Abstract
Backgroundn-Butyraldehyde is a high-production volume chemical produced exclusively from hydroformylation of propylene. It is a versatile chemical used in the synthesis of diverse C4–C8 alcohols, carboxylic acids, esters, and amines. Its high demand and broad applications make it an ideal chemical to be produced from biomass.ResultsAn Escherichia coli strain was engineered to produce n-butyraldehyde directly from glucose by expressing a modified Clostridium CoA-dependent n-butanol production pathway with mono-functional Coenzyme A-acylating aldehyde dehydrogenase (Aldh) instead of the natural bifunctional aldehyde/alcohol dehydrogenase. Aldh from Clostridium beijerinckii outperformed the other tested homologues. However, the presence of native alcohol dehydrogenase led to spontaneous conversion of n-butyraldehyde to n-butanol. This problem was addressed by knocking out native E. coli alcohol dehydrogenases, significantly improving the butyraldehyde-to-butanol ratio. This ratio was further increased reducing media complexity from Terrific broth to M9 media containing 2% yeast extract. To increase production titer, in situ liquid–liquid extraction using dodecane and oleyl alcohol was investigated. Results showed oleyl alcohol as a better extractant, increasing the titer of n-butyraldehyde produced to 630 mg/L.ConclusionThis study demonstrated n-butyraldehyde production from glucose. Through sequential strain and condition optimizations, butyraldehyde-to-butanol ratio was improved significantly compared to the parent strain. Results from this work may serve as a basis for further development of renewable n-butyraldehyde production.
Highlights
Commodity chemical industry relies almost entirely on non-renewable petroleum and other fossil fuel resources, leading to environmental pollutions and the inevitable depletion crisis
Selection of CoA‐acylating aldehyde dehydrogenase for n‐butyraldehyde production n-Butyraldehyde is an intermediate in the Clostridium CoA-dependent n-butanol production pathway [9, 13, 14] (Fig. 1b)
Based on the sequence of aldh from C. beijerinckii [18], we selected two additional homologues from C. saccharolyticum and C. saccharoperbutylacetonicum, as well as a mutant aldh from C. beijerinckii which we isolated previously in our lab. These four aldh genes were individually cloned into synthetic operons with the genes necessary to convert acetyl-CoA to butyryl-CoA (Fig. 1b). These synthetic operons were driven by native E. coli promoter of ack and adhE genes, Pack and PadhE, respectively, which have been previously shown to produce higher titers of butanol compared to using IPTGinducible PLlacO1 promoter [19]
Summary
Commodity chemical industry relies almost entirely on non-renewable petroleum and other fossil fuel resources, leading to environmental pollutions and the inevitable depletion crisis. Renewable chemical production through microbial conversion of biomass is an attractive direction for sustainability. Chemicals natural to biological production are limited in quantity and type. Synthetic metabolic pathways are designed and engineered into microorganisms for increasing both the amounts and types of chemicals accessible to bio-based conversion. N-Butyraldehyde is a large volume chemical produced exclusively from hydroformylation of propylene (Fig. 1a). Biological production of aldehydes is limited due to toxicity and reactivity. While few aldehyde products have been produced by engineered microbes [3,4,5,6], the biochemical repertoire for aldehydes needs to be expanded.
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