Diacylglycerol structure

Figure 12.13 Glycosyl diacylglycerol structure. The carbohydrate, which may be either a monosaccharide or a disaccharide residue, is attached by a glycosidic linkage to the sn-3 position of glycerol.

De Jong, S. 1980. Diacylglycerol structures and fatty acid conformations. A theoretical approach. Ph.D. thesis University of Ulrecht. The Netherlands 171... 

Lipids are essential components in the diet and nutritionists should be more active in arguing the positive aspects of these molecules. They are required in their own right and also for the important minor components such as carotenoids, tocols, and sterols which accompany the oils and fats. The first chapter in the book is an overview of the development of functional foods and this is followed by chapters devoted to materials which are either lipids or are compounds that are fat-soluble and co-occur with oils and fats. These include carotenoids, tocopherols, phytosterols, omega-3 (n-3) oils, diacylglycerols, structured lipids, sesame oil, rice bran oil, and oils or preparations containing palmitoleic acid, y-linolenic acid, stearidonic acid, or conjugated linoleic acid. These topics will be of interest to those who produce lipids and those who seek to incorporate them into appropriate foods, and this book is directed primarily to these two groups. 

Olid carbon is asymmetric. The various acylglycerols are normally soluble in benzene, chloroform, ether, and hot ethanol. Although triacylglycerols are insoluble in water, mono- and diacylglycerols readily form organized structures in water (discussed later), owing to the polarity of their free hydroxyl groups. 

Let us first consider the lipid molecular structures required. First is the hydrophobic matching. The length of the hydrophobic chain determines the thickness of the hydrophobic part of the lipid bilayer, this should correspond closely to the dimension of the native membrane. As most biological membranes contain diacylglycerol lipids with hydrophobic chain lengths of 16 18 carbon atoms. Thus, synthetic lipids should possess relatively long hydrocarbon chain length, e.g., 16-18 carbon atoms. 

The nature of the reaction catalyzed by PLC i( in which phosphatidylcholine is split into diacylglycerol and phosphorylcholine (Fig. 11) requires two proton transfer steps The first is the deprotonation of an active site water to generate the attacking hydroxide nucleophile, and the second is the protonation of the alkoxide leaving group. Although analyses of the X-ray structures of PLCSc and... 

Glycerophospholipids are used for membrane synthesis and for producing a hydrophilic surface layer on lipoproteins such as VLDL. In cell membranes, they also serve as a reservoir of second messengers such as diacylglycerol, inositol 1,4,5-triphosphate, and arachidonic acid. Their structure is similar to triglycerides, except that the last fatty acid is replaced by phosphate and a water-soluble group such as choline (phosphatidylcholine, lecithin) or inositol (phosphatidyl-inositol). 

Figure 11.25 Structure of phosphatidylinositol bisphosphate and position of hydrolysis by phospholipase. Phosphatidylinositol 4,5-bisphosphate (PIP2) is a component of the inner leaflet of a cell membrane from which inositol 1,4,5-trisphosphate (IP3) is released by hydrolysis catalysed by phospholipase where indicated. The diacylglycerol is retained in the membrane.

Diacylglycerol, on the other hand, is lipid soluble and remains in the lipid bilayer of the membrane. There it can activate protein kinase C (PKC), a very important and widely distributed enzyme which serves many systems through phosphorylation, including neurotransmitters (acetylcholine, a,- and P-adrenoceptors, serotonin), peptide hormones (insulin, epidermal growth hormone, somatomedin), and various cellular functions (glycogen metabolism, muscle activity, structural proteins, etc.), and also interacts with guanylate cyclase. In addition to diacylglycerol, another normal membrane lipid, phos-phatidylserine, is needed for activation of PKC. The DG-IP3 limbs of the pathway usually proceed simultaneously. 

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. 

The major lipoproteins of insect hemolymph, the lipophorins, transport diacylglycerols. The apolipo-phorins have molecular masses of -250, 80, and sometimes 18 kDa.34-37a The three-dimensional structure of a small 166-residue lipophorin (apolipophorin-III) is that of a four-helix bundle. It has been suggested that it may partially unfold into an extended form, whose amphipathic helices may bind to a phospholipid surface of the lipid micelle of the lipophorin 35 A similar behavior may be involved in binding of mammalian apolipoproteins. Four-helix lipid-binding proteins have also been isolated from plants.38 See also Box 21-A. Specialized lipoproteins known as lipovitellins... 

A third group of lipid-binding proteins have a four-helix bundle structure. They include the insect lipophorins, which transport diacylglycerols in the hemolymph (see main text), and nonspecific lipid carriers of green plants.q An 87-residue four-helix protein with a more open structure binds acyl-coenzyme A molecules in liver.r... 

Figure 23-29 (A) Stereoscopic drawing of light-harvesting complex from the dinoflagellate protozoan Amphidinium carterae. The central cavity contains eight molecules of peridinin, two of which can be seen protruding from the top. Deeply buried toward the bottom are two molecules of Chi a. Also present are two molecules of digalactosyl diacylglycerol. From Hofmann et al.268 Courtesy of Wolfram Welte. (B) Structure of peridinin.

Outline of pathways for the biosynthesis of major cellular lipids (other than cholesterol) in a mammalian cell. Most of the metabolism of these lipids occurs on membrane surfaces because of the insoluble nature of the substrates and products. These lipids play three major roles (l) they act as a storehouse of chemical energy, as with triacylglycerols (2) they are structural components of membranes (boxed compounds) and (3) they act as regulatory compounds (underlined), either as eicosanoids, which act as local hormones, or as phosphorylated inositols and diacylglycerols, which function as second messengers. 

Lipid synthesis is unique in that it is almost exclusively localized to the surface of membrane structures. The reason for this restriction is the amphipathic nature of the lipid molecules. Phospholipids are biosynthesized by acylation of either glycerol-3-phosphate or dihydroxyacetone phosphate to form phosphatidic acid. This central intermediate can be converted into phospholipids by two different pathways. In one of these, phosphatidic acid reacts with CTP to yield CDP-diacylglycerol, which in bacteria is converted to phosphatidylserine, phosphatidylglycerol, or diphos-... 

Full fat milk contains about 3% fat and triacylglycerols account for about 95% of the lipid fraction. Other components of the lipid fraction are diacylglycerols, cholesterol, phospholipids, and free fatty acids. The lipid structures contain many fatty acids from all major classes, that is, saturated, monounsaturated, and polyunsaturated fatty acids (Haug et al., 2007). More than 60% of the fatty acids in cow s milk and consequently in dairy products are saturated, including shorter and medium... 

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