Lipid Compositions in Plants and Microorganisms
Comprehensive discussion of the enormous literature on compositions of animal, plant and microbial tissues would be a daunting task, and it is only possible here to summarize briefly some of the more significant features of lipid composition, with some highly selective (and simplified) examples of relevant analyses. Further data are available for specific lipids in most of the web pages in this part of the website.
One problem in comparing data from different sources is the method of presentation, which is often dependent on the nature of the analytical methods used. For example, it is easier technically to analyse the phospholipids and glycolipids following isolation as distinct groups, separately from the simple lipids. Often there is no attempt subsequently to integrate the data. To add to the confusion, results of analyses of simple lipids are often reported in terms of weight percent of each lipid class, since the data are acquired in this form, while the results for phospholipids are most frequently recorded as molar percent, especially when phosphorus analysis is used as the means of quantification. Many of the data listed below are from publications that may appear old, but the results from modern lipidomics analyses are often difficult to summarize in a simple tabular form.
2. Lipid Class Compositions of Plant Tissues
The compositions of the lipids of plant tissues have been reviewed . Some results are listed in Table 1. Those plant tissues that serve as major food materials have received most study obviously. Triacylglycerols tend to be the most abundant class of storage lipid in tissues that are rich in lipids, such as the commercially important oil seeds. On the other hand, there are exceptions and jojoba oil, for example, consists mainly of wax esters. Many storage tissues in plants have starch as the main constituent rather than lipid, and in potato tubers and apples the complex glycolipids and phospholipids are the only lipids present at low levels. In addition to those lipids listed, sterols, sterol esters, acylated sterol glycosides, phytoglycolipid, ceramide, glucosylceramide, phosphatidic acid, N-acylphosphatidylethanolamine, and phosphatidylserine, amongst others, may be found. It should be recognized that seeds and tubers do not have a homogeneous lipid distribution, the endosperm, germ, bran and other organelles each having a distinctive composition.
|Table 1. The lipid class compositions (weight % of the total lipids) of various plant tissues|
|Lipid class||Potato tuber||Apple fruit||Soybean seed||Clover leaves||Rye grass||Spinach chloroplasts|
The glycosyldiacylglycerols, i.e. mono- and digalactosyldiacylglycerols and sulfoquinovosyldiacylglycerol, are the most abundant lipid classes in leaf (photosynthetic) tissues. Glycerophospholipids, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylglycerol, are also present and other complex lipids are occasionally reported. Phosphatidylglycerol appears to be especially characteristic of photosynthetic tissue, and it can be the main glycerophospholipid in certain green algae. Triacylglycerols are virtually absent from leaves.
As in animal tissues, each of the membranes or organelles in the leaf has a characteristic lipid composition. Spinach chloroplasts have received a great deal of attention, because they can be prepared relatively easily for biochemical experiments, and like the intact leaf they contain appreciable amounts of the glycolipids and a smaller proportion of glycerophospholipids. In contrast, as in animal tissues, the plasma membrane has a high content of phosphatidylcholine, while the mitochondria contain cardiolipin.
A further region of plants with a distinctive composition is the epidermis or cuticle. The lipids here tend to be rich in waxes, and can include cutin and suberin, which are complex polyesters of hydroxy fatty acids. Leaf surface lipids are largely non-polar waxes. It has been argued that these may be the most abundant lipids on Earth, considering that leaves cover much of the land surface.
