Lipid, unsaturated

Nichols DS, MR Miller, NW Davies, A Goodchild, M Raferty, R Caviccholi (2004) Cold adaptation in the Antarctic arch eon Methanococcoides burtonii involves membrane lipid unsaturation. J Bacterial 186 8508-8515. 

In biological membranes, the class of fully saturated PC lipids is just one of many. Indeed, variations in the lipid architecture are frequently encountered. There are variations with respect to the head-group structure as well as variations in the tail architecture. For example, in the tails, single or multiple unsaturated bonds are very common. There is relatively little knowledge about the role of lipid unsaturation in biomembranes. There are relatively few MD simulations on bilayers composed of lipids with unsaturated bonds. Nevertheless, single unsaturation as well as multiple unsaturated chains have been of some interest. For a recent review see reference [102], We will now present a selection of the results. We will also take this opportunity to compare the results from the MD simulations with those from the SCF calculations. 

For the NO studies that show inhibition of lipid peroxidation, we used HL-60 cells made more oxidizable by adding 32 pM docosahexaenoic acid (22 6q)3 ) to the growth media. When unenriched cells were examined, inhibition of oxidation was qualitatively identical, but the magnitude of the effect was less, and therefore more difficult to measure. The effect of lipid unsaturation will be developed below. 

Ivanov (1929) found a close inverse relationship between the value of lipid unsaturation of plants (determined by iodine value) and the latitude of their habitat. It was found that, with lower environmental temperatures, the iodine numbers of the lipids were greater (i.e. there was more unsaturation). The effect of increasing unsaturation is to lower the temperature at which the lipids change from liquids to a liquid-crystalline state, and so to lower the temperature at which the cells remain flexible. 

Figure 33 Seasonal dynamics of lipid unsaturation in anchovy. (After Shulman, 1978a Yuneva, 1990.) Continuous line, iodine value broken line, total polyenoic acids.

All kinds of selectivity of hydrogenation are important here, notably chemoselec-tivity (polyunsaturated versus monounsaturated lipids, unsaturated fatty acids versus quinones), regioselectivity (terminal versus internal alkeneic units), and stereochemical selectivity (cis- versus trans-fatty acid units). An ideal catalyst is able to reach particular membranes in various compartments of the cell ( targeting ), does not effect transformations other than hydrogenation, efficiently reduces the unsaturated fatty acid units at low temperatures (0-40°C) in an aqueous environment, can be totally removed from the cell after the reaction is completed, and has no self-effect, such as toxicity. 

Sakamoto, T., Los, D.A., ITigashi, S., Wada, IT., Nishida, I., Ohmori, M. and Murata, N. Cloning of omega 3 desaturase from cyanobacteria and its use in altering the degree of membrane-lipid unsaturation. Plant Mol Biol, 26 (1994a) 249-263. 

C. butyricum appears to regulate the stability of the bilayer arrangement of membranes by altering the ratio of ether versus acyl ethanolamine phospholipids in response to changes in the degree of lipid unsaturation of the membranes. Experiments with bacteria indicate that substitution of plasmenylethanolamine for phosphatidylethanolamine in biomembranes would have only small effects on lipid melting transitions, whereas the tendency to form non-lamellar lipid structures would be significantly increased. 

Characterization of unsaturated FA, as well as those FA that possess some substitution in the chain, is clearly a difficult task. As it was indicated in the previous section on derivatization, a continual suggestion of new derivatives from different laboratories over the years point to some deficiencies in this area [221]. In addition, better methods are needed to recover quantitatively polyunsaturated acids from various lipids [210]. Lipid unsaturation is of much current interest in relation to medicine and dietary research. 

Neidleman, S.L. Effects of temperature on lipid unsaturation. Biotechnol. Genet. Eng. Rev. 1987, 5, 245-268. 

Exceptions to cold temperature adaptation being accompanied by shorter chain or more unsaturated fatty acids Include mycelial phospholipids with greater unsaturation when grown at 36 C vs 20 C (77), and the lack of correlation of cold temperature acclimation with Increased lipid unsaturation or membrane fluidity by four fungi representative of four different fungal classes (78). 

Fluorescence techniques are very sensitive. On a molar basis, the amount of malonaldehyde detected by fluorescence is 10 to 1(X) times more sensitive than the colorimetric TB A assay. However, this method is not specific, as it measures complex mixtures resulting from the interactions of oxidized lipids, unsaturated aldehydes and malonaldehyde with proteins, peptides, amino acids, phospholipids, DNA and nucleic acids. These interactions involve oxidized or polymerized species of proteins or amino acids. The fluorescence method provides a non-speciflc but sensitive measure of oxidative deterioration in complex foods such as meats and fish, which are often difficult to analyse for... 

Synthetic mixtures of neutral lipids, skin lipids, unsaturated fatty acids, serum lipids Neutral lipids in Biom-phalaria glabrata... 

A new series of studies [84] has focused on bilayers composed of unsaturated lipids. This was motivated in part by the fact that (1) bilayer permeability to glycerol was previously found to increase with lipid unsaturation [8] and (2) the solubility of water in alkenes is slightly greater than in alkanes. Transport of various molecules across the bilayer occurs by diffusion down a concentration gradient. The permeability, that characterizes this mass transfer is a property of the membrane composition, the solute, and the temperature,... 

Browse, JB (1988) Reduced levels of lipid unsaturation in mutants of Arabidopsis. Plant Physiol i 39... 

Marinova EM, Yanishheva NV. 1994. Effect of lipid unsaturation on the oxidative activity of some phenohc acids. Journal of American Oil Chemists Society, 71 427-434. 

Figure 2. A scheme for control of 16 2 and 18 2 desaturation by the fadP gene product. We suggest that fadP controls a chloroplast desaturase which accepts both 16 2 and 18 2 as substrates. The specificity with respect to head group is not known but must include MGP since 16 3 is formed on MGP. In order to affect cytoplasmic lipid unsaturation either the acyl group must be exported (shown here) or the glycerolipid moiety must be transferred (not shown). In the latter case the transfer process would have to exclude MGP with 16-carbon acyl groups at the sn-2 position since such lipids are not found in cytoplasmic membranes (Roughan and Slack, 1982).

It appears that plant lipid metabolism may be amenable to experimental manipulation by the isolation of a wide spectrum of mutants. We anticipate that the mutants described here will provide useful tools for studies of the regulation and functional significance of lipid unsaturation. The anticipated development of more advanced methods of genetic analysis in Arabidopsis (Estelle and Somerville, 1986) may also facilitate the isolation of the genes which complement these mutations. 

Another mutant, LK3, is deficient in activity of w-6 desaturase and palmitoleic and oleic fatty acyl residues dominate the composition of thylakoid galactolipids [4]. Biochemical studies of the mutant have shown that the reduced level of lipid unsaturation is associated with marked changes in protein and chlorophyll content of the thylakoid membranes and a reduction in photosynthetic electron transport rates. Like the JB67 mutant, photosynthetic electron transport of the LK3 mutant is more heat stable compared to the wild-type [6]. 

Hugly S, Kunst L, Browse J, Somerville CR. Enhanced thermal tolerance and altered chloroplast ultrastructure in a mutant of Arabldopsls deficient in lipid unsaturation. Plant Physiol 1989 90 1134-1142. 

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