Simultaneous optimization of donor and acceptor substrate specificity for transketolase by a small but smart library

A narrow substrate range is a major limitation in exploiting enzymes more widely as catalysts in synthetic organic chemistry. For enzymes using two substrates, the simultaneous optimization of both substrate specificities, is also required for the rapid expansion of accepted substrates. Transketolase catalyses the reversible transfer of a C2-ketol unit from a donor substrate to an aldehyde acceptor and suffers the limitation of narrow substrate scope for widely industrial applications. Herein, transketolase from E. coli was engineered to simultaneously accept both pyruvate as a novel donor substrate, and unnatural acceptor aldehydes, including propanal, pentanal, hexanal and 3-formylbenzoic acid. Twenty single-mutant variants were firstly designed and characterized experimentally. Beneficial mutations were then recombined to construct a small but smart library. Screening of this library identified the best variant with a 9.2-fold improvement in the yield towards pyruvate and propionaldehyde, relative to WT. Pentanal and hexanal were used as acceptors to determine stereoselectivities of the reactions, which were found to be higher than 98% ee for the S configuration. Three variants were identified to be active for the reaction between pyruvate and 3-formylbenzoic acid. The best variant was able to convert 47% of substrate into product within 24 h, whereas no conversion was observed for WT. Docking experiments suggested a cooperation between the mutations responsible for donor and acceptor acceptances, that would promote the activity towards both the acceptor and donor. The variants obtained have the potential to be used for developing catalytic pathways to a diverse range of high-value products.