Triacylglycerols have many
attractive features as lubricants, but have limited ability to replace fossil
oil based lubricants due to poor hydrolytic and oxidative stability. In
contrast wax esters, which are esters between a fatty acid and a fatty alcohol,
have excellent resistance to hydrolysis and oxidation (300).
Different molecular species of wax
esters can display diverse lubrication properties. A lubricant should ideally
have a low melting point and high oxidation stability. Therefore, it is
desirable to have species of medium or long chain lengths having not more than
one double bond in each part of the molecule (301).
Spermaceti oil from sperm whales,
which is mainly composed of wax esters, was widely used in high performance
lubricants in the past. After the global ban of sperm whale hunting, synthetic
alternatives based on fossil oil is on focus. An alternative biological source is
jojoba, which stores wax esters in its seeds as an energy source in contrast to
all other known oil seeds, which store triacylglycerols (302). Since jojoba
is not suitable for high-yield cultivation in moderate zones, jojoba oil is
highly priced and the supply of jojoba oil is limited. Additionally, jojoba oil
is a blend of different very long-chained (mostly C20, C22 and C24)
monounsaturated wax esters, which may represent an unfavourable composition for
the development of jojoba-based technical lubricants (303).There is currently no large-scale
biological source for wax esters. Wax esters deriving from oil seed compatible
to industrial agriculture in temperate zones could represent an attractive
alternative source for industrial applications (303).
To produce plant-derived sperm oil like
liquid wax esters, Iven
et al. (303) expressed the wax
synthase from Mus musculus or Simmondsia chinensis in combination
with the fatty acid reductase from Mus musculus or Marinobacter
aquaeolei in seeds of Arabidopsis thaliana and Camelina sativa.
They demonstrate that the low-input oil seed crop Camelina sativa can
serve as a biotechnological platform for environmentally benign wax ester
production. Expression of the enzyme combinations in the Arabidopsis fae1 fad2
mutant background high in oleic acid resulted in wax ester accumulation
enriched in oleyl oleate (18:1/18:1 > 60%), suggesting that similar values
may be obtained with a Camelina high oleic acid line (303).
Chloroplast metabolism has been engineered
to divert intermediates from de novo fatty acid biosynthesis to wax ester
synthesis.To acomplish this, chloroplast targeted fatty acyl-reductases and wax
ester synthases were transiently expressed in Nicotiana benthamiana
leaves. Wax esters of different qualities and quantities were produced providing
in sights to the properties and interaction of the individual enzymes used (302).