Acyl lipids, formation

Acyl carrier protein, fatty acid synthase sequence, 45-46 Acyl carrier protein derivatives, desaturation, 13,l4f Acyl lipids, formation, 47-48 Aglycone skeletons saponins, 288f Solanum glycoalkaloids, 288f... 

Acetyl-CoA carboxylase is recognised generally as being a key enzyme in acyl lipid formation. In plants it has been shown to be important as a regulatory enzyme in light-driven lipid synthesis [1] and has a high flux control coefficient under such conditions [2]. Recently, characterisation of different isoforms of acetyl-CoA carboxylase from plants has been made. In Poaceae such as maize there are two multifunctional proteins [3]. By contrast, the dicotyledon pea contains a multienzyme complex form of acetyl-CoA carboxylase in the mesophyll chloroplasts but a multifunctional protein Isoform In the cytosol of epidermal cells [4]. 

No herbicide has yet been demonstrated to have a specific action against one of the enzymes responsible for acyl lipid formation. Nevertheless, because many of the reactions involved (particularly in glycosylglyceride synthesis) have unique features, or are not found in significant amounts... 

Raven, A. M., van Bergen, P. F., Stott, A. W., Dudd, S. N., and Evershed, R. P. (1997). Formation of long-chain ketones in archaeological pottery vessels by pyrolysis of acyl lipids. Journal of Analytical and Applied Pyrolysis 40 267-285. 

Fig. 7.1 shows a typical H-NMR spectrum obtained with the partially purified, microbubble surfactant mixture prior to monolayer formation. For comparison, Table 7.1 gives the chemical-shift data for the proton resonances that can be readily identified in the 1 H-NMR spectra of long-chain acyl lipids (ref. 395-401). 

More recent studies have shown that a novel hepta-acylated lipid A (m/z 1855, Fig. 11.2c) is present in a subset of clinical isolates from patients with severe CF pulmonary disease. Formation of hepta-acylated lipid A results from loss of an enzymatic activity (PogL) that ordinarily deacylates the 3-position of the diglucosamine backbone. Retention of this fatty acid at this position is associated with enhanced resistance to i-lactam antibiotics but not to aminoglycosides. 

A variety of proteins are acylated by formation of thioesters to cysteine and esters to serine and threonine. Acylation may serve either to anchor the proteins in membranes (e.g., rhodopsin Section 2.3.1) and the mannosidase of the Golgi, or to increase lipophilicity and thus enhance the solubilization of lipids being transported (e.g., the plasma apolipoproteins and milk globule proteins). Proteolipids with fatty acids esterified to threonine residues occur in the myelin sheath in nerves. 

The major precursors in meat flavors are die water-soluble components such as carbohydrates, nucleotides, thiamine, peptides, amino acids, and the lipids, and Maillard reaction and lipid oxidation are the main reactions that convert these precursors in aroma volatiles. The thermal decomposition of amino acids and peptides, and the caramelization of sugars normally require temperatures over 150C for aroma generation. Such temperatures are higher than those normally encountered in meat cooking. During cooking of meat, thermal oxidation of lipids results in the formation of many volatile compounds. The oxidative breakdown of acyl lipids involve a free radical mechanism and the formation of... 

The endoplasmic reticulum (ER) has traditionally been viewed as the primary source of phospholipids in plant cells. With the exception of cardiolipin, all of the common phospholipids can be produced by microsomal fractions. The ER also serves as the major site of fatty acid diversification. Although plastids do have the ability to synthesize polyunsaturated fatty acids, they are formed on acyl lipid substrates and are not typically exported. Thus, the ER desaturation pathways are of particular importance for developing seeds that store large quantities of 18 2 and 18 3. Pathways for the production of unusual fatty acids found primarily in seed oils have likewise been described in microsomes. Not surprisingly, the ER also appears to be instrumental in the formation of TGs and the lipid bodies in which they are stored (Section 7). 

Homung E, Pemstich C, Feussner 1. (2002) Formation of conjugated deltal ldeltal3-double bonds by deltal2-hnoleic acid (l,4)-acyl-lipid-desaturase in pomegranate seeds. Eur J Biochem 269 4852-4859. 

In summary, therefore, formation of the photosynthetic apparatus causes a very marked change in the acyl lipid composition of the organism concerned. This stems from the unique lipid content of photosynthetic mem-... 

