Structure glycerolipids

The structural glycerolipids of all plant membranes contain predominantly five fatty acids (18 1, 18 2,18 3, 16 0, and in some species, 16 3). However, the fatty acid composition of storage oils varies far more than in membrane glycerolipids. Altogether more than 300... 

Ikeda, I., Tomari, Y., Sugano, M., Watanabe, S., and Nagata, J. 1991. Lymphatic absorption of structured glycerolipids containing medium-chain fatty acids and linoleic acid, and their effect on cholesterol absorption in rats. Lipids 26 369-373. 

More complex structures, often related to natural products are prepared by organic synthesis. Among them can be mentioned (f )-3-hydroxytetradecanoic acid (the double-tail hydrophobic moiety of lipid A), sphingosine derivatives related to the ceramides or 1,2- and l,3-dialkyl(acyl)glycerols related to glyco-glycerolipids, glycerophospholipids, and GPI anchors of membrane proteins. The preparations of the above derivatives were reported several years ago but some improvements have been published more recently. 

Fig. 1. Structure and posttranslational processing of PrP. (Upper) Structure of the primary translation product of mammalian PrP. The five proline/glycine-rich repeats in mouse PrP have the sequence P(Q/H)GG(T/G/S)WGQ. (Lower) Structure of the mature protein. The GPI anchor attaches the polypeptide chain to the membrane. (See Fig. 4B for a schematic of the core anchor structure.) Arrows A and B indicate the positions of cleavage sites in PrP, and arrow C a cleavage site in PrP. Site A lies within the GPI anchor, between the glycerolipid moiety and the ethanolamine residue that is attached to the C-terminal amino acid. Site B lies near position HO, and site C near position 89. (Reprinted with permission from Harris, 1999).

Early studies by Overath and Stumpf (P. Overath, 1964) established not only that the constituents of the avocado fatty acid synthesis system could be dissociated and reconstituted, but also that the heat stable fraction from E. coli known as acyl carrier protein (ACP) could replace the corresponding fraction from avocado. Plant ACPs share both extensive sequence homology and significant elements of three-dimensional structure with their bacterial counterparts. In plants, this small, acidic protein not only holds the growing acyl chain during fatty acid synthesis, but also is required for synthesis of monounsaturated fatty acids and plastidial glycerolipids. 

Fig. 2. Chemical stractures of platelet-activating factor (PAF) and structurally related ether-linked glycerolipids possessing biological activities.

The competitive binding experiments of Tischer and Strotmann (4) suggest that the phenylureas, biscarbamates, triazines, tria-zinones, and pyridazinones inhibit electron transport by interaction with the same component of PS II. Action at this site seemed to account for the phytotoxicity of pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone]. In addition to action at this site, compounds with molecular substitutions onto the structure of pyrazon (Figure 1) also interfere with the formation of chloroplast membrane lipids, namely the chlorophylls, carotenoids, and glycerolipids. 

Glycerolipids are lipids that contain glycerol in which the three hydroxyl groups are substituted in some way. In terms of quantity, these are by far the most abundant lipids in mammals. Somewhat similar in structure, but occurring at concentrations of less than 1% of the glycerolipids, are lipids that contain diols, i.e., ethylene glycol (ethane diol) and 1,2- and 1,3-propanediol. Because of their rarity, lipids based on diols are not discussed further here. 

Whereas the biodiesel synthesis from natural fats or vegetable oils does not alter the ester structure of glycerolipids, the intentionally conducted overhydrogenation has recently created considerable interest as a new synthesis... 

Most glycerolipids and sphingolipids in aqueous dispersions form closed vesicles, limited by lipids in the lamellar (bilayer) disposition. Depending on the lipid structure, different thermotropic transitions may be observed, of which the following are the most common. 

In plant seeds, glycerolipids can be synthesized in two similar pathways, which are known as prokaryotic and eukaryotic systems. These pathways are localized in different subcellular compartments and are characterized by a similar two-stage enzymatic conversion of 5 n-glycero-3-phosphate (G3P) into phosphatidic acid (PA), but different further conversions of PA into structural, storage, or signaling lipids [65]. Despite the closeness of these two biosynthetic pathways, enzymes that catalyze acylation reactions are unique to each system. 

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