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Issue 11, 2012
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Photosynthetic production of 2-methyl-1-butanol from CO2 in cyanobacterium Synechococcus elongatus PCC7942 and characterization of the native acetohydroxyacid synthase

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Abstract

Direct conversion of CO2 to bio-based fuels and chemicals has emerged as a significant thrust to address the energy and environmental concerns caused by over-reliance on fossil fuels and the increasing level of atmospheric CO2. Here we report the first photosynthetic production of 2-methyl-1-butanol (2MB), an energy-dense fuel molecule, from CO2 in the genetically engineered cyanobacterium Synechococcus elongatus PCC7942. 2MB is synthesized through the isoleucine pathway by decarboxylation of 2-keto-3-methylvalerate followed by reduction and has been produced from glucose by recombinant Escherichia coli with 1-propanol and isobutanol as the major by-products. However, direct photosynthetic production of 2MB from CO2 has not been reported. In this work, introduction of a ketoacid decarboxylase (Kivd), an alcohol dehydrogenase (YqhD), and the citramalate pathway, which produces the isoleucine precursor 2-ketobutyrate (2KB), in S. elongatus PCC7942 successfully redirected the flux to 2MB biosynthesis with significant productivity (an average of 20 mg per L per day). Interestingly, the native isoleucine pathway activity was able to compete with the overexpressed Kivd activity for the same substrate 2KB, such that 1-propanol formation was minimal. Kinetic analysis of the key enzyme in the isoleucine pathway, acetohydroxyacid synthase (AHAS) from S. elongatus PCC7942, yielded a Vmax(2KB) of 1.21 ± 0.03 U mg−1 and a Km(2KB) of 1.9 ± 0.3 mM using the purified protein and demonstrated preferential selectivity towards 2KB. The final titer of 2MB reached 200 mg L−1 in 12 days with minor accumulation of other alcohols. The high in vivo activity of the native S. elongatus PCC7942 AHAS suggests the advantage of utilizing branched-chain amino acid pathways in this organism for the production of fuels and chemicals.

Graphical abstract: Photosynthetic production of 2-methyl-1-butanol from CO2 in cyanobacterium Synechococcus elongatus PCC7942 and characterization of the native acetohydroxyacid synthase

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Publication details

The article was received on 11 Aug 2012, accepted on 04 Sep 2012 and first published on 04 Sep 2012


Article type: Paper
DOI: 10.1039/C2EE23148D
Citation: Energy Environ. Sci., 2012,5, 9574-9583
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    Photosynthetic production of 2-methyl-1-butanol from CO2 in cyanobacterium Synechococcus elongatus PCC7942 and characterization of the native acetohydroxyacid synthase

    C. R. Shen and J. C. Liao, Energy Environ. Sci., 2012, 5, 9574
    DOI: 10.1039/C2EE23148D

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