Food Lipids: Chemistry, Nutrition, and Biotechnology

enzyme with high activity for placing laurate into the second position. When the
cDNA corresponding to the mRNA for the coconut endosperm LPAT was cloned,
adapted to canola gene expression controlling elements, and transferred into canola
along with the bay tree thioesterase gene construct, the resulting oil from transgenic
seeds contained laurate in all three positions of the triacylglycerols. Thus, one type
of lauric oil was obtained by engineering canola with the bay tree thioesterase enzyme,
and a second type of oil was obtained by combining the bay tree lauroyl–
ACP thioesterase with the coconut endosperm LPAT in seed of transgenic canola.
It has also been possible to create novel oils in seeds of transgenic plants with
increased levels of other saturated fatty acids of chain lengths less than 16 carbons.
By using cDNA clones derived from mRNA from different species of New World
genus Cuphea plants, seed oils enriched in C8 and C10 fatty acids have been obtained.
Using cDNA clones derived from mRNA from developing seed of Cuphea
palustrus or from nutmeg, oils enriched in C14 fatty acids have been obtained. The
latter myristate-type oils also tend to have higher (C16:0 (palmitic) levels. Again,
the transgenic canola triacylglycerols tend to have these newly introduced, engineered
fatty acids in the first and third positions on the glycerol backbone. Thus,
these oils and fats may have special functional properties due to the underlying
structure.
2. Naturally Solid Fats
The temperate zone oilseed crops tend to have storage lipids rich in unsaturated fatty
acids, and thus liquid oils that are in themselves not suitable for products like shortenings
and margarine. This shortcoming in typically addressed by a postharvest treatment
of partial hydrogenation of vegetable oils. Partial hydrogenation does carry
some incremental cost and generally results in the introduction of trans-unsaturated
fatty acids, which may be undesirable in foods for health reasons. Of the tropical
tree oils, only oil palm is produced on a scale large enough to provide a ‘‘fat’’ fraction
rich in palmitic acid as a hardstock. Illipe, shea, and sal are tropical trees with seeds
rich in the C18 fatty acid stearate, which contributes more solids to a fat; however,
such tropical fats are limited in availability and generally too expensive to use as
hardstock ingredients. Inspection of the pathway for biosynthesis of fatty acids reveals
that both palmitic and stearic fatty acids are precursors to the unsaturated fatty
acids making up the bulk of the oil found in seeds from temperate zone crops. Not
surprisingly, then, it has been possible to engineer the pathway so that fewer unsaturated
fatty acids are made and more of the precursor palmitic and stearic acids
accumulate in the triacylglycerols.
Numerous thioesterase enzymes have been identified by cloning cDNAs from
plant mRNAs that share DNA homology with the California bay tree thioesterase
cDNA described above. Every plant so far examined appears to have an enzyme
with high activity on oleoyl–ACP as a substrate, and this enzyme presumably is the
basic thioesterase in the fatty acid biosynthesis pathway shown in Figure 1. The
oleoyl–ACP thioesterase typically has less but significant activity on both palmitoyl–
ACP and stearoyl–ACP substrates. Another class of thioesterase enzymes has been
found in many plants, however, with enhanced activity on palmitoyl–ACP substrates;
genetically engineered overexpression of such enzymes in seed of transgenic canola
plants results in seed oils enriched in palmitic acid. On the other hand, thioesterase
enzymes with low activity on palmitoyl–ACP substrates and relatively high activity
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.