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Miscellaneous Lipid Sulfates and Sulfonates



Sulfation is a common reaction in lipid biochemistry for manifold reasons. For example, it can enable solubilization of a lipid or it can convert a complex lipid to a highly anionic form for specific purposes in membranes. Sulfonolipids have the sulfur atom linked directly to a carbon atom, while in lipid sulfates the sulfur is linked to the lipid component by an oxygen atom. Sphingolipid sulfates are vital for the function of the brain and kidney, amongst other tissues, and they have their own web page on this site. Similarly, seminolipid, cholesterol sulfate and lipo-chitooligosaccharides (Nod factors) are important sulfur-containing lipids, which are discussed on other web pages here for reasons of their relevance to other topics. The best known and most abundant of the sulfonolipids in nature is sulfoquinovosyldiacylglycerol or 1,2-di-O-acyl-3-O-(6'-deoxy-6'-sulfo-α-D-glucopyranosyl)-sn-glycerol, a key component of the photosynthetic mechanism of higher plants and other photosynthetic organisms. Because of its biosynthetic and functional relationship to the mono- and digalactosyldiacylglycerols, it is discussed in those web pages. However, there remain many interesting lipid sulfates and sulfonates, which are of crucial importance to the organisms that produce them.


1.   Chlorosulfolipids

The phytoflagellate Ochromonas danica (a chrysophyte alga) contains a number of linear alkanes (C22 and C24) substituted with sulfate groups and with both sulfate groups and chlorine, i.e. chlorosulfolipids, the discovery and exploration of which are largely associated with Thomas H. Haines. Generally, there are two sulfate groups in the 1 and 14 positions of the C22 alkyl chain (