Microbial production of short and medium chain esters: Enzymes, pathways, and applications.

Sustainable production of bulk chemicals is one of the major challenges in the chemical industry, particularly due to their low market prices. This includes short and medium chain esters, which are used in a wide range of applications, for example fragrance compounds, solvents, lubricants or biofuels. However, these esters are produced mainly through unsustainable, energy intensive processes. Microbial conversion of biomass-derived sugars into esters may provide a sustainable alternative. This review provides a broad overview of natural ester production by microorganisms. The underlying ester-forming enzymatic mechanisms are discussed and compared, with particular focus on alcohol acyltransferases (AATs). This large and versatile group of enzymes condense an alcohol and an acyl-CoA to form esters. Natural production of esters typically cannot compete with existing petrochemical processes. Much effort has therefore been invested in improving in vivo ester production through metabolic engineering. Identification of suitable AATs and efficient alcohol and acyl-CoA supply are critical to the success of such strategies and are reviewed in detail. The review also focusses on the physical properties of short and medium chain esters, which may simplify downstream processing, while limiting the effects of product toxicity. Furthermore, the esters could serve as intermediates for the synthesis of other compounds, such as alcohols, acids or diols. Finally, the perspectives and major challenges of microorganism-derived ester synthesis are presented.