Food Lipids: Chemistry, Nutrition, and Biotechnology

of reactivity is also supported by an Arabidopsis mutant lacking fad2 (165). The
reactivity of 18:2 �9,12-PC �15 desaturase (�15DES) is almost identical to that of
18:2 �9,12-PC as just described for the �12DES on 18:1 �9-PC (166). However, there
is also evidence for multiple forms of the �15DES, one with reactivity toward both
positions and the other reactive only on the sn-2 18:2 �9,12 residue, leading to an
enrichment in 18:3 �9,12,15 at the sn-2 position of glycerolipids. The �12DES and
�15DES are not capable of recognizing 18:2 �6,9 and 18:3 �6,9,12, respectively, as substrates
(14).
A less common membrane desaturase occurs in seeds of borage (Borago officinalis),
and also likely, evening primrose (Oenothera biennis) and black currant
(Ribes nigrum), all of which accumulate �-linolenic acid (18:3 �6,9,12) in seed oils
(Table 2). The �6 double bond is introduced by a �6-desaturase (�6DES) located
in the endoplasmic reticulum (Fig. 4, Refs. 167 and 168). The �6DES also requires
O 2 and NADH as cosubstrates and acts on PC-linked 18:2 �9,12. However, in contrast
to the positional selectivity of the �12DES and �15DES, the �6DES from borage
recognizes only the sn-2 residue of 18:2 �9,12-PC for the desaturation reaction.
The role of the desaturases in triacylglycerol assembly is elucidated further in
Sec. V. Despite inability to purify these desaturases, the identification of Arabidopsis
mutants (15,16) has allowed genetic transformation of various hosts to proceed in
attempts to modify the profile of unsaturated fatty acids in seed/fruit oils (see
Sec. VI).
V. TRIACYLGLYCEROL ASSEMBLY
A. Overview
Following plastidic biosynthesis (Fig. 1), and modification of the acyl chains in the
plastid (Fig. 3) and endoplasmic reticulum (Fig. 4), the various fatty acids are finally
destined for incorporation into storage (triacylglycerols) or functional (polar) glycerolipids.
In oil-bearing seed or fruit tissues, assembly of lipids of both types takes
place in the endoplasmic reticulum (some polar glycerolipid assembly also takes
place in the plastid of ‘‘16:3’’ plants (35) (Sec. III.B.7 and Fig. 3). With both functional
and storage lipids assembled in the endoplasmic reticulum, it quickly becomes
obvious that mechanisms must exist to allow for discrimination in the pool of fatty
acids to be used for these divergent purposes. Despite the diversity of fatty acids in
the plant kingdom (Table 2), a strict fatty acid profile among the polar glycerolipids
that are destined to be functional unit in cellular membranes must be maintained to
ensure survival of the organism (10,13,14). Thus, the divergent pathways of storage
and functional lipid assembly must have safeguards to prevent nonproprietary utilization
of fatty acids.
There are two basic routes of triacylglycerol synthesis or assembly (Fig. 5).
One route involves the exclusive use of the so-called Kennedy (169) pathway, which
operates in a rather linear fashion, drawing from the acyl-CoA pool to satisfy the
needs for triacylglycerol assembly. The other route for triacylglycerol assembly uses
the Kennedy pathway as modulated by the involvement of phosphatidylcholine (PC)
and its derivatives. Different organisms evoke these pathways to different degrees,
and this is reflected in their characteristic triacylglycerol compositions. This section
first describes regulatory features of triacylglycerol assembly and then examines the
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.