Tuesday, April 23, 2013

Making long chain hydrocarbons directly from E. Coli

Work of John Love of the University of Exeter in England published in PNAS on April 22 on making long chain hydrocarbons from E. Coli is featured in news reports, some coming via Scientific American.

A post on Scientific American titled Gut Microbe Makes Diesel Biofuel contains the text

That hardiness also seems to be helping the bacterium survive its own production of such longer-chain hydrocarbons, which could have proved toxic to the microbes, in the way brewer's yeast cells are killed off by the alcohol they ferment.

Long chain hydrocarbons are a lot less toxic than ethanol, so the analogy in the article is not a good one.

Separately, of course, is what "food" for the E. coli is cheap enough to make cheap fuel. The SA article notes:

The E. coli are currently fed on sugar and yeast extract, which suggests that the resulting fuel would be expensive compared with the kind refined from oil found in the ground. "We are hopeful that we could change their diet to something less valuable to humanity," Love suggests. "For example, organic wastes from agriculture or even sewage."

The abstract at PNAS states:

Biofuels are the most immediate, practical solution for mitigating dependence on fossil hydrocarbons, but current biofuels (alcohols and biodiesels) require significant downstream processing and are not fully compatible with modern, mass-market internal combustion engines. Rather, the ideal biofuels are structurally and chemically identical to the fossil fuels they seek to replace (i.e., aliphatic n- and iso-alkanes and -alkenes of various chain lengths). Here we report on production of such petroleum-replica hydrocarbons in Escherichia coli. The activity of the fatty acid (FA) reductase complex from Photorhabdus luminescens was coupled with aldehyde decarbonylase from Nostoc punctiforme to use free FAs as substrates for alkane biosynthesis. This combination of genes enabled rational alterations to hydrocarbon chain length (Cn) and the production of branched alkanes through upstream genetic and exogenous manipulations of the FA pool. Genetic components for targeted manipulation of the FA pool included expression of a thioesterase from Cinnamomum camphora (camphor) to alter alkane Cn and expression of the branched-chain α-keto acid dehydrogenase complex and β-keto acyl-acyl carrier protein synthase III from Bacillus subtilis to synthesize branched (iso-) alkanes. Rather than simply reconstituting existing metabolic routes to alkane production found in nature, these results demonstrate the ability to design and implement artificial molecular pathways for the production of renewable, industrially relevant fuel molecules.

Of yields:

In the system presented here, alkane titers
from both the cyanobacterial pathway and the artificial CEDDEC
pathway were ∼2–5 mg·L−1 24 h after induction (Fig. 1 and
Fig. S5). These values are in agreement with a recent report for
expression of the cyanobacterial pathway in BL21* (DE3) cells

full article

Reference 31 in the article by Love is Akhtar MK, Turner NJ, Jones PR (2013) Carboxylic acid reductase is a versatile enzyme
for the conversion of fatty acids into fuels and chemical commodities. Proc Natl Acad
Sci USA 110(1):87–92. [Akhtar has an affiliation with the University of Manchester .] The abstract of the Akhtar paper includes the
text This concept was applied in vivo, in combination with a chain-length-specific thioesterase, to engineer Escherichia coli BL21(DE3) strains that were capable of synthesizing fatty alcohols and alkanes. A fatty alcohol titer exceeding 350 mg·L−1 was obtained in minimal media supplemented with glucose. Moreover, by combining the CAR-dependent pathway with an exogenous fatty acid-generating lipase, natural oils (coconut oil, palm oil, and algal oil bodies) were enzymatically converted into fatty alcohols across a broad chain-length range (C8–C18). Together with complementing enzymes, the broad substrate specificity and kinetic characteristics of CAR opens the road for direct and tailored enzyme-catalyzed conversion of lipids into user-ready chemical commodities. Note the Akhtar article was available in 2012: Published online before print December 17, 2012, doi: 10.1073/pnas.1216516110
PNAS December 17, 2012


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