Fatty acid at 2-position

Table 2.4 Fatty acid composition of olive oil-glycerol tristearate blends and proportion of fatty acids at 2-position after inter-esterification...

Kawashima, A., Shimada, Y, Yamamoto, M., et al. 2001. Enzymatic synthesis of high-purity structured lipids with caprylic acid at 1,3-positions and polyunsaturated fatty acid at 2-position. J. Am. Oil Chem. Soc. 78 611-616. 

As noted earlier, there are some 400 fatty acids in milk fat, which means that theoretically milk fat could contain many thousand triacylglycerols. Even if one considers only the 15 or so fatty acids that are present at concentrations above 1% (Table 1.2), and ignores the placement of these fatty acids at specific positions on the triacylglycerol molecule, there are still 680 compositionally different triacylglycerols. 

Triacylglycerols are synthesised in the mammary gland by enzymic mechanisms that exert some selectivity over the esterification of different fatty acids at each position of the. v/t-glycerol moiety (Moore and Christie, 1979). A triacylglycerol molecule showing the three -positions is shown in Figure 1.2. 

T3 seed TAGs from selected independent SLCl-1 transgenics contained increased proportions of VLCFAs (e.g. 20 1) at the sn-2 position (Table 2). Furthermore, these increases were correlated with decreases in the proportions of polyunsaturated Cjg fatty acids at this position. 

Phospholipids. Glycerides esterified by fatty acids at the 1,2 positions and a phosphoric acid residue at the 3 position constitute the class called phosphoHpids (3). In older Hterature and in commercial practice, these materials are described as phosphatides. 

The venoms of poisonous snakes contain (among other things) a class of enzymes known as phospholipases, enzymes that cause the breakdown of phospholipids. For example, the venoms of the eastern diamondback rattlesnake (Crotalus adamanteus) and the Indian cobra Naja naja) both contain phospholipase Ag, which catalyzes the hydrolysis of fatty acids at the C-2 position of glyc-erophospholipids. 

FIGURE 24.3 (a) A duct at the junction of the pancreas and duodenum secretes pancreatic juice into the duodenum, the first portion of the small intestine, (b) Hydrolysis of triacylglycerols by pancreatic and intestinal lipases. Pancreatic lipases cleave fatty acids at the C-1 and C-3 positions. Resulting monoacylglycerols with fatty acids at C-2 are hydrolyzed by intestinal lipases. Fatty acids and monoacylglycerols are absorbed through the intestinal wall and assembled into lipoprotein aggregates termed chylomicrons (discussed in Chapter 25). 

Diacylglycerol is glycerol esterified to two fatty acids at the sn-1 and sn-2 positions. It is a membrane-embedded product of phospholipase C action and an activator of protein kinase C. It is also an intermediate in the biosynthesis of triacylglycerol, phosphatidyletha-nolamine and phosphatidylcholine. 

Due to the specificities of the acyltransferases in the pathways, the fatty acid at position one of glycerol is saturated whereas that at position 2 is monounsaturated (e.g. oleic acid), although in most glycerophospholipids, the fatty acid at position 2 is polyunsaturated (e.g. arachidonic or eicosapentaenoic acids). This is important for the... 

Mechanism for changing the type of fatty acid at position 2 of phosphoglycerides... 

Fats (triacylglycerols) are mainly attacked by pancreatic lipase at positions 1 and 3 of the glycerol moiety. Cleavage of two fatty acid residues gives rise to fatty acids and 2-mono-acylglycerols, which are quantitatively the most important products. However, a certain amount of glycerol is also formed by complete hydrolysis. These cleavage products are resorbed by a non-ATP-dependent process that has not yet been explained in detail. 

T6. The Action of Phospholipases The venom of the Eastern diamondback rattler and the Indian cobra contains phospholipase A2, which catalyzes the hydrolysis of fatty acids at the C-2 position of glycerophospholipids. The phospholipid breakdown product of this reaction is lysolecithin (lecithin is phosphatidylcholine). At high concentrations, this and other lysophospholipids act as detergents, dissolving the membranes of erythrocytes and lysing the cells. Extensive hemolysis may be life-threatening. 

