Lipids from genetically-
Lipid Snippets is a regular Inform column that features select content
from The AOCS Lipid Library ( http://lipidlibrary.aocs.org/).
Text excerpted from http://lipidlibrary.aocs.org/Biochemistry/
content.cfm?ItemNumber=41495; full version includes tables
Remarkable process has been made during
the last decade regarding genetic optimization
of E. coli as well as cyanobacteria to overpro-duce and secrete free fatty acids (FFA)s.
The secretion of fatty acids into the cultivation medium
would eliminate the effort of harvesting, drying and sol-vent-extracting lipid-containing cells and, thus, greatly reduce
the overall production costs of bacterial lipids. Although not
naturally capable of accumulating lipids, E. coli is an attractive
production organism as its lipid metabolism has been examined in the most detail amongst prokaryotes and numerous
tools have been well established for its genetic modification.
Thus, by overexpression of genes involved in fatty acid de
novo synthesis and/or blocking genes of fatty acid degradation, significant fatty acid overproduction can be achieved. The
overexpression of (native or recombinant) thioesterases then
releases the acyl residue from acyl binding protein (ACP) and
FFA accumulate. Up to 4. 8 g FFA/L has already been obtained
by engineered E. coli strains using glycerol or glucose in fed-batch fermentation, or even 3. 8 g/L from woody biomass
The cyanobacterium, Synechocystis sp. SD277, was extensively modified by deleting several genes involved in competing processes (such as the undesirable synthesis of PHB or
cyanophycin instead of fatty acids) or to weaken the cell wall
integrity to facilitate FFA release into the medium. After overexpressing several thioesterase genes, this engineered strain
produced up to 200 mg FFA/L. Although the yield is far below
the values obtained with E. coli, this process would nevertheless be much more sustainable because CO2 can serve as the
sole carbon source.
R. opacus can accumulate large amounts of TAG. Thus, most
genetic engineering approaches have been aimed at extending
its substrate range to enable a more sustainable lipid production from glycerol, (hemi)cellulose-derived sugars or hydrolyzed plant material. By means of targeted modifications and
an adaptive evolution strategy, more than 50% (w/w) TAG
(corresponding to ca. 16 g/L) was achieved using corn stover
hydrolysate as sole carbon source. These yields are significantly above the amounts obtained by wild-type R. opacus with different hydrolyzed plant materials (of approx. 28%
fatty acids) which clearly underlines the importance of strain
optimization for a competitive bacterial lipid production from
a challenging feedstock.
A number of studies also focused on establishing TAG
accumulation in E. coli; for example by combining increased
fatty acid and diacylglycerol synthesis with a recombinant
expression of essential wax ester synthase/acyl coenzyme A