RHAMNOLIPIDS, SOPHOROLIPIDS AND OTHER SIMPLE GLYCOLIPIDS

STRUCTURES, OCCURRENCE AND BIOLOGY

Innumerable simple glycolipids, comprising simply fatty acids esterified to a carbohydrate moiety have been described in nature, from animals, plants and microorganisms, and it is impossible to discuss more than a few of these here. They can vary in structure from monosaccharides with one or more fatty acyl substituents to complex carbohydrates that can in turn be linked to terpenoids, aromatic compounds or nucleosides, as well as having multiple points of attachment to fatty acids via ester or glycosidic linkages. Some are integral components of tissues, while others produced by microorganisms are secreted into the growth medium. It is only possible to describe a few of the more important of these in this review. Because of their amphipathic nature, simple glycolipids are natural biodegradable detergents. In addition, some are reputed to have valuable pharmaceutical properties, for example as antibiotic, anti-fungal or even anticancer agents. A number of these lipids are major products of certain organisms, and have appreciable commercial importance. Substantial amounts of simple fatty acyl derivatives of sugars, e.g. sucrose esters, are produced in industry by chemical synthesis, but the discussion here is restricted to natural glycolipids.

1. Simple Carbohydrate-Fatty Acid Conjugates

Simple conjugates of mono- and disaccharides with fatty acids via glycosidic or ester bonds (alkyl or acyl glycosides) are common in nature, but especially in marine organisms and in plants. Little or nothing is known of their biological functions or biosynthesis and the reviews by Dembitsky cited below cover the literature thoroughly. In contrast, a glucopyranosyl derivative of tuberonic acid is known to induce tuber formation in potatoes (see our webpage on plant oxylipins). Mycobacteria produce 6-O-acylglucosides of mycolic acids in addition to the more complex trehalose lipids described below.

Linoleic acid is oxidized in the human liver by a P450 mono-oxygenase to a mixture of 9,10 and 12,13 epoxides, which are converted to the corresponding diols, termed leukotoxin and isoleukotoxin, by an epoxide hydrolase. Specific enantiomers of each of the four possible hydroxyl groups can then be converted to glucuronides by the action of a UDP-glucuronosyltransferase. The products from 9,10-dihydroxyoctadec-12-enoate are illustrated.

Structure - fatty acid glucuronide

A small proportion of the dihydroxy metabolites are also converted to glucuronide esters. As the precursor monoepoxides of linoleic acid are produced at high levels during acute inflammation, and in patients with adult respiratory distress syndrome or suffering from severe burns, it is believed that glucuronidation may be a detoxification mechanism, facilitating excretion. However, there are also suggestions that some fatty acid glucuronides, for example of phytanic and docosahexaenoic acids, may be ligands for hormone receptors in the nucleus or have signalling functions.

2. Rhamnolipids

Pseudomonads are rod-shaped gram-negative bacteria found in soils that produce extracellular lipids known as rhamnolipids. The term is indicative of the fact that these lipids contain one or two rhamnose units, linked glycosidically to a 3-hydroxy acid, thence by an ester bond to a further 3-hydroxy acid as illustrated. Thus, the monorhamnolipid from Pseudomonas aeruginosa grown on hydrocarbons is 2-O-α-L-rhamnopyranosyl-α-L-3-hydroxydecanoyl-3-hydroxydecanoic acid.

formulae of rhamnolipids

3- or β-Hydroxydecanoic acid is the most common fatty acid constituent, but other fatty acids may be found depending on the Pseudomonas species or growth conditions, including 12:0, 12:1, 12:2 and 8:2 (each with a 3-hydroxyl group), resulting in a number of distinct molecular species. All of these lipids have antifungal and antiviral properties, and they exhibit bactericidal properties to Gram-positive bacteria. Because of their potent detergent properties, they are produce commercially as soil remediation agents and to combat marine oil pollution. Although the exact mechanism is not clear, it is evident that rhamnolipids are able to bind to substrates with low degrees of aqueous solubility including hydrophobic pollutants. Rhamnolipids are also used as a source of L-rhamnose. Specific genetically modified Pseudomonas species can produce as much as 100g/L of culture medium under optimum conditions. While the wild organisms are pathogenic so must be cultured in a strictly regulated environment, the recombinant Pseudomonads appear to be safe.

The biosynthesis of monorhamnolipid involves two sequential glycosyl-transfer reactions catalysed by specific rhamnosyltransferases, in which 3-hydroxydecanoyl-3-hydroxydecanoate is linked to an activated rhamnose moiety (thymidine diphospho-rhamnose).

3. Sophorolipids

Some yeast species, and in particular Candida (Torulopsis) bombicola, secrete extracellular glycolipids known as sophorolipids (or sophorosides), as they contain the sugar sophorose (β-D-Glc-(1→2)-D-Glc). This is linked glycosidically to the hydroxyl group of a 17-hydroxy-C₁₈ saturated or monoenoic (cis-9) fatty acid, the carboxyl group of which is usually linked to the 4’-hydroxyl group of the second glucose unit to form a lactone, though it can also remain in free form and then have more powerful detergency properties. One or both of the 6-hydroxyl groups on the glucose units are acetylated. With the organism C. bognoriensis, the fatty acid is 13-hydroxydocosanoate, while in C. batistae it is 18-hydroxy-stearic acid (and the acidic form of the lipid predominates).

