Food Lipids: Chemistry, Nutrition, and Biotechnology

12-MO, the cDNA for the enzyme has been isolated (109) and rapeseed transformants
have been prepared for field evaluation (148).
Fatty acid oxygenating and desaturating enzymes in plants appear to share
similar mechanisms. The oleate epoxidase (12-EPOX) (in Euphorbia lagascae
and, likely, Vernonia spp.) causes epoxidation of 18:1 �9 to yield vernolic acid
(18:1 �9,12/13-epoxy) and requires NADPH and PC-linked 18:1 �9 (149). Furthermore, antibodies
to cytochrome b 5 inhibited both epoxidation and desaturation (see Sec. IV.C)
reactions with PC-linked 18:1 �9.
B. Elongation
Chain elongation steps are responsible for the production of the series of gondoic
(20:1 �11), erucic (22:1 �13), and nervonic (24:1 �15) acids in selected Brassica, Crambe,
and related species, in some of the same fatty acids plus 20:1 �5 and 22:2 �5,13 in
meadowfoam (Limnanthes alba), in the series of arachidic (20:0), behenic (22:0),
and lignoceric (24:0) acids in groundnut (Arachis hypogaea) oils, and in other saturated
and monounsaturated long chain fatty acids that become incorporated as surface
waxes (28). Very long chain fatty acid (VLCFA) synthesis has been primarily
studied in seed of rape, meadowfoam, and jojoba (Simmondsia chinensis), and in
epidermal tissue of leek (Allium porrum) (150). Generally, only saturated and monounsaturated
acyl-CoA derivatives are substrates for elongation in seeds. The basic
process of elongation (of 18:0 or 18:1 �9) involves successive two carbon additions
supplied by malonyl-CoA to the acyl-CoA chain in the presence of NAD(P)H and
membraneous acyl-CoA elongase (ACE) enzymes (Table 4 and Fig. 4) (151–154).
[The jojoba elongation system is distinct in that it recognizes both ACP and CoA
thioesters of the lengthening acyl chain as substrate (150).] For surface wax formation,
the subsequent actions of a reductase and a ligating enzyme are required, as
is a means of transporting the acyl chains to the cellular surface for wax deposition
(146,150).
It is not certain whether a single or multiple ACE(s) is/are involved in lengthening
18:X to 24:X, but the various elongation steps appear to share common functional
units (152). Studies with Arabidopsis mutants revealed that the product of the
FAE1 (fatty acid elongase) gene was a synthetase/condensing enzyme, based on its
homology with KAS III and E. coli (155). Furthermore, this ACE was concluded
to be singularly capable of both condensation steps from 18 to 22 acyl carbons.
Similar to FAS systems, the complete reduction cycle and intermediate steps are
believed to occur after each condensation step (150). Indeed, a partially purified ACE
system possessed the full complement of expected condensing and reducing activities
(156) (as shown in Fig. 1 for FAS). The emerging organizational structure of ACE
systems is based on multienzyme membrane complexes held by strong interactions
with the acyl-CoA substrates partitioning at the interface to gain access to the elongase
(150). The ACE in Arabidopsis, which is expressed in developing seeds and
not leaves (155), is likely modulated by the surrounding membrane environment. At
least two contentious issues remain regarding the nature of ACE action. It has been
questioned whether acyl-CoA are the putative substrates, and instead, lipid-linked
acyl groups or free acids have been suggested as possible substrates for elongation
in oil bodies of rape (157). A second and, perhaps, related issue is the ambiguity
surrounding whether a covalent acyl-enzyme intermediate is involved in the reaction
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.