Ether-linked glycerolipids containing a betaine moiety occur naturally in algae, bryophytes, fungi and in some primitive protozoa and photosynthetic bacteria. They are not found in flowering plants, but have been detected in some spore-producing plants, such as ferns and species belonging to the Equisetophyta. These lipids contain a polar group linked by an ether bond at the sn-3 position of the glycerol moiety, with the fatty acids esterified in the sn-1 and sn-2 positions. There is no phosphorus or carbohydrate group.
Three related lipids of this type have been described with differing permethylated hydroxyamino acids linked to diacylglycerols through an ether bond. They have a positively charged trimethylammonium group and a negatively charged carboxyl group, and they are therefore zwitterionic at neutral pH. The three types of betaine lipid are 1,2-diacylglyceryl-3-O-4'-(N,N,N-trimethyl)-homoserine, 1,2-diacylglyceryl-3-O-2'-(hydroxymethyl)-(N,N,N-trimethyl)-β-alanine and 1,2-diacylglyceryl-3-O-carboxy-(hydroxymethyl)-choline. Of these, the first is by far the most common in nature, and taxonomic studies suggest that it may have been the first lipid of this type to be formed during evolution.
In the diacylglyceryltrimethylhomoserine of most algae studied, the fatty acids in position sn-1 of the glyceryl moiety tend to be saturated (mainly 14:0 and 16:0), while those in position 2 are C18 unsaturated (predominantly 18:2(n-6) and 18:3(n-3)). However, marine algae can contain high proportions of polyunsaturated fatty acids (e.g. 20:5(n-3)) in both positions.
There is an obvious similarity between the structures of betaine lipids and that of the glycerophospholipid phosphatidylcholine. Although the phase transition temperature for the former was found to be slightly higher than that of phosphatidylcholine with an identical fatty acid composition, the physical phase behaviour of both lipids in mixtures with water is in general similar. There is some evidence for an inverse relationship between the presence of betaine lipids and phosphatidylcholine in the membranes of some organisms (but not in all), indicating that they can substitute for each other in part at least. For example, betaine lipids can accumulate in membranes to a level of 20% at the expense of phosphatidylcholine when phosphorus is limiting. This is accompanied by increases in the concentration of cyclopropyl fatty acid constituents in all lipids in the plant pathogen Agrobacterium tumefaciens.
The biosynthesis of diacylglyceryl-N,N,N-trimethylhomoserine has been studied in phosphate-starved cells of the purple bacterium Rhodobacter sphaeroides. Two enzyme systems have been identified as essential to the process. The first transfers the 3-amino-3-carboxypropyl group of S-adenosylmethionine to the 3-hydroxyl of a 1,2-diacyl-sn-glycerol to form the intermediate diacylglycerylhomoserine. The second enzyme system transfers methyl groups from S-adenosylmethionine in three successive steps to form the final product diacylglyceryl-N,N,N-trimethylhomoserine.
There is evidence from experiments with algae that the betaine lipids are involved in the transfer of fatty acids from the cytoplasm to the chloroplast, and that they may be the primary acceptor of fatty acids formed de novo before they are processed and redistributed to other lipids.
A similar type of lipid in which lysine is esterified to 1,2-diacylglycerol via an ester linkage (lysyl-diacylglycerol) has been isolated from Mycobacterium phlei strain IST, with palmitic and tuberculostearic acids as the fatty acid constituents.
Like the choline-containing lipids, betaine lipids display a blue coloration when sprayed with Dragendorff reagent. However, they are not stained by the typical reagents used to detect lipid-bound phosphorus. They are usually identified by this means on examination by thin-layer chromatography.
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Updated May 21, 2014