2. Fatty Acid Compositions of Plant Tissues
The fatty acid compositions of the seed oils of importance to commerce have been reviewed , and data for a few typical ones are listed in Table 2. Maize (corn), sunflower and safflower oils are of nutritional value since they contain appreciable amounts of the essential fatty acid - linoleic acid. Excessive amounts of linolenic acid, as in soybean oil, can lower the commercial value of an oil because it is then more susceptible to rancidity problems caused by autoxidation; it is therefore a common industrial practice to subject the oil to hydrogenation. In contrast, there are no such problems with olive oil, an important lipid constituent of the ‘Mediterranean’ diet, with its high content of oleic acid. Palm oil contains a higher proportion of saturated fatty acids than most seed oils. Similarly, cocoa butter consists largely of molecular species with saturated fatty acids in positions sn-1 and -3 and oleic in position sn-2. Rapeseed is one of the few oil crops capable of being grown in northerly climates. In its native form, it tends to have a high content of erucic acid (22:1(n-9)), which may have some properties that may be harmful to the consumer, although this is still a matter for controversy. However, new cultivars with negligible levels of this component (‘Canola’) are now widely grown. Cottonseed oil resembles maize oil in its composition, but also contains small amounts of the cyclopropene fatty acid, ‘sterculic’ or 9,10-methyleneoctadecenoic acid, which has well-established toxic properties and must be removed during refining. Palm kernel and coconut oils are noteworthy for a high content of saturated fatty acids of medium chain-length.
|Table 2. The fatty acid compositions (weight % of the total) of some seed oils |
|Fatty acid||Seed oil|
|a Newer cultivars can contain much less erucic acid.|
The picture will become more complex as new genetically modified seed oils are introduced. In addition, there are many seed oils which may have limited or negligible commercial value at present, but contain fatty acids with unusual substituent groups and are of great interest to biochemists.
Each of the lipids in a plant tissue can have a characteristic fatty acid composition and for illustrative purposes, some results for spinach leaf lipids are listed in Table 3.
|Table 3. The fatty acid compositions (weight % of the total) of the individual lipids of spinach leaves |
|Fatty acid||Lipid class|
|a MGDG, monogalactosyldiacylglycerol; DGDG, digalactosyldiacylglycerol; SQDG, sulfoquinovosyldiacylglycerol; PG, phosphatidylglycerol; PC, phosphatidylcholine; PI, phosphatidylinositol; PE, phosphatidylethanolamine.|
Glycosyldiacylglycerols tend to consist mainly of the unsaturated fatty acids, linoleic acid and especially linolenic acid; a hexadecatrienoic (16:3(n-3)) acid may be present also with certain species. On the other hand, the glycerophospholipids contain higher proportions of saturated fatty acids, generally palmitic acid, in addition to the unsaturated components. Phosphatidylglycerol is unique in that it contains a substantial amount of an unusual fatty acid, i.e. trans-3-hexadecenoic acid. The fatty acid compositions of plant tissues can vary with climatic and other cultivation conditions, and with the stage of development of the tissue, and major species differences occur. However, the results listed in Table 3 are typical.
3. Fatty Acid Distributions within Glycerolipids in Plant Tissues
Once more, the triacylglycerols of the commercial seed oils are those that have been subjected most frequently to detailed structural analyses. In general, there tends to be little difference between the compositions of positions sn-1 and sn-3 of the glycerol moiety, but the saturated fatty acids are concentrated in the primary positions and the unsaturated are in greatest abundance in position sn-2. In some instances, there appears to be a higher proportion of longer-chain fatty acids (C20 to C22) in position sn-3 than in position sn-1, and sometimes the more unusual fatty acids are concentrated in position sn-3.
The positional distributions of fatty acids in many of the glycerophospholipids of plants seem to resemble those of animal tissues in that the saturated fatty acids are concentrated in position sn-1 and the unsaturated in position sn-2. However, phosphatidylglycerol from spinach leaves and the ‘model’ plant Arabidopsis thaliana is unusual in that the major molecular species contains linolenic acid in position sn-1 and trans-3-hexadecenoic acid in position sn-2 [9,10]. Data for both glycoglycerolipids and glycerophospholipids are listed in Table 4.
|Table 4. Composition (mol %) of fatty acids in positions sn-1 and sn-2 of mono- and digalactosyldiacylglycerols, phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol from leaves of Arabidopsis thaliana |
|Lipid class||Position||Fatty acids|
In those plants containing 16:3(n-3), the monogalactosyldiacylglycerols consist mainly of the 16:3-18:3 combination (with all the 16:3 in position sn-2), while the digalactosyldiacylglycerols have the more common 18:3-18:3 species. The distinctive compositions of the phosphatidylglycerol and monogalactosyldiacylglycerol are believed to result from the existence of a primitive prokaryotic biosynthetic pathway in addition to the more usual eukaryotic pathway in the chloroplasts . This is discussed further in our web page on glycosyldiacylglycerols.
