(acyl-CoA):diacylglycerol acyltransferase (WS/DGAT) genes.
However, the achievable yields still need to be significantly
improved in order to compete with natural lipid accumulators.
FATTY ACID ETHYL ESTER SYNTHESIS
The vast majority of biodiesel (fatty acid methyl ester, FAME) is
currently produced with methanol derived from fossil sources,
although ethanol can alternatively be utilized to gain biodiesel
made from fatty acid ethyl esters (FAEE) which can show superior physicochemical properties compared to FAME. In 2006,
a strategy was developed to completely synthesize FAEE de
novo (“microdiesel”) from unrelated carbon sources in E. coli.
In this first approach and follow-up studies, the overexpression
of a WS/DGAT gene was usually combined with the ethanol
synthesis pathway from Zymomonas mobilis (adhB encoding
alcohol dehydrogenase B, and pdc for pyruvate decarboxylase).
Similar to strategies employed for FFA overproduction, increasing fatty acid de novo synthesis and blocking the beta-oxida-tion of fatty acids, could further increase FAEE levels in more
So far, highest FAEE amounts of 1. 5 g/L were only
achieved with glucose and in small-scale cultivations. A major
disadvantage of E. coli is its inability to metabolize lignocellu-
lose, although a first proof-of-principle could be achieved by
constructing an optimized strain which utilizes switchgrass
Another promising approach is the synthesis of branched-chain fatty acid alkyl esters from (branched) fatty acids and
branched short-chain alcohols (e.g. isoamyl alcohol or isobuta-nol), exploiting the biosynthesis pathway for branched-chain
amino acid synthesis. These esters show improved properties
at low temperatures, under which conventional biodiesel usually has a rather poor performance. However, the synthesis
yield of branched FAEE still needs to be considerably increased.
Alternatively, bacteria which naturally incorporate branched
fatty acids into TAGs, such as Streptomyces isolate G25, could
be advantageous for the production of biodiesel with a mixture of straight- and branched-chain fatty acid residues.
Annika Röttig is with the Institut für Molekulare Mikrobiologie
und Biotechnologie, Westfälische Wilhelms-Universität Münster,
D-48149 Münster, Germany.
Alexander Steinbüchel is faculty in environmental sciences at
King Abdulaziz University, Jeddah, Saudi Arabia.
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