Synthesis of Complex Lipids

The complex lipids in milk fat are comprised of the phosphoglycerides, phosphatidyl choline, phosphatidyl serine, phosphatidyl ethanolamine, phosphatidylinositol and plasmalogens. Also, the non-glyceride phospholipid, sphingomyelin, occurs in important amounts (Jensen, 2002). Bitman and Wood (1990) described the distribution of phospholipid classes in bovine milk and their fatty acid composition. The phospholipids comprise about 1% and cholesterol 0.4—0.5% of the total milk fat. These occur almost completely in the milk fat globule membrane. 

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A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

In the organism tissues, fatty acids are continually renewed in order to provide not only for the energy requirements, but also for the synthesis of multicomponent lipids (triacylglycerides, phospholipids, etc.). In the organism cells, fatty acids are resynthetized from simpler compounds through the aid of a supramolecular multienzyme complex referred to as fatty acid synthetase. At the Lynen laboratory, this synthetase was first isolated from yeast and then from the liver of birds and mammals. Since in mammals palmitic acid in this process is a major product, this multienzyme complex is also called palmitate synthetase. 

Little is known about the regulation mechanisms of the synthesis of complex carbohydrate in plants, through lipid intermediates. However, partial evidence indicates that lipid-mediated glycosylation in proteins could be a regulatory step. When glycosylation of carboxypeptidase Y is inhibited... 

Archaea are a group of organisms, previously classified as bacteria, from which eubacte-ria and other life may have evolved. A melavonic acid (six carbon)-building block is used for synthesis instead of acetic acid. The generated phytanyl chains are attached to glycerol moieties of complex lipids by ether linkages. Thus, these lipids are unlike anything found in eubacteria or eukaryotes today. 

Once host lipid or lipid constituents are acquired, both organisms respond in a similar fashion—modification of the gross lipid composition and, in some cases, the fatty acyl chain composition of complex lipids. In most cases it is unknown why these changes are needed. The malaria parasite may be attempting to alter the permeability of the parasitized cell to allow more facile entry of nutrients. A significant part of the African typanosomes lipid metabolism is directed toward synthesis of not only membrane lipid, but supply of the glycolipid anchor of its unique surface coat. 

Recent studies have revealed that marine viruses encode unexpected and novel proteins which would not be expected to occur within a virus genome. For example, the giant algal viruses have been shown to encode novel glycosylases, potassium pumps, and a pathway for the synthesis of complex sphingolipids. This biochemical diversity indicates that marine viruses could be a rich source for exploitation in the future for new types of carbohydrate and lipid as well as new proteins and enzymes. 

Roseman S. The synthesis of complex carbohydrates by multiglycosyltrans-ferase systems and their potential function in intercellular adhesion. Chem Phys Lipids 1970 5 270-297. 

The most important saturated fatty acid in higher plants is palmitic acid. Stearic acid, in contrast, occurs in low amounts as an acyl component of complex lipids. While the individual enzymes for the synthesis of palmitic acid have not been examined, there is no evidence at present that they are associated. There is now increasing evidence that the initial product of de novo fatty acid synthesis in plants is palmitoyl-ACP. The components of systems for the synthesis of palmitoyl-ACP include ACP, NADPH, NADH, acetyl-CoA, and malonyl CoA. In support of this, the following observations may be cited ... 

Scheme 8.9). The C NMR and optical rotation of trisaccharide 26 were identical to those reported by Lichtenthaler (vide supra) [36]. Crich and co-workers have also successfully applied their methodology towards the synthesis of complex P-mannan structures [48] and the mannosyl erythritol lipid MEL A [49]. 

Recent studies in humans and animal models have revealed that modulation of stearoyl-CoA desaturase-1 (SCDl) activity by dietary intervention or genetic manipulation strongly influences several facets of energy metabolism to affect the susceptibility to obesity, insulin resistance, diabetes and hyperlipidaemia (Flowers and Ntambi, 2008, 2009 Paton and Ntambi, 2008). SCDl catalyzes the DO-di desaturation of a range of fatty acyl-CoA substrates. The preferred substrate is stearoyl-CoA, which produces OA from stearic acid (18 0). The major product of SCDl, OA (18 ln-9), is the key substrate for the formation of complex lipids such as phospholipids, TAG, cholesterol esters, wax esters and alkyl-2,3-diacylglycerols. Reduced OA synthesis is associated with several metabolic changes that elicit protection from obesity, cellular Upid accumulation and insulin resistance (Miyazaki et al., 2000 Ntambi et al., 2002 Sampath et al., 2007). 

A survey of a number of spp. in the family Boraginaceae has shown that if the leaf contains both a andY-18 3, then 18 4 is always present. It seems, therefore, that there is a close correlation between the presence of 18 4 and the activity of theA6 and A 15-desaturase enzymes. These observations could be useful in understanding the relationship between the synthesis of unsaturated C18-fatty acids in the plant cell and the assembly of complex lipids for specific membrane (chloroplast) formation. Certainly such tissue provides a rather unique system with which it should be easier to interpret the movement of acyl species between the cell compartments and to assess more precisely the regulatory mechanisms involved. To this end we speculate here on the relationship between 46... 

Two forms of ACP have been purified from spinach and barley leaf, but, only one ACP Isoform predominates In spinach seed tissue (2, 3). Oleic acid Is the major product of fatty acid synthesis by Isolated chloroplasts (4). The regulation of plant lipid metabolism Is believed to Involve the export of oleic acid from Its site of synthesis (plastld) to sites of complex lipid metabolism (eg. ER). This pathway requires the Initial release of oleic acid from ACP by oleoyl-ACP thloesterase (5). In addition, the oleoyl moiety may also enter glycerollpld synthesis within the plastld through the action of acyl-ACP acyl-transferase (6). The preferred substrate for both the thloesterase and glycerol—3-phosphate acyl transferase reactions Is 18 1-ACP (6,7). 

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