Formulae of sophorolipids

Biosynthesis involves sequential transfer of activated glucose molecules, UDP-glucose (see our webpage on glycosyldiacylglycerols), to a hydroxy acid in processes catalysed by two different glycosyltransferases. Finally, the molecule is acetylated by an acetyltransferase. The fatty acid constituents can be synthesised de novo from acetate or by modifying alkanes in the growth medium.

While the physiological role of sophorolipids in yeast species is uncertain, it seems likely that they serve for extracellular carbon storage (reducing the cellular sugar content) and as a defense against competing microorganisms.

These lipids are produced on a commercial scale when the organism is cultured on substrates containing glucose and a source of alkyl moieties, such as alkanes or seed oils, which influence the nature of the fatty acid constituent. Yields can be as much as 300g/L from organisms in the stationary phase. Sophorolipids are used in commerce in cosmetics as deodorant, anti-dandruff and bacteriostatic agents, and they are also known to possess antifungal, antiviral and spermicidal properties. The hydroxy acid constituents are in demand for lactonization for use in perfumes

4. Mannosylerythritol and Cellobiose Lipids

The yeast Candida (Pseudozyma) antarctica secretes an extracellular mannosylerythritol lipid, with biosurfactant properties, when grown on a vegetable oil substrate. When grown on glucose, the same lipid accumulates intra-cellularly until it amounts to 10% or more of the dry weight of the cell. One or two of the hydroxyls on the mannose residue are acetylated, and there are two esterified fatty acids, which are both are odd- and even-numbered from C₈ to C₁₂ in chain-length. While this organism gives the greatest yields of these lipids, they were first found in the fungus Ustilago maydis and termed ‘ustilipids’. In this instance, the 2-hydroxyl group of the mannose residue is esterified with a C2 to C8 fatty acid, while the 3-hydroxyl group is esterified by a C₁₂ to C₂₀ fatty acid. Several other organisms are now known to produce similar lipids. Mannosylerythritol lipids have been shown to have a number of profound biological effects, but especially to induce the differentiation of certain cancer cells.

formulae - mannosylerythritol and cellobiose lipids

Ustilago maydis also contains distinctive cellobiose lipids (or ‘ustilagic acid’), consisting of the disaccharide cellobiose linked O-glycosidically to the ω-hydroxyl group of the unusual long-chain fatty acid 15,16-dihydroxyhexadecanoic acid or 2,15,16-trihydroxyhexadecanoic acid. Others of the hydroxyl groups are esterified either to acetate or a medium-chain 3-hydroxy fatty acid. A further unusual cellobiose lipid is produced by the fungal biocontrol agent, Pseudozyma flocculosa, and has been show to be 2-(2',4'-diacetoxy-5'-carboxy-pentanoyl)octadecyl cellobioside (flocculosin), the compound responsible for the antifungal activities of the organism.

5. Trehalose Lipids

Trehalose is a non-reducing disaccharide in which the two glucose units are linked in an α,α-1,1-glycosidic linkage. It is the basic component of a number of cell wall glycolipids in Mycobacteria and Corynebacteria. Of these trehalose lipids, cord factor is the best known. It is a component of the cell wall lipid of M. tuberculosis and comprises a distinctive branched-chain mycolic acid esterified to the 6-hydroxyl group of each glucose to give trehalose 6,6’-dimycolate. In addition to being one of the major toxic components of the cell wall, it is believed to be responsible for the low permeability of the membranes conferring appreciable drug resistance to the organisms.

structure - cord factor

During biosynthesis, trehalose is first esterified to form the monomycolate, which is believed to be the precursor to the dimycolate, although via the action of a mycolyl transferase it also may be the donor of mycolic acid residues to the cell wall arabinogalactan to produce the mycolyl-arabinogalactan-peptidoglycan complex.

Among the other antigenic glycolipids in the mycobacterial cell wall based upon trehalose, there are acylated trehaloses with various fatty acids attached to the 2 and 3 hydroxyl groups of the same glucose. These fatty acids include n-C_16–19 saturated fatty acids, C_21–25 α-methyl branched fatty acids, and C_24-28 α-methyl-branched, β-hydroxy fatty acids. Trehalose lipids produced by Corynebacteria and Nocardia are similar in structure but contain the corynomycolic or nocardomycolic acids, respectively, which are related in structure to the mycolic acids. A strain of Rhodococcus erythropolis produces extra-cellular trehalose lipids containing succinic acid, i.e. 2,3,4,2''-di-O-succinoyl-di-O-alkanoyl-α,α-trehalose and 2,3,4-mono-O-succinoyl-di-O-alkanoyl-α,α-trehalose. Presumably, each of these lipids has a distinct but as yet unknown function. More complex sulfated trehalose lipids are also known.

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