4. Lipids of Microorganisms
Lipid Class Compositions
It is not easy to generalize about the lipids of microorganisms, as each family tends to have a distinct and characteristic lipid composition, and there are many lipid classes that are unique to particular groups. In addition, the fatty acid components are often very different from those of animal or plant tissues, and there are some that have as yet been found in certain rare species of microbes only. For this reason, I have not attempted to tabulate data here. Microbial lipids have been reviewed elsewhere in great detail . The nature and composition of microbial lipids have proved to be of great taxonomic value, and their study has assisted towards an understanding of the molecular basis of evolution.
Simple glycerolipids such as triacylglycerols are often the most abundant lipids in fungi, but bacteria do not normally accumulate storage lipids of this type, although they are known to be present in a few species; diacylglycerols may be formed transiently as intermediates in the biosynthesis of glycerophospholipids. Sterols are found in yeasts, fungi and algae, but in essence are absent from bacterial membranes.
Most of the common glycerophospholipids of plants and animals that are described above are present in microorganisms. For example, phosphatidylethanolamine is often the most abundant lipid class in many bacterial species (including both Gram-negative and Gram-positive bacteria), and may be accompanied by phosphatidylserine, phosphatidylglycerol, and phosphatidylinositol. The N-mono- and N,N-di-methyl derivatives of phosphatidylethanolamine are found in some bacterial genera. On the other hand, phosphatidylcholine is not a ubiquitous constituent of microbial lipids, rarely being found in bacteria, although it is often the major lipid class in eukaryotes. In bacteria, phosphatidylglycerol especially is widely distributed and it is found in most genera; uniquely it is the only glycerophospholipid in the membranes of certain cyanobacteria. Phosphatidylglycerol is usually accompanied by cardiolipin, which is often a major lipid component of bacteria, although it is only found as a constituent of mitochondrial membranes in eukaryotes. In addition, derivatives of phosphatidylglycerol (lipoamino acids) in which the glycerol moiety is esterified to an amino acid, such as lysine, ornithine, or alanine, are common in Gram-positive bacteria.
The plant glycosyldiacylglycerols are also found as major constituents of the membranes of algae and cyanobacteria, although glucose may replace a galactose unit in some species. Diglycosyldiacylglycerols, in which the carbohydrate moiety may be two glucose, two mannose or less often two galactose units, are found as minor components of the membranes of Gram-positive and occasionally of Gram-negative bacteria. Similarly, the plant sulfolipid, sulfoquinovosyldiacylglycerol, is present in small amounts in certain photosynthetic bacteria. Other bacterial sulfolipids tend to be sulfate esters of glycolipids.
Complex phosphorylceramides containing inositol are major components of yeast lipids, but sphingolipids are rarely found in bacteria. On the other hand, there always appear to be exceptions to any rule of this kind, and ceramide phosphorylethanolamine and ceramide phosphorylglycerol have been detected in some anaerobic bacteria. Free ceramides have been found in some species of Bacteriodes. Aminolipids containing alkylamines, but lacking the hydroxyl groups of the sphingoid bases, occur in some bacteria, while others contain sulfonolipids (capnoids), which appear to be related structurally to the long-chain bases. For example, capnine is 2-amino-3-hydroxy-15-methylhexadecane-1-sulfonic acid and is found in the free form or as the N-acyl derivative in some species of gliding bacteria.
