Betaine lipids

Diacylglyceryltrimethylhomoserine (DGTS) and diacylglyceryl hydroxymethyltrimethyl-β-alanine (DGTA) belong to a new type of glycerolipids called betaine lipids, which are composed of diacylglycerol andN-permethylated hydroxyamino acids, that are linked by an ether linkage. Betaine lipids are widely distributed in lower plants and algae as well as in some non-photosynthetic microorganisms. They are no longer regarded as “unusual” lipids but are important constituents of membranes of lower organisms. The present state of knowledge of the phylogenetic distribution, the fatty acid composition, the thermal properties, and the biosynthesis of the bataine lipids is briefly summarized in this review in a perspective of a future search for the biological roles and activities of betaine lipids. Recipient of the Botanical Society Award of Young Scientists, 1991.     

Archaeal lipids

The major archaeal membrane glycerolipids are distinguished from those of bacteria and eukaryotes by the contrasting stereochemistry of their glycerol backbones, and by the use of ether-linked isoprenoid-based alkyl chains rather than ester-linked fatty acyl chains for their hydrophobic moieties. These fascinating compounds play important roles in the extremophile lifestyles of many species, but are also present in the growing numbers of recently discovered mesophilic archaea. The past decade has witnessed significant advances in our understanding of archaea in general and their lipids in particular. Much of the new information has come from the ability to screen large microbial populations via environmental metagenomics, which has revolutionised our understanding of the extent of archaeal biodiversity that is coupled with a strict conservation of their membrane lipid compositions. Significant additional progress has come from new culturing and analytical techniques that are gradually enabling archaeal physiology and biochemistry to be studied in real time. These studies are beginning to shed light on the much-discussed and still-controversial process of eukaryogenesis, which probably involved both bacterial and archaeal progenitors. Puzzlingly, although eukaryotes retain many attributes of their putative archaeal ancestors, their lipid compositions only reflect their bacterial progenitors. Finally, elucidation of archaeal lipids and their metabolic pathways have revealed potentially interesting applications that have opened up new frontiers for biotechnological exploitation of these organisms. This review is concerned with the analysis, structure, function, evolution and biotechnology of archaeal lipids and their associated metabolic pathways.     

Cycling lipids

When synaptic vesicles fuse with the presynaptic plasma membrane, their membrane constituents are recycled by internalisation into clathrin-coated vesicles. To form new synaptic vesicles, the clathrin coat must be shed and recent studies reveal that a lipid phosphatase, synaptojanin, plays a central role in this process.     

Ether lipids

The naturally occurring 1-O-alkyl-sn-glycerols and their methoxylated congeners, 1-O-(2′-methoxyalkyl)-sn-glycerols, are biologically active compounds, ubiquitously found in nature as diacyl glyceryl ether lipids and phosphoether lipids. The chief objective of this article is to provide a comprehensive and up to date review on such ether lipids. The occurrence and distribution of these compounds in nature are extensively reviewed, their chemical structure and molecular variety, their biosynthesis and chemical synthesis and, finally, their various biological effects are described and discussed. An unprecedented biosynthesis of the 2′-methoxylated alkylglycerols is proposed. The first synthesis of enantiopure (Z)-(2′R)-1-O-(2′-methoxyhexadec-4′-enyl)-sn-glycerol, the most prevalent 2′-methoxylated type alkylglycerol present in cartilaginous fish, is described. It was accomplished by a highly convergent five step process.     

Yolk lipids.

The mature egg yolk of the domestic hen possesses remarkably constant lipid and lipoprotein composition despite much variation in dietary and environmental conditions. The greatest differences are seen in the fatty acid composition of the triacylglycerols which may show significant alterations in the content of the minor acids including certain polyunsaturated acids. The lipid class composition appears to be minimally affected by dietary influences, including the cholesterol content of the diet. The limited dietary influence on the yolk lipid composition extends to different strains of the hens. Genetic selection has led to some increase in the cholesterol content of the egg, but the desired lowering of the cholesterol content of egg yolk has not been realized. Likewise, production of a polyunsaturated fatty acid egg does not appear to be practical. As a result the egg yolk continues to provide a food product of nearly constant composition, which serves to maintain its chemical and physico-chemical properties for reliable utilization in the baking, cosmetic and pharmaceutical industries. The great uniformity in the composition of the egg yolk phospholipids makes them desirable starting materials for partial chemical resynthesis of glycerophospholipids. Partial hydrogenation of the egg yolk lipids promises to further increase the utility of the product as a desirable material for the manufacture of liposomes and liposome based drug products. In contrast, the constancy of the egg yolk composition and the inability to alter it significantly by dietary or genetic means also renders egg yolk undesirable for unlimited human consumption. Excessive ingestion of egg yolk raises plasma lipid and cholesterol levels which are believed to contribute to the development of heart disease. The physico-chemical and biological properties of egg yolk apoproteins have been less extensively investigated and their function is less well understood. The finding that phosvitin is a effective chelator of metal ions and thus an effective antioxidant demonstrates that egg yolk lipoproteins possess as yet unexplored potential for beneficial nutritional, medical and industrial application.     

Microorganisms for lipids

Micro-organisms have often been considered for the production of oils and fats as an alternative to agricultural commodities. However, with the continuing low cost of agricultural production of oil-seeds, biotechnology has little to offer in the way of competition against such items as soybean oil, groundnut oil, and even polyunsaturated oil such as sunflower oil even though good facsimiles of these oils could be produced. It is now clear that if we are to use microorganisms to produce lipids, i.e. Single Cell Oil, then these must be highly specific ones which are currently expensive to obtain from agricultural sources. In particular, the possible production of a cocoa-butter substitute using mutated yeast cells is reviewed here as well as the production of highly polyunsaturated fatty acids (PUFAs) using various species of mould. With regard to producing a cocoa-butter equivalent fat, it is necessary to identify not only a cheap carbon source on which the yeast may grow (this is now considered likely to be lactose arising from whey) as well as deriving a mutant strain of the yeast which will accumulate large amounts of stearic acid within the oil. The recent work carried out in The Netherlands and New Zealand to achieve these goals is described. With regard to polyunsaturated fatty acid production, various prospects exist and indeed, the commercial production of one acid, gamma-linolenic acid, is now being produced commercially in both the U.K. and Japan. The background work leading to this product is described and opportunities for other PUFAs being produced biotechnologically are discussed.