Recent Advances In Aldolase-Catalyzed Synthesis Of Unnatural Sugars And Iminocyclitols

Facile chemoenzymatic methods for the synthesis of a variety of D- and L- iminocyclitols or L-sugars have been developed. The practicality of formerly reported methods using dihydroxyacetone phosphate (DHAP) aldolase was limited by the high cost and instability of DHAP. Here we discuss three strategies toward the facile synthesis of sugar analogues using DHA(P) aldolases from readily available non-phosphoryrated donor substrates.(1) Directed evolution of the L-rhamnulose 1-phosphate aldolase (RhaD) was employed to alter the donor substrate specificity of RhaD aldolase from DHAP to DHA. In vivo selection for the directed evolution using genetically engineered E. coli strain was constructed.(2) RhaD aldolase was found to accept non-phosphorylated dihydroxyacetone (DHA) as a donor substrate in the presence of borate. We applied this discovery to develop a practical one-step synthesis of L-fructose and two-step synthesis of L-iminocyclitols.(3) A one-pot synthesis was achieved using the recently discovered D-fructose 6-phosphate aldolse (FSA). FSA utilized DHA, hydroxyacetone, and 1-hydroxy-2-butanone as donor substrates to allow the synthesis of a variety of novel D-iminocyclitols in a concise fashion.