Food Lipids: Chemistry, Nutrition, and Biotechnology

6. Desaturation
The dominant fatty acid to be exported from the plastid in plants is oleic acid
(18:1�9). It is formed by the action of �stearoyl-ACP desaturase (�9DES) on
18:0-ACT (Table 3 and Fig. 3). The �9DES is a soluble, homodimeric diiron-oxo
enzyme localized in the stroma of the plastid (52,95,96). The soluble nature of
�9DES is a unique feature of this plant enzyme relative to desaturases in general
in higher organisms (10). Desaturation is mediated through electron transfer from
NADPH (or reducing equivalents from PS I/II in chloroplasts) → ferredoxin-NADPH
reductase → ferridoxin → desaturase � O2 (10,93). Although the �9DES is 100
times more active toward 18:0-ACP than 16:0-ACP, it will also act on 14:0- and
16:0-ACP, to yield myristoleic (14:1�9)- and palmitoleic (16:1�9)-ACP derivatives.
Two isoforms have been identified in sesame (Sesamum indicum), with one form
restricted to seeds (97).
Seeds of some species of Araliaceae, Garryaceae, and Umbelliferae, such as
coriander (Coriandrum sativum), dill (Anethum graveolens), parsley (Phytophthora
megasperma), and carrot (Daucus carota), are known to yield oils unusually rich in
petroselenic acid (18:1�6) (Table 2 and Refs. 24 and 25). This can be explained by
the presence of �4 acyl-ACP desaturase (�4DES) (98,99). The �4DES is believed
to be a stromal enzyme (based on homology to the �9DES) that acts principally on
16:0-ACP. Subsequent elongation by KAS II (or possibly, an unidentified KAS
isoform) gives rise to 18:1�6. Conversion of 18:1�6 by a cleavage process would
provide an industrial source of lauric and adipic acids.
7. Termination
Termination reactions are mediated by thioesterase (oleoyl-ACP hydrolase or TE)
or acyl-ACP acyltransferase activities (Fig. 3).
a. Acyl-ACP Hydrolysis. Thioesterases (TEs) yield the corresponding fatty acid
and ACP, and the dominant TE isoform in plants exhibits a 5- to 10-fold preference
for 18:1�9-ACP over 16:0- and 18:0-ACP (53,54,95). This TE isoform is required
of all plants to provide a ‘‘housekeeping’’ function (100) and to channel 18:1�9 toward further desaturation processes for incorporation into polar glycerolipids to
serve as functional units of cellular membranes (13,15,100). This specificity accounts
for the preponderance of 16:0 and 18:1�9 (and 18:2�9,12/18:3�9,12,15), and combined
with the �9DES, a lack of 18:0 in plant lipids. However, this TE does not account
for the presence of shorter chain length fatty acids that exist in oils of tissues of
coconut, palm kernel, and Cuphea spp.
The existence in plants of medium chain TE isoforms has long been speculated
(74), partly on the basis of an extrapolation of a medium chain TE (and transcylase)
activity in lactating mammary gland tissues (101). However, the existence of medium
chain TE isoforms in plants has been confirmed only in the last 5 years (102–104),
and the currently recognized diversity of TE isoforms in plant FAS systems may be
one of the most exploitable traits. Some TE isoforms, such as that from California
bay [Umbellurlaria californica (102,103)], Cuphea wrightii (58), camphor (Cinnamomum
camphora), and elm (Ulmus americana) (104), are selective for hydrolyzing
10:0- and 12:0-ACP, resulting in accumulation of oils rich in lauric acid. The TE
isoform(s) in coconut is more broadly selective for 8- to 12-carbon acyl-ACP substrates
(104), and two TE isoforms from Cuphea palustris have selectivities toward
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