The denominator for Equation E6.2 is calculated by performing steps 1 and 2 for all of the FAMEs present in a single triacylglycerol sample. The mole-corrected peak areas are added together to obtain the total of all mole-corrected areas. If we assume that the extent of conversion of free fatty acids to FAMEs is essentially quantitative, or at least equal for all fatty acids in our experiment, the above calculation leads directly to the combined mole percent composition of fatty acids in all positions of the triacylglycerol. 

The pathway for the synthesis of dipalmitoyl-phos-phatidylcholine is illustrated in figure 19.5. The starting species of phosphatidylcholine is made by the CDP-choline pathway (see fig. 19.4). The fatty acid at the sn-2 position, which is usually unsaturated, is hydrolyzed by phospholi-pase A2, and the lysophosphatidylcholine is reacylated with palmitoyl-CoA. This modification permits alteration of the properties of the phospholipid without resynthesis of the entire molecule, a strategy called remodeling. Deacylation-reacylation of phosphatidylcholine occurs in other tissues and provides an important route for alteration of the fatty acid substituents at both the sn-1 and sn-2 positions. For example, fatty acids at the sn-2 position can be replaced by arachidonic acid, which is stored there until needed for eicosanoid biosynthesis, as we discuss later in this chapter. 

Biosynthesis of dipalmitoylphosphatidylcholine. R2 is usually an unsaturated fatty acid. Thus this two-step reaction results in the replacement of an unsaturated by a saturated fatty acid at the C-2 position on the glycerol backbone. Dipalmitoylphosphatidylcholine is... 

Enzymes that degrade phospholipids are called phospholipases. They are classified according to the bond cleaved in a phospholipid (fig. 19.11). Phospholipases Al and A2 selec- tively remove fatty acids from the sn-1 and sn-2 positions, i respectively. Phospholipase C cleaves between glycerol and the phosphate moieties phospholipase D hydrolyzes the [ head-group moiety X from the phospholipid. Lysophospho- lipids, which lack a fatty acid at the sn-1 or sn-2 position, j are degraded by lysophospholipases. f Phospholipases are found in all types of cells and in... 

Arachidonic acid is not present in significant amounts in tissues as the free acid but is stored as a fatty acid at the sn-2 position of phospholipids. Prostaglandin biosynthesis is initiated by the interaction of a stimulus with the cell surface. Depending on the cell type, the stimulus can take the form of a hormone, such as angiotensin II or antidiuretic hormone, or a protease such as thrombin (involved in blood clotting), or both hormone and protease. These agents bind to a specific receptor that activates a phospholipase A2 that specifically releases the arachidonic acid from a phospholipid such as phosphatidylcholine. The release of arachidonic acid by phospholipase A2 is believed to be the rate-limiting step for the biosynthesis of eicosanoids. 

Fig. 1.3 Various structures of lipid A in different bacteria. The most conserved part of lipid A is its backbone, disaccharide of glucosamine. The groups connecting to the backbone of lipid A could be different from one bacterium to another. (A) In E. coli lipid A there are two phosphates and six fatty acid chains connecting to the backbone. (B) In F. tularensis lipid A there are only one phosphate and four fatty acid chains. (C) In the lipid A of S. typhimurium there is an additional second fatty acid chain at 2-position. (D) In R. etli lipid A there is a very long fatty acid chain at...

The structure of PA lipid A has been elucidated using both mass spectrometry (MS) and NMR techniques (Goldman et al., 1988 Kulshin et al., 1991 Karunaratne et al., 1992). As with lipid A of other Gram-negative organisms, PA lipid A consists of a 6-(l ,6)-linked diglucosamine backbone with phosphates at the 1 and 4 positions, amide-linked fatty acids at the 2 and 2 positions, and ester-linked fatty acids at the 3 and 3 positions (Fig. 11.1). The chain lengths of the fatty acids attached to the PA... 

Fatty acids at the glycerol 2-position by lipase method... 

After inter-esterification an olive oil will present higher concentrations of saturated fatty acids at the 2-position than in the original oil, and this increase can be correlated with the overall percentage of fatty acids. In a real case like that described by Gavriilidou and Boskou (1991) for olive oil-tristearin blends, it is shown how the percentage of saturated fatty acids at the sn-2 position of monoglycerides increases significantly in relation to the blend ratios (Table 2.4). 

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