Fatty acyl-CoA synthetase. Cytosolic processing of fatty acid and lipid formation is a highly integrated process. An active acyl-CoA pool exists and is dependent upon a supply of fatty acid from the plastid. The composition of this pool can be analyzed by capillary electrophoresis (MacKenzie and Taylor, 1990). Formation of acyl-CoAs involves fatty acyl-CoA synthetase (EC 6.2.1.3) and appears to occur on the inner face of the plastidic envelope. While a broad specificity for fatty acids exists in various oilseeds, a preference is shown for medium-chain fatty acids in Cuphea and for very-long-chain fatty acids in Crambe (Battey and... 

S.2 Fatty acyl-CoA transferases. The enzyme systems involved with fatty acyl-CoA utilization in the cytosol appear to be membrane-bound. Consequently, detailed knowledge of their individual structure, specificity and genetic control is generally lacking due to the particular inability to obtain ready isolation and purification of the relevant proteins. Studies, however, support the concept of the operation of the eukaryotic pathway for the production of glycerolipids and polyunsaturated fatty acid (Browse et al., 1990 Stymne et al., 1990). While this pathway may contribute a significant quantity of fatty acid for use in membrane synthesis in the plastid (chloroplast) (Browse et al., 1990), its major importance would seem to lie with the production of unsaturated oils (Frentzen, 1986). On the other hand the occurrence of the prokaryotic pathway in the plastid permits more direct membrane lipid formation in both 16 3 and 18 3 plants (Browse et al., 1990 Somerville and Browse, 1991). Different sets of acyltransferase may be associated with the two pathways (Hills and Murphy, 1991). 

FIGURE 4.4 Proposed pathway for the formation of short chain carbonyls by enzymatic degradation of acyl lipids in disrupted tomato fruits. (From Galliard, R, J.A. Matthew, A.J. Wright, M.J. Fishwick, J. Sci. Food Agric., 28, p. 863, 1977. With permission.)... 

The activity of antioxidants reverses under certain conditions they become prooxidants. One way in which a-tocopherol can become perox-idatively active is shown in Formula 3.81. Another way is through the formation of the chro-manoxyl radical in concentrations high enough to overcome the inertness mentioned in 3.7.3.1 and abstract H-atoms from unsaturated acyl lipids to a definite extent, starting lipid peroxidation. This activity reversion, which is also undesirable from a nutritional and physiological point of... 

Acyl lipids are transesterified very rapidly in anhydrous methanol in the presence of a basic catalyst. Free fatty acids are not normally esterified, however, and care must be taken to exclude water from the reaction medium to prevent their formation as a result of hydrolysis of lipids. 0.5 M Sodium methoxide in anhydrous methanol, prepared simply by dissolving fresh clean sodium in dry methanol, is the most popular reagent, but potassium methoxide or hydroxide have also been used as catalysts. The reagent is stable for some months at room temperature, especially if oxygen-free methanol is used in its preparation. The reaction is very rapid phosphoglycerides, for example, are completely transesterified in a few minutes at room temperature. It is commonly performed as follows ... 

This conclusion can also be rationalized on the basis of a comparison of the monolayer properties of the two acyl lipids. The collapse pressure of DGDG is considerably greater than DSPC presumably because of hydrogen bonding between the sugar residues of the galactoUpid (Tomoaia-Coti l et al., 1983). Interm olecular interactions between DSPC molecules, on the other hand, are weaker and provide scope for AX molecules to increase cohesion via the formation of DSPCrAX associations. 

Once oleate has been formed, in the majority of plants it appears to be exported to the extraplastidic compartment. This movement requires hydrolysis of oleoyl-ACP by a stromal thioesterase followed by formation of oleoyl-CoA on the chloroplast envelope. Oleoyl-CoA is then used to acylate lipids on the endoplasmic reticulum. The esterification to the sn-2 position of PC is particularly important since l-acyl-2-oleoyl-PC is the major substrate for the A12-desaturase. The linoleate generated by this desaturation can then be further desaturated on PC in oilseeds or returned to the chloroplast. The latter case will naturally occur in leaf tissue, where linoleate is esterified to MGDG in order to provide the main substrate for the A15-desaturase. Little is known about the enzymatic characteristics of the A12- and A15-desaturases, not least because no purification has yet been achieved and, in addition, the A15-desaturase is particularly labile. 

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