**4. Mono-, di- and triacylglycerides**

6 Lipid Metabolism

origin (EPA and DHA) [32].

considering the notation " ω".

point.

pointing the location of the first double bond, it will automatically determined the location of the subsequent double bonds [29]. Thus, C18: 1 ω-9, which has a single double bond at C9 counted from the methyl end, correspond to oleic acid (OA), which is the main exponent of the ω-9 family. Oleic acid is highly abundant both in vegetable and animal tissues. C18: 2 ω-6 corresponds to a fatty acid having double bonds at the C6 and C9 (for unconjugated fatty acids it is not necessary to indicate the position of the second or successive double bonds). This is linoleic acid (LA), the main exponent of the ω-6 family and which is very abundant in vegetable oils and to a lesser extent in animal fats [30]. C18: 3, ω-3 corresponds to a fatty acid having double bonds at C3, C6 and C9. It is alpha-linolenic acid (ALA), the leading exponent of the ω-3 family. ALA is a less abundant fatty acid, almost exclusively present in the vegetable kingdom and specifically in land-based plants [31]. Within (LCPUFAs), C20: 4, ω-6 or arachidonic acid (AA); C20: 5, ω-3 or eicosapentaenoic acid (EPA) and; C22 : 6, ω-3 or docosahexaenoic acid (DHA), are of great nutritional importance and are only found in ground animal tissues (AA) and in aquatic animal tissues (AA, EPA and DHA) and in plants of marine

The increase of double bonds in fatty acids significantly reduces its melting point. Thus, for a structure of the same number of carbon atoms, if it is saturated may give rise to a solid or semisolid product at room temperature, but if the same structure is unsaturated, may originate a liquid or less solid product at room temperature. Figure 1 shows the classification of fatty acids according to their degree of saturation and unsaturation and considering the notation "ω", and table 1 shows different fatty acids, showing the C nomenclature, their systematic name, their common name and the respective melting

**Figure 1.** Classification of fatty acids according to their degree of saturation and unsaturation and

The structural organization of fatty acids in food and in the body is mainly determined by the binding to glycerol by ester linkages. The reaction of a hydroxyl group of glycerol, at any of its three groups, with a fatty acid gives rise to a monoacylglyceride. The linking of a second fatty acid, which may be similar or different from the existing fatty acid, gives rise to a diacylglyceride. If all three hydroxyl groups of glycerol are linked by fatty acids, then this will be a triacylglyceride [33]. Monoacylglycerides, by having free hydroxyl groups (two) are relatively polar and therefore partially soluble in water. Different monoacylglycerides linked to fatty acids of different lengths are used as emulsifiers in the food and pharmaceutical industry [34]. The less polar diacylglycerides which have only one free hydroxyl group are less polar than monoacylglycerides and less soluble in water. Finally, triacylglycerides, which lack of free hydroxyl groups are completely non-polar, but highly soluble in non-polar solvents, which are frequently used for their extraction from vegetable or animal tissues, because constitutes the energy reserve in these tissues [35]. Diacylglycerides and monoacylglycerides are important intermediates in the digestive and absorption process of fats and oils in animals. In turn, some of these molecules also perform other metabolic functions, such as diacylglycerides which may act as "second messengers" at the intracellular level and are also part of the composition of a new generation of oils nutritionally designed as "low calorie oils" [36]. When glycerol forms mono-, di-, or

#### 8 Lipid Metabolism

triacylglycerides, its carbon atoms are not chemically and structurally equivalent. Thus, carbon 1 of the glycerol is referred as carbon (α), or sn-1 (from "stereochemical number"); carbon 2 is referred as carbon (β), or sn-2, and carbon 3 as (γ), or sn-3. It is important to note that the notation "sn" is currently the most frequently used [37]. This spatial structure (or conformation) of mono-, di- and triacyglycerides is relevant in the digestive process of fats and oils (ref). Figure 2 shows the structure of a monoacylglceride, a diacylglyceride and a triacylglyceride, specifying the "sn-" notation.

Overview About Lipid Structure 9

structure of a SAFA, such as the stearic acid (C18:0), AO, LA and ALA, exemplifying the "ω" notation of each and indicating the essential condition in relation to the position of their

**Figure 3.** The chemical structure of a SAFA, such as the stearic acid (C18:0), AO, LA and ALA,

exemplifying the "ω" notation of each and indicating the essential condition in relation to the position of

According to the distribution of double bonds in a fatty acid and to its spatial structure, unsaturated fatty acids may have two types of isomerism: geometrical isomerism and positional isomerism. By isomerism it is referred to the existence two or more molecules having the same structural elements (atoms), the same chemical formula and combined in equal proportions, but having a different position or spatial distribution of some atoms in

Carbon atoms forming the structure of the fatty acids possess a three-dimensional spatial structure which forms a perfect tetrahedron. However, when two carbons having tetrahedral structure are joined together through a double bond, the spatial conformation of the double bond is modified adopting a flat or plane structure [41]. Rotation around single bonds (C**-**C) is entirely free, but when they are forming a double bond (C**=**C), this rotation is impeded and the hydrogen atoms that are linked to each carbon involved in the bond may

unsaturated bonds.

their unsaturated bonds

the molecule [40].

**6. Isomerism of fatty acids** 

**6.1. Geometrical isomers of fatty acids** 

**Figure 2.** Structure of a monoacylglyceride, a diacylglyceride and a triacylglyceride, specifying the "sn-" notation
