Novel Pathway Designs for Production of Value-Added Biochemicals

The growing interest in a “biomass-based” economy has led to new efforts to construct and improve microorganisms capable of producing chemicals.  The current focus is largely on liquid biofuels; however, a successful “biorefinery” is likely to be a mixed-product facility, with many compounds produced from one or more biomass-derived feeds.  Identifying methods for the production of both novel biofuels and “value-added” compounds is both a challenge and an opportunity.  The potential for biological conversion of feedstocks to bulk chemicals is enhanced by the availability of tools and techniques from the established discipline of Metabolic Engineering, which has enjoyed tremendous successes in the development of highly productive microorganisms for a variety of products of interest.  We can also gain insights from Biocatalysis, where the choice of enzymes to mediate biotransformation of chemical substrates is based largely on consideration of the required functional group conversion without being limited by prior evidence of transformation of the full structure.  This “retro-biosynthetic design” approach is aided by advancements in the development of new tools under the umbrella of Synthetic Biology that facilitate re-engineering of biological systems. 

Our group has applied principles from each of these intellectual arenas towards the design and construction of novel biosynthetic pathways for specified target compounds, in particular, 3-hydroxyalkanoic acids.  Hydroxyacids and lactones are versatile, chiral compounds that can readily be modified into several useful derivatives.  Specifically, these compounds see use in the synthesis of antibiotics, β- and γ-aminoacids and peptides, in fragrances, and as chiral synthetic building blocks.  These compounds can also be used directly as nutritional supplements and can be polymerized into biodegradable polyesters with interesting physical properties.  In this work, we describe a novel microbial system for the biosynthesis of several 3-hydroxyalkanoic acids and lactones through the condensation of various acyl-CoA moieties as well as the various tools we use to manipulate acyl-CoA biochemistry.

One product of particular importance made in this work, 3-hydroxybutyrolactone (3-HBL), is listed in a 2004 report from the Department of Energy as one of the top ten value added chemicals from biomass.  The compounds is frequently used in the pharmaceutical industry as the central building block for many blockbuster drugs like Zetia® and in most statins (cholesterol-reducing drugs) such as atorvastatin and rosuvastatin.  There are no known enzymes or metabolic pathways, natural or engineered, for the biosynthesis of 3-HBL.  Here we describe the biosynthesis of (S)-3-HBL and its hydrolyzed form 3,4-dihydroxybutyric acid (DHBA) in E. coli.  Work will be presented on the development of this 3-HBL pathway as well as ongoing efforts to improve pathway productivity and extend it towards other novel compounds.