Many bacterial species contain certain glycerophospholipids and glycolipids (including glycophospholipids), which are apparently found nowhere else in nature. Glycoglycerophospholipids are common constituents of bacterial membranes. For example, although phosphatidylinositol itself is rarely found, various mannoside derivatives do occur in some species. Glycosylated forms of phosphatidylglycerol are perhaps the most common of all. To add to the complexity, the glyceryl moiety can also be esterified with an amino acid, the nature of which is dependent on the bacterial species, and the stereochemistry of the glyceryl group can vary between species in these lipids. Glucosaminylphosphatidylglycerol has been found in both Gram-negative and Gram-positive bacteria. Certain rumen bacteria contain lipids with complex glycerophosphoryl groups and glycosylglycerol groups, linked by a C32 dicarboxylic acid (‘diabolic acid’), and may span the membrane bilayer. The lipoteichoic acids of the cell walls of Gram-positive bacteria are polymers of glycerol-1-phosphate and other complex organic groups linked to a diglycosyldiacylglycerol.
Relatively simple glycolipids, such as di- and triacylglucoses, and an analogous rhamnolipid are present in some bacteria. Much more complex lipid polysaccharides, for example ‘Lipid A’, occur in the cell envelope of certain microbial species with phosphatidylinositol as the anchor moiety in the membrane. Similarly, the cell walls of the Mycobacteria contain a wide range of lipid polymers, including trehalose derivatives, phosphate esters, peptidolipids and phenolic lipids, and these may be esterified to a bewildering array of unusual fatty acids (see below).
A further group of distinctive bacterial lipids are the acylornithines. In these, the amine group of ornithine is attached via an amide bond to a fatty acid with a 2- or 3-hydroxyl group, which can in turn be esterified to a further fatty acid. The carboxyl group of the ornithine can also be linked to an aliphatic alcohol. The precise nature of the aliphatic groups and of the linkages can be characteristic of particular species.
The glycerophospholipids of bacteria exist not only in the diacyl forms but also as ether lipids, most often the plasmalogen forms. In addition, many different complex di- and tetra-ether phospholipids and glycolipids, in which the aliphatic moieties are phytanyl or related structures, are present in archaebacteria, thermoacidophiles and thermohalophiles .
Fatty Acid Compositions
Polyunsaturated fatty acids of the kind found in plant lipids occur also in algae (green and brown), fungi and cyanobacteria, but are not often present in other bacteria (some marine species are exceptions). In general, bacterial lipids tend to contain appreciable amounts of C14 to C18 straight-chain saturated and monoenoic fatty acids. The common C18 monoenoic acid is not oleic acid, however, but cis-vaccenic acid (18:1(n-7)). In addition, bacterial lipids can contain odd-chain, branched-chain (mainly iso- and anteiso-methyl, but 10-methyloctadecanoic or ‘tuberculostearic’ acid is characteristic of some species), cyclopropane and 3-hydroxy fatty acids, which are only rarely synthesised by other organisms. The presence of a methyl branch or of a cyclopropane ring in the fatty acids in a membrane increases its fluidity in an analogous manner to that of double bonds in polyunsaturated fatty acids in the membranes of higher organisms. In comparing the detailed fatty acid compositions of bacteria, it is important to recognize that they can vary greatly with culture conditions and with stage of growth.
The Mycobacteria and certain related species contain a highly distinctive range of very-long-chain α-branched β-hydroxy fatty acids. They occur in the bacterial cell walls in the free form, as wax esters and as components of complex lipids.
5. Lipidomic Analyses
As cautioned earlier, it is not easy to present the results of modern lipidomics analyses in a concise tabular fashion, but the immense contribution the newer mass spectrometric methods are making to our understanding of lipid compositions cannot be ignored. However, Ruth Welti discusses plant lipidomics elsewhere on this site. Information on the techniques of lipidomics in general is available in our web pages here..
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Acknowledgement: This document is based on part of Chapter 1 of the Third edition of Lipid Analysis by the author and published by P.J. Barnes & Associates (The Oily Press Ltd). It was omitted from the Fourth (and latest) edition of the book to save space.
Updated May 2, 2011