Food Lipids: Chemistry, Nutrition, and Biotechnology

mechanism. The absence of such an intermediate has been suggested (150), but there
is also evidence of involvement of a thioester intermediate (156); the latter possibility
is favored by the homology of the ACE to KAS III (155).
C. Desaturation
The initial desaturation step to form 18:1�9 is mediated by the soluble (stromal)
�9DES of the plastid, while the acyl chain is on the ‘‘ACP track’’ (Fig. 3 and Sec.
III.B.6). Both the plastid and the endoplasmic reticulum possess membrane desaturase
systems that yield the di- and triunsaturated fatty acids characteristic of plants.
Both membrane systems use glycerolipid-linked fatty acyl groups as substrates [an
exception is found in meadowfoam seeds, which also have an acyl-CoA desaturase
(15)]. The plastidic system acts on acyl groups linked to galactoglycerolipids, sulfoglycerolipids,
and phosphatidylglycerol as substrates and gives rise to membrane
lipids assembled in a ‘‘prokaryotic’’ configuration (see Sec. III.B.7). Since these
glycerolipids have little impact on triacylglycerol composition, the plastid desaturase
systems are not considered further.
The endoplasmic reticulum system almost exclusively uses phosphatidylcholine
(PC) as the carrier for 18:1�9 desaturation to form 18:2�9,12 and then 18:3�9,12,15 (Fig.
4). In oil-bearing seeds and fruit, a major fraction of these polyunsaturated fatty
acids becomes incorporated into storage triacylglycerols. Accordingly, the balance of
this section is devoted to the endoplasmic reticular desaturases. Much of the understanding
of the molecular biology of membrance desaturases has been derived from
work on mutants of the small mustard plant Arabidopsis thaliana (15). This organism
is suitable for genetic studies of plants because it has a relatively small genome and
mutants are easy to prepare; moreover, it is phlyogenetically related to Brassica
species, such as rape (148).
The two membrane desaturases of the endoplasmic reticulum are defined as
products of the fad2 and fad3 genes (15). Mobilization for desaturation of 18:1�9- CoA, imported from the plastid, requires the sequential action of GPAT and lysophosphatidic
acid acyltransferase (LPAAT) to form the 18:1�9-glycerol esters, and
then phosphatidic acid phosphatase (PTP) and CDP-choline:diacylglycerol cholinephosphotransferase
(CPT) to yield PC-linked 18:1�9 (20) (Fig. 4; these enzymes
are described in more detail in Sec. V, Table 5 and Fig. 5). The natures of GPAT
and LPAAT give rise to the ‘‘eukaryotic’’ configuration of glycerolipids in that saturated
units of 16 or 18 acyl carbons are virtually excluded from the sn-2 position
in favor of 18:1�9 is positioned at both sn-1,2 sites (158).
Since these membrane desaturases have been difficult to isolate (13,14,159),
much of the understanding of their properties has been based on the behavior of
microsomal preparations (14,160–162). Evidence strongly weighs in favor of multiple
desaturases, each specific for a particular site (�6, �12, �15), rather than one
desaturase capable of successive desaturation steps (14,146) (Fig. 4). The desaturases
are iron-containing enzymes that require O2 as cosubstrate (terminal electron acceptor)
and reducing equivalents transferred from NAD(P)H, likely through cytochromes
(b5 in particular) and cytochrome flavoprotein reductases (14,161,163). Sequence
analyses have revealed histidine-rich motifs, which could serve as the iron-coordinating
region of the protein (164).
For the 18:1-PC �12 desaturase, (�12DES) both sn-1,2-18:1�9 residues are
reactive, with the sn-2 residue more favorable for reaction (160,161). This pattern
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