Food Lipids: Chemistry, Nutrition, and Biotechnology

KAS, and KAR while the �-chain has ACAT, MCAT, KHAD, EAR, and an acyl-
ACP:CoA acyltransferase (instead of TE; these FAS systems produce acyl CoA
instead of fatty acid as the final product of FAS).
Type II or ‘‘dissociable’’ FAS systems are characteristic of prokaryotes (Escherichia
coli being the working example) and plants (10). The similarity of FAS in
bacteria and plants is founded on the endosymbiont or ‘‘plastid–reticular’’ theory of
evolutionary linkage between chloroplasts (and plastids in general) and the bacterial
cell (62). Because of this relationship, studies on E. coli have often been viewed as
a model for FAS in plants (11). Thus, some of the characteristic distinctions between
E. coli and plant FAS systems are identified in this chapter because these differences
help promote an understanding of the nature and control of fatty acid biosynthesis
in plants.
FAS is organized in plants by virtue of being almost exclusively localized in
plastidic compartments (63), although traces of FAS activity and ACP have been
reported in mitochondria (64,65). Despite little evidence for the association of the
unit activities of type II FAS systems as multienzyme complexes in plants in vivo
(15), there are several features of control of FAS in plants (13,15,65). Unlike type I
FAS systems, which exist as a single (or dual) isoform or protein domain, many of
the component enzymes of type II FAS systems often exist as several isoforms, with
each isoform having different properties/selectivities that affect the nature of the
specific reaction they mediate. Second, many of the individual enzymes of type II
FAS systems are extracted and purified as dimers and tetramers, suggesting that
oligomers exist in vivo and may be more subject to control than monomeric forms.
Third, the different enzymes of type II FAS systems are not present at stoichiometric
levels (66,67), and individual levels and ratios of activities of the enzymes can allow
for dynamic, multilocus control over fatty acid biosynthesis. The details of some of
these features of control of FAS systems in plants are provided in Sec. III.C.
B. Biosynthesis of the Acyl Group and Other Plastidic Reactions
1. Basic Features
The reactions involved in synthesis of the acyl chain are shown in Figure 1. Historically,
the different phases of fatty acid biosynthesis have been referred to as activation,
initiation, condensation, elongation, and termination. The essential features
of fatty acid synthesis are (a) initial and sustained commitment of carbon to this
biosynthetic pathway, (b) utilization of acetyl and malonyl fragments as building
blocks of fatty acid synthesis, and (c) lengthening of the acyl chain in two-carbon
increments.
Acetyl CoA is the basic building block of fatty acid synthesis (Fig. 1). Acetate
can be imported from extraplastidic sources, or alternatively, intraplastidic acetyl
CoA can be formed from pyruvate by pyruvate dehydrogenase action (68). This
latter pathway is likely to be of greater importance in nongreen plastids than in
chloroplasts because of the more active glycolytic and amylolytic systems in the
former (13,15,68–71).
2. Activation
Activation of acetate for fatty acid metabolism (either catabolic or anabolic) involves
the formation of a high energy thiol ester bond, supplied by conjugation of acetate
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.