Fatty acid positional specification

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. 

Except for enzyme-directed processes to place certain fatty acids in specific positions on TAG, such as production of coating fats, cocoa butter substitutes, or reduced-calorie fats,135 the... 

TGs are composed of a 3 carbon glycerol to which 3 fatty acids are esterified, and the glycerol carbons are stereo-specifically numbered as snl, sn2, and sn3 (Iimis 2011). The mammary gland actively uptakes FAs from plasma for the de novo synthesis of TGs (Neville and Picciano 1997), where, there is a preferential acylation of fatty acids to specific positions in TGs, making it seem as there is a biological purpose for investing in the enzymes needed to achieve the nonrandom positioning of TG fatty acids (Innis 2011). 

LOXs catalyse the addition of dioxygen to methyl-interrupted cis double bond [(Z,Z)-pentadiene] in a polyunsaturated fatty acid to produce a hydroperoxy fatty acid containing a Z,E conjugated double bond system. Where multiple pentadienes occur in a single molecule such as arachidonic acid, position-specific oxygenation can take place, resulting in the production of 5-, 12- or 15-hydroperoxyeicosa-tetraenoic acids. These hydroperoxy acids may be subsequently converted into other oxylipins by enzymic or non-enzymic reactions [3]. Table 1 shows some examples of LOX products from fungi. 

Four synthetic triacylglycerols containing linoleate (L) and linolenate (Ln) in specific 1,2 or 1,3-positions (LLLn, LLnL, LnLnL and LnLLn) were oxidized to determine the effect of fatty acid position on rates and products of autoxidation. The rates and yields of oxidation products decreased in the order LnLnL > LnLLn > LLnL > LLLn (Figure 2.18). Therefore, triacylglycerols containing two Ln and one L were more easily oxidized when the Ln is in the... 

Lipases can be divided into three classes based on their specificity and/or selectivity regio- or positional specific lipases, fatty acid—type specific lipases, and specific lipases for a certain class of acylglycerols (mono-, di-, or triglycerides). In terms of regioselectivity, lipases have been divided into three types sn-1,3-specific (hydrolyze ester bonds in positions R1 or R3), sn-2-specific (hydrolyze ester bond in position R2), and nonspecific (do not distinguish between positions of ester bonds to be cleaved). Most known lipases are 1,3-regiospecific with activity on terminal positions. 

Amides can be titrated direcdy by perchloric acid ia a nonaqueous solvent (60,61) and by potentiometric titration (62), which gives the sum of amide and amine salts. Infrared spectroscopy has been used to characterize fatty acid amides (63). Mass spectroscopy has been able to iadicate the position of the unsaturation ia unsaturated fatty amides (64). Typical specifications of some primary fatty acid amides and properties of bisamides are shown ia Tables 5 and 6. 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

The specific behaviour of unsaturated fatty acids under oxidation is determined by the position and the number of double bonds in the fatty acid molecule. The stepwise oxidation of an unsaturated acid to the position of a double bond in it proceeds in a manner similar to that of saturated acid oxidation. If the double bond retains the same configuration (trans-configuration) and position (A2,3) as those of the enoyl-CoA, which is produced during the oxidation of saturated fatty acids, the subsequent oxidation proceeds via conventional route. Otherwise, the oxidation reaction proceeds with the involvement of an accessory enzyme, A3,4-CiS-A2,3jrans-enoyl-CoA isomerase this facilitates the translocation of the double bond to an appropriate position and alters the double-bond configuration from cis to trans. 

Figure 21 shows the LC-MS separation of fatty acid lubricants detected using positive ESI with specific ion monitoring as (M—H). The following conditions were employed. 

Interpretation/results A GC-isotope ratio MS technique is used on known animal fats and the ancient samples. (In this technique, each peak eluting from the GC is combusted to C02 and its 12C-13C ratio is measured by a mass spectrometer [7].) The 13C ratio of the C-16 0 and C-18 0 fatty acids are plotted. The knowns are concentrated in specific areas on the plot, shown as ellipses in Fig. 21.16. The position of the cream sample points on this pattern recognition plot indicates that the fatty portion of the cream is from ruminant adipose tissue. 

The initial acylation at the 1-position of glycerol 3-phosphate is catalysed by glycerol 3-phosphate acyl-transferase-1, abbreviated to GPAT-1. This enzyme is specific for a saturated fatty acid (in the acyl form). 

The second acylation at position 2 is catalysed by GPAT-2, which is specific for a fatty acid with one or two double bonds. This produces phosphatidic acid, for which the phosphate must be removed prior to the final acylation. 

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