Phosphoglycerides structure

Figure 12.11 Phosphoglyceride structure. The members of this group are derivatives of the parent compound, l,2-diacyl-src-glycerol-3-phosphate (phosphatidic acid) in which X is a hydrogen atom. This is replaced by either an amino alcohol or a polyhydroxy residue. In phosphoglycerides derived from animal tissues R1 is usually a saturated acyl chain of between 16 and 20 carbon atoms and R2 is usually unsaturated. Polyunsaturated acyl chains containing 16 or 18 carbon atoms predominate in leaf phosphoglycerides and those of bacterial origin are often more complex.

Figure 2.2 Variations in phosphoglyceride structures. For abbreviations see Table 2.1.

Each phospholipid class in a given tissue has a characteristic fatty acid composition. Though the same fatty acid may be present in a number of lipids, the quantitative fatty acid composition is different for each class of lipids and remains fairly constant during the growth and development of the brain. A typical distribution profile of the major fatty acids in rat brain phospholipids is given in Table 3.1. Not only do the phosphoglycerides differ in the structure of the polar head groups, or phospholipid... 

Platelet activating factor (PAF) is the term used to describe a family of structurally-related lipids that are all acetylated phosphoglycerides (Fig. 

The different phosphoglycerides are often named by placing the constituent attached to the phosphate group after phosphatidyl , e.g. phosphatidyl choline (3-in-phosphatidylcholine or l,2-diacyl-sn-glycero-3-phosphoryl-choline). There are many phosphoglycerides because of the possible variation in the fatty acid chains, and when the full chemical structure is known, it should be used (e.g. l-palmitoyl-2-oleoyl-phosphatidylcholine). Nomenclature that entails the use of the DL system should be avoided. 

Cholesterol The pathway for synthesis of cholesterol is described in Appendix 11.9. Cholesterol is important in the structure of membranes since it can occupy the space that is available between the polyunsaturated fatty acids in the phospholipid (Chapter 4). In this position, cholesterol restricts movement of the fatty acids that are components of the phosphoglycerides and hence reduces membrane fluidity. Cholesterol can be synthesised de novo in proliferating cells but it can also be derived from uptake of LDL by the cells, which will depend on the presence of receptors for the relevant apoUpoproteins on the membranes of these cells (Appendix 11.3). 

Liposomes (Fig. 8) are completely closed vesicular bilayer structures formed by exposing phosphoglycerides in water suspension to sonic oscil-... 

Kuksis, A. 1972. Newer developments in determination of structure of glycerides and phosphoglycerides. In Progress in the Chemistry of Fats and Other Lipids, Vol. 12. R.T. Holman (Editor). Pergamon Press, New York, p. 82. 

Two main types of lipids occur in biological membranes phospholipids and sterols. The predominant phospholipids in most membranes are phosphoglycerides, which are phosphate esters of the three-carbon alcohol, glycerol. A typical structure is that of phosphatidylcholine (lecithin) ... 

In addition to phosphoglycerides, membranes from animal cells usually contain a second group of phospholipids, the sphingolipids. Sphingomyelin, which is representative of this group, has the structure... 

Structure of phosphatidic acid, a phosphoglyceride. A saturated fatty acid is esterified at carbon 1 of the glycerol, and a cA-unsaturated fatty acid at carbon 2. The cluster of polar and charged oxygens makes the head-group hydrophilic, in marked contrast to the hydrophobic fatty acid chains. 

Hydrolysis is the other important mechanism for some lipids (glycerides and phosphoglycerides) to degrade. Lowering of pH can arise from both hydrolysis and lipid peroxidation (Arakane et al., 1995), as both processes can give acid products. It has also been reported that hydrolysis and peroxidation can act in synergy with one another in liposome bilayers (Swern, 1995), but it is uncertain whether this can happen for other structures since different mechanisms are operative for... 

Structural formula A is typical of a group of compounds commonly called phosphoglycerides. In this particular depiction the symbols R1 and R2 are long-chain carboxylic acids linked in an ester linkage to the primary and secondary alcohol residues of glycerol. The symbol X is a mixed venue with a wide variety of bases—for example, choline (HOCH2CH2 N (CH3)3) or... 

The nomenclature rules just cited for phosphatidylcholine and its analogs also hold for ethanolamine phosphoglycerides. Mixtures of the diacyl, alkenyl acyl, and alkyl ether forms are found, for example, in macrophages and neutrophils but in much different ratios than observed for the choline-containing types (Figure 1-11). Again, only the (diacyl) phosphatidylethanol-amine is found in liver cells. These structures will be discussed in more detail in Chapter 5. 

The question can be posed as to whether the stereochemical conformation of a phosphoglyceride can be determined by FAB-MS. While no rigorous examination of synthetic sn-3 or sn-1 compounds by this technique (or any other mass spectrometric procedure) has been reported, it seems highly unlikely that any detectable or significant differences would be noted. Nonetheless, FAB-MS and other forms of mass spectrometry are powerful tools useful in the structural identification of organic compounds, and this possibility may develop. 

The proof of structure on the naturally occurring phosphoglycerides is by no means a simple task, but it can be done with perseverance. Perhaps the best way to initiate such experiments is to explore in order the topics briefly discussed in the section entitled The Ubiquitous One (Diacyl)Phosphatidyl-choline. These are the essential components of a structure proof for these compounds. 

As mentioned in an earlier section, there are no chemical reagents that can establish with any certainty the specific position of attachment of an acyl ester on a diacylphosphatidylcholine or any other phosphoglyceride. The same is true in any attempt to establish the stereochemical conformation of phosphatidylcholine, whether sn-1 or sn-3, by strictly chemical means. The solution to this dilemma is to use synthetic phosphoglycerides of defined structure as well as phospholipase A2 to establish the correct stereospecificity of the latter enzyme. Certainly, this is reminiscent of the chicken or egg argument, yet it does work as will be explained below. 

An Approach to Proof of Structure of Ether-linked Phosphoglycerides... 

At the present time there is no satisfactory route to isolation of diacyl-, alkylacyl-, or alkenylacylcholine phosphoglycerides as individual components— for example, only the diacyl type or the alkylacyl form, and so on. In general, a differential analytical approach must be invoked and this will be discussed later. However, since diacylphosphatidylcholine has been discussed, proof of structure of ether-linked choline phosphoglycerides as individual species will be described as well as some of their structural features. Later, as the big picture emerges, the route to analysis of a naturally occurring mixture will be examined. 

Taken together, the above information strongly supports the structural formula of naturally occurring alkylacylcholine phosphoglycerides to be 1-0-alkyl-2-0-acyl-.vn-glycero-3-phosphocholine. 

Several other analytical procedures can be of value, to varying degrees, in the proof of structure of glyceryl ethers and the ether-linked phosphoglycerides. Among these are the old-fashioned approaches melting points and elemental... 

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