Abstract
In this work, we describe the construction of a synthetic metabolic pathway enabling direct biosynthesis of 1,3-propanediol (PDO) from glucose via the Krebs cycle intermediate malate. This non-natural pathway extends a previously published synthetic pathway for the synthesis of (L)-2,4-dihydroxybutyrate (L-DHB) from malate by three additional reaction steps catalyzed respectively, by a DHB dehydrogenase, a 2-keto-4-hydroxybutyrate (OHB) dehydrogenase and a PDO oxidoreductase. Screening and structure-guided protein engineering provided a (L)-DHB dehydrogenase from the membrane-associated (L)-lactate dehydrogenase of E. coli and OHB decarboxylase variants derived from the branched-chain keto-acid decarboxylase encoded by kdcA from Lactococcus lactis or pyruvate decarboxylase from Zymomonas mobilis. The simultaneous overexpression of the genes encoding these enzymes together with the endogenous ydhD-encoded aldehyde reductase enabled PDO biosynthesis from (L)-DHB. While the simultaneous expression of the six enzymatic activities in a single engineered E. coli strain resulted in a low production of 0.1 mM PDO from 110 mM glucose, a 40-fold increased PDO titer was obtained by co-cultivation of an E. coli strain expressing the malate-DHB pathway with another strain harboring the DHB-to-PDO pathway.