Environmental Engineering Reference
In-Depth Information
yields farnesane, which has a cetane number of 58 and good cold-flow properties and
can be blended up to 35 vol.%. In contrast, pentadecane has a cetane number of 95 and
may be used directly as fuel. Both farnesane and pentadecane have been reported to be
produced extracellularly. The main potential advantages of these routes are the extra-
cellular production, making cell retention possible and facilitating product recovery
in a separate lipid phase, and the possibility of tailoring the type of product obtained.
13.5.2.1 Fatty Acid Ethyl Esters (FAEE): in vivo Transesterification
This route
aims at the complete synthesis of fatty acid ethyl esters (FAEE) from renewable car-
bon sources. This has been achieved by modifying
E. coli
in two steps: firstly, by
establishing a pathway for ethanol synthesis and, secondly, by including an enzyme
for the transesterification of ethanol and the fatty acid moiety of acyl-coenzyme
A (acyl-CoA) to FAEE. The latter is especially important, since it saves the costly
chemical transesterification step of first-generation biodiesel production.
Results to date have shown intracellular FAEE accumulation of up to 26% of dry
cell mass (Kalscheuer et al., 2006). However, substantial FAEE biosynthesis was only
achieved when fatty acids were also present in the feedstock. Current research is
looking for alternative host microorganisms, such as oleaginous microorganisms,
so that the flux of fatty acids can be directed from TAG toward FAEE biosynthesis.
13.5.2.2 Production of Isoprenoids
Isoprenoids are a diverse class of chemicals
derived from isopentenyl pyrophosphate (IPP) and comprise one or more five-carbon
isoprene units. Isoprenoids are involved in many cellular processes including
respiration, cell membrane structure, signaling, photosynthesis, cell defense, and
vitamin production. Plant isoprenoids, in particular, have long been used as flavor
and fragrance agents.
In nature, two independent biosynthetic pathways, the mevalonate (MEV) pathway
and the methylerythritol phosphate (MEP) pathway, are responsible for the production
of the key intermediate, IPP. Although both pathways exist in plants, the MEV
pathway is responsible for all the isoprenoid production in archaea (prokaryotic
single-cell microorganisms), some bacteria, and most eukaryotes (including the yeast
S. cerevisiae
), while the MEP pathway is present in most bacteria (including
E. coli
)
and green algae. The biosynthesis of farnesene, however, involves the enzymatic
conversion of one of the pathway intermediates, farnesyl pyrophosphate (FPP). In
S. cerevisiae
, this has been achieved by incorporating the enzyme from a plant,
Artemisia annua
, into the pathway and by overexpressing the enzymes from the native
MEV pathway. Farnesene is secreted to the production medium and after recovery is
converted by chemical hydrogenation to farnesane. This approach has been demon-
strated at pilot and production scales (Chandran et al., 2011). It is also being used for
the production of flavors and fragrances.
13.5.2.3 Production of Alkanes and Alkenes
This route focuses on the biosynthe-
sis of long-chain hydrocarbons, such as pentadecane, derived from a fatty acid path-
way. This has been achieved by incorporating an alkane biosynthesis pathway from
cyanobacteria in
E. coli
(Schirmer et al., 2010). The pathway converts the fatty
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