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  Section: Molecular Biology of Plant Pathways » Metabolic Engineering of the Content and Fatty Acid Composition
  of Vegetable Oils
 
 
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Introduction

 
     
 
Oils and fats tend to be the predominant energy reserves in mobile organisms because of their high energy value per unit weight. Plants, given a sessile lifestyle, limit oil production primarily to portable reproductive structures. Nevertheless, more than 120 million metric tons of vegetable oil reach world markets per year (United States Department of Agriculture, Foreign Agricultural Service, 2007). Oilseeds such as soybean, sunflower, and rapeseed are the major oil crops in temperate regions, although fruits of olive and especially of oil palm are significant sources on a world basis (Table 7.1).



FIGURE 7.1 Structure of a typical triacylglycerol (TAG) molecule of vegetable oil. A TAG molecule consists of fatty acids attached by ester linkages to each of the three stereospecific or sn positions of a glycerol backbone. As shown, the <em>sn-</em>2 position of a typical plant TAG is occupied by an unsaturated fatty acid. Saturated fatty acids generally occupy only the <em>sn-</em>1 or <em>sn-</em>3 positions, but unsaturated fatty acids can be found at any of the three stereospecific positions.
FIGURE 7.1 Structure of a typical triacylglycerol (TAG) molecule of vegetable oil. A TAG molecule consists of fatty acids attached by ester linkages to each of the three stereospecific or sn positions of a glycerol backbone. As shown, the sn-2 position of a typical plant TAG is occupied by an unsaturated fatty acid. Saturated fatty acids generally occupy only the sn-1 or sn-3 positions, but unsaturated fatty acids can be found at any of the three stereospecific positions.

FIGURE 7.2 Structure of linoleic acid. This structure illustrates the basis for the shorthand notation that is often used for fatty acids. The 18:2Δ9,12 abbreviation indicates that linoleic acid contains 18 carbon atoms and 2 double bonds, which are located at the C-9 and C-12 atoms relative to the carboxyl end of the fatty acid. Linoleic acid is often referred to as an ω-6 fatty acid, which indicates that the last double bond is positioned six carbon atoms from the methyl end of the fatty acid. Vegetable oils rich in linoleic acid, such as soybean oil, are sometimes called ω-6 oils.
FIGURE 7.2 Structure of linoleic acid. This
structure illustrates the basis for the shorthand
notation that is often used for fatty acids.
The 18:2Δ9,12 abbreviation indicates that linoleic
acid contains 18 carbon atoms and 2 double
bonds, which are located at the C-9 and C-12
atoms relative to the carboxyl end of the fatty
acid. Linoleic acid is often referred to as an ω-6
fatty acid, which indicates that the last double
bond is positioned six carbon atoms from the
methyl end of the fatty acid. Vegetable oils rich in
linoleic acid, such as soybean oil, are sometimes
called ω-6 oils.
At the molecular level, the typical oil molecule is a triacylglycerol (TAG), a glycerol molecule with a fatty acid esterified to each of the three hydroxyl groups (Fig. 7.1). The three carbon atoms of the glycerol backbone of TAG are referred to using the stereospecific numbering system as sn-1, sn-2, and sn-3 (Fig. 7.1). As indicated by this nomenclature, the three carbons of glycerol are stereochemically distinct. It is the fatty acid composition that determines the physical characteristics of a given oil. For example, a sufficient proportion of saturated fatty acids, which lack carbon–carbon double bonds, can raise the melting point of an oil until it is solid at room temperature, as required in some baked goods. Palmitic acid, abbreviated 16:0 because it has 16 carbons and 0 double bonds, is the most abundant of the saturated fatty acids in plants, although at least some stearic acid (18:0) occurs in most edible oils (Table 7.2). The unsaturated fatty acids of typical plant oils feature one or more cis-double bonds, which introduce kinks into the fatty acid chain and increase fluidity more effectively than would trans-double bonds. Oleic acid (18:1Δ9), the most prominent monounsaturated fatty acid, has a cis-double bond nine carbons from its carboxyl terminus (see Fig. 7.2 for explanation of numerical fatty acid nomenclature). It can comprise 65–85% of the olive (Olea) oil for which it was named, but contributes a mere 20% of traditional sunflower or soybean oils (Gunstone et al., 2007). Thus, high oleic acid seed oils mimicking the qualities of olive oil as a cooking and salad oil are under development. Plant oils are also important sources of polyunsaturated fatty acids including linoleic acid (18:2Δ9,12; Fig. 7.2) and α-linolenic acid (18:3Δ9,12,15). Since increasing unsaturation decreases oxidative stability, oils high in 18:3 become rancid quickly and are unsuitable for frying. However, both linoleic and α-linolenic acids are essential to the human diet. Finally, some qualities of vegetable oils reflect the arrangement of fatty acids on glycerol as well as absolute fatty acid composition. For example, the positive ‘‘mouthfeel’’ of cocoa butter is largely attributed to TAG having saturated fatty acids at positions 1 and 3, but 18:1Δ9 at position 2 (Jandacek, 1992). The positional distribution of fatty acids in dietary TAG also has clinical implications (Kubow, 1996).


Although vegetable oils are primarily used in foods, they also serve as industrial feedstocks (Table 7.3). A few oils are targeted entirely to such uses. Highly unsaturated ‘‘drying oils’’ such as linseed oil are desirable for paints and coatings; lauric acid (12:0) in coconut and palm kernel oil is a vital component of soaps and detergents; castor oil, which contains the unusual hydroxy-fatty acid ricinoleic acid (12-hydroxy-18:1Δ9), is used for certain plastics and lubricants; and high erucate (22:1Δ13) rapeseed oil contains the raw material for Nylon 1313 and slip agents used in the manufacture of sheet plastic. Edible oils may likewise serve industrial purposes. For example, in the United States, 12% of soybean oil is currently channeled to products ranging from lubricants and biodiesel fuels to inks, polyurethane, and candles (American Soybean Association, 2007). As petroleum stocks dwindle, it is likely that vegetable oils will play a greater industrial role.


In addition to control of oil composition, improvement of total yield of oil crops is a major goal of breeders and molecular biologists. To some extent, such improvement can involve parameters beyond the scope of this discussion. Flower number and seed set, disease resistance and fruit or seed size are only a few examples of factors indirectly affecting oil production. At a more direct level, scientists are attempting to identify control points for carbon flux into fatty acids, factors influencing partitioning of fatty acids between structural lipids and TAG, and regulatory elements determining overall expression of lipid biosynthesis genes.
 
     
 
 
     



     
 
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