In triacylglycerols

Strictly speaking, the term fatty acid is restricted to those carboxylic acids that occur naturally in triacylglycerols. Many chemists and biochemists, however, refer to all unbranched carboxylic acids, irrespective of their origin and chain length, as fatty acids. 

Hu X, Jandacek RJ and White WS. 2000. Intestinal absortion of (3-carotene ingested with a meal rich in sunflower oil or beef tallow postprandial appearance in triacylglycerol-rich lipoproteins in women. Am J Clin Nutr 71 1170-1180. 

Waxes are biosynthesized by plants (e.g., leaf cuticular coatings) and insects (e.g., beeswax). Their chemical constituents vary with plant or animal type, but are mainly esters made from long-chain alcohols (C22-C34) and fatty acids with even carbon numbers dominant (Fig. 7.11). They may also contain alkanes, secondary alcohols, and ketones. The majority of wax components are fully saturated. The ester in waxes is more resistant to hydrolysis than the ester in triacylglycerols, which makes waxes less vulnerable to degradation, and therefore more likely to survive archaeologically. 

Saturated fatty acids do not contain double bonds in the hydrocarbon chain. Unsaturated fatty acids contain from one to hve double bonds. Those with one double bond are known as monounsaturated, those with two as diunsatu-rated and those with more than two as polyunsaturated fatty acids. A brief summary of the roles of saturated and unsaturated fatty acids is given in Table 11.1. The proportion of these fatty acids in triacylglycerol in human adipose tissue is presented in Table 11.2. 

The abundance of fatty acids in triacylglycerols (TAG) can be determined by conversion of the acyl groups into tertiary alcohols on reaction with an alkyl Grignard reagent, followed by chromatographic separation. The method was found to be of advantage over saponification and conversion to a methyl ester, especially in the determination of short-chain fatty acids, which suffer losses by volatilization. ... 

Certain classes of lipids are susceptible to degradation under specific conditions. For example, all ester-linked fatty acids in triacylglycerols, phospholipids, and sterol esters are released by mild acid or alkaline treatment, and somewhat harsher hydrolysis conditions release amide-bound fatty acids from sphingolipids. Enzymes that specifically hydrolyze certain lipids are also useful in the determination of lipid structure. Phospholipases A, C, and D (Fig. 10-15) each split particular bonds in phospholipids and yield products with characteristic solubilities and chromatographic behaviors. Phospholipase C, for example, releases a water-soluble phosphoryl alcohol (such as phosphocholine from phosphatidylcholine) and a chloroform-soluble diacylglycerol, each of which can be characterized separately to determine the structure of the intact phospholipid. The combination of specific hydrolysis with characterization of the products by thin-layer, gas-liquid, or high-performance liquid chromatography often allows determination of a lipid structure. 

Assembly of chylomicrons The enzymes involved in triacylglycerol, cholesterol, and phospholipid synthesis are located in Ihe smooth ER. Assembly of the apolipoproteins and lipid into chylomicrons requires microsomal triacylglycerol transfer protein (see p. 229), which loads apo B-48 with lipid. This occurs during transition from the ER to the Golgi, where the particles are packaged in secretory vesicles. These fuse with the plasma membrane releasing the lipoproteins, which then enter the lymphatic system and, ultimately, the blood. 

Release of VLDLs VLDLs are secreted directly into the blood by the liver as nascent VLDL particles containing apolipoprotein B-100. They must obtain apo C-ll and apo E from circulating HDL (see Figure 18.17). As with chylomicrons, apo C-ll is required for activation of lipoprotein lipase. [Note Abetalipoproteinemia is a rare hypolipoproteinemia caused by a defect in triacylglycerol transfer protein, leading to an inability to load apo B with lipid. As a consequence, no chylomicrons or VLDLs are formed, and tria-cylglycerols accumulate in the liver and intestine.]... 

FIGURE 4.14 BC NMR (500 MHz) carbonyl region (173.4-172.0 ppm) of lipids extracted from salmon muscle of four different origins (from top) wild salmon from Norway (NW), Scotland (SW), and Ireland (IW) and farmed salmon from Norway (NF). The position of fatty acids in triacylglycerols is designated (snl,3 or sn2). (From Aursand et al, 2009.)... 

S Cases, S J Smith, Y-W Zheng, HM Myers, SR Lear, E Sande, S Novak, C Collins, CB Welch, AJ Lusis, SK Erickson, RV Farese Jr. Identification of a gene encoding an acyl CoA diacylglycerol acyltransferase, a key enzyme in triacylglycerol synthesis. Proc Natl Acad Sci (USA) 95 13018-13023, 1998. 

Breakdown The fatty acids in triacylglycerols are released from the glycerol backbone by the action of lipases. The free fatty acids can then be degraded by (3-oxidation to produce energy. The glycerol is converted into dihydroxyacetone phosphate which enters glycolysis. 

Ferrari, R., Esteves, W. and Mukherjee, K. (1997) Alteration of steryl ester content and positional distribution of fatty acids in triacylglycerols by chemical and enzymatic interesterification of plant oils. J. Am. Oil Chem. Soc., 74(2), 93—96. 

The specific phospholipids differ in the degree of unsaturation (Table 1.12). Phosphatidylethanolamine has a low content ( 26%) of saturated fatty acids, and has especially a high content of linoleic acid, far higher than found in triacylglycerols of milk fat. Phosphatidylinositol and phosphatidylserine have 37 40% saturated fatty acids, whereas phosphatidylcholine has over 50%. The fatty acids in sphingomyelin are almost all completely saturated. 

Ueno, S., Yano, J., Seto, H., Amemiya, Y., Sato, K. 2000. Synchrotron radiation X-ray diffraction study of polymorphic crystallization in triacylglycerols. In, Physical Properties of Fats, Oils and Emulsifiers (N. Widlak, ed.), pp. 64-78, AOCS Press, Champaign. 

---