Food Lipids: Chemistry, Nutrition, and Biotechnology

tools for using genetic transformation as a means to study the process and control
of lipid biosynthesis in plants. In turn, this will provide greater understanding of
how to manipulate fatty acid composition in plant storage lipids for specific purposes
or end uses.
It is also well known that the arrangement or distribution of fatty acyl groups
along the glycerol backbone of triacylglycerols (i.e., profile of molecular species of
triacylglycerols) also impacts greatly on the composite physiochemical or nutritional
properties of a particular lipid (33,34). Lipids of similar fatty acid composition may
have markedly different physiochemical properties because of how the fatty acids
are distributed. The origins of positional distribution of fatty acids lie in triacylglycerol
assembly in plants, an area in which definitive research is barely a decade old
(20,35). However, a working knowledge of how triacylglycerols are specifically assembled
in different species will be paramount to identifying strategies by which to
modify the process in vivo and to direct triacylglycerol assembly in a manner designed
to improve the functional properties of the lipid resource in foods. Section V
explores this phase of lipid biosynthesis in detail.
III. FATTY ACID BIOSYNTHESIS
A. Comparative Biochemistry of Fatty Acid Synthetase Systems
1. Historical Perspective
It has been about 50 years since fatty acid synthesis was first demonstrated in cellfree
extracts (36). Some of the earliest studies revealed the essential features of fatty
acid biosynthesis: it required ATP, NADPH, CO2 ( ), CoA, Mn 2� �
HCO3 , acetate, and
source of heat-labile (FAS enzymes) and heat-stable (ultimately determined to be
acyl carrier protein) proteinaceous factors isolated from the organism of interest.
Slowly, each discrete, protein-associated unit activity of the type I and II fatty acid
synthetase (FAS) systems was identified. Table 3 gives some of the seminal citations
(37) that formalized the identification of these components and lists the unit activities
(38–55). Even though virtually all the FAS enzymes, and corresponding steps in
fatty acid biosynthesis, had been isolated and identified by the early 1980s, discoveries
and challenges to dogmatic views regarding this intensely researched area of
primary metabolism have continued. A case in point is the discovery and elucidation
over the last decade of an enzyme isoform, now believed to cause the initial condensation
reaction in type II FAS systems in plants and bacteria (43,44,56), and the
repeated speculation that there is another condensing enzyme yet to be discovered
(44,56–59).
In contrast to the wealth of history of research on fatty acid biosynthesis,
definitive studies on triacylglycerol assembly and control of this process extend
barely a decade into the past (15,20,35). The study of both fatty acid biosynthesis
and assembly into triacylglycerols (and other glycerolipids) is essentially research in
enzymology. The advent of genetic manipulation of organisms has made progress in
both these areas of lipid synthesis extremely rapid since about 1990. While the theme
of this chapter is founded on the characteristics of the individual enzymes involved
in storage lipid biosynthesis, the superseding objective is to convey how these individual
steps are coordinated as part of this overall process.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.