Food Lipids: Chemistry, Nutrition, and Biotechnology

10-carbon and 14- to 16-carbon acyl-ACP substrates, respectively, accounting for
the bimodal distribution of 8:0 and 14:0 in the seed oil (97). Further diversity has
been recognized in a TE isoform selective for 18:1 �6 that allows for accumulation
of 18:1 �6 in seed oils of coriander and other Umbelliferae (105).
b. Acyl Transfer to Plastidic Glycerol-3-phosphate. The other possible fate of
neosynthesized acyl groups by plastidic FAS is the incorporation into functional
glycerolipids in the plastid. Acyl–ACP acyltransferases exist in plastids and include
acyl-ACP: sn-glycerol-3-phosphate acyltransferase (GPAT) and acyl-ACP: 1acyl-sn-glycerol-3-phosphate
acyltransferase (or lysophosphatidic acid acyltransferase,
LPAAT) (Fig. 3 and Refs. 15 and 35). These two enzymes serve to channel
newly synthesized acyl groups toward polar glycerolipid synthesis within the plastid
(principally for phosphatidylglycerol, galactoglycerolipid, and sulfoglycerolipid assembly)
(15,16). These plastidic acyltransferases are in competition with TE, which
mediates the release of fatty acids for export to the endoplasmic reticulum for further
processing. The relative activities and selectivities of plastidic acyl–ACP acyltransferases,
TE, �9DES, and KAS II provide the basis for partitioning of fatty acids
among competing processes or fates (Fig. 3).
The pattern of fatty acid flux that occurs provides the basis for a chemotaxonomic
classification of plants as ‘‘prokaryotic’’ or ‘‘16:3’’ plants, or alternatively,
‘‘eukaryotic’’ or ‘‘18:3’’ plants, based on the enrichment of 16-carbon acyl residues
at the sn-2 position for the former class (106,107). Plants having physiologically
relevant plastidic acyl-ACP acyltransferase activities and a specific ACP isoform
(see Sec. III.C.4) yield the ‘‘prokaryotic’’ configuration among some of the plastidic
polar glycerolipids and are characterized as ‘‘16:3’’ species. Since plastidic acyltransferases
have little to do with triacylglycerol synthesis and oil deposition, they are
not considered in much detail here and the reader is referred to reviews covering
this topic (15,106,108). However, plastidic acyltransferases are identical (same EC
numbers: see Ref. 37) to those involved in triacylglycerol assembly (see below, Sec.
V.B, Table 5), except that the plastidic forms recognize acyl-ACP, but not acyl-CoA
substrates (35).
The relevance of plastidic acyltransferase to FAS and triacylglycerol assembly
lies in their impact on the profile of fatty acids that are exported from the plastid
for triacylglycerol assembly in the endoplasmic reticulum, and in one case, they have
been subject to genetic transformation in a manner that has the potential to improve
resistance of chilling-sensitive plants (Sec. VI). In addition, the apparent strict specificity
of the plastidic GPAT to recognize the �9 double bond (105) helps ensure
that unusual fatty acids, like petroselenic acid (18:1 �6), are channeled toward triacylglycerol
assembly in the endoplasmic reticulum, and not toward plastidic polar
glycerolipid assembly. This feature maintains the profile of unsaturated 16- to 18carbon
fatty acids in membrane lipids to allow for normal physiological functioning
(13,15,23).
8. Fate of Plastidic 16:0- (18:0-) and 18:1�9-ACP Although 16- to 18-acyl carbon fatty acids are the principal products of plant FAS,
the diversity and abundance of fatty acid chain lengths of 8–24 among storage lipids
indicates that some plant species are distinguished by premature termination of fatty
acid synthesis or postsynthetic modification.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.