding D‐threo configured aldol adduct in >95:5 dr as assessed by NMR. After reductive amination, 47 different iminocyclitols were identified and characterized. In some examples partial racemization of the corresponding aldehyde was observed, which appears to be produced mostly during the aldol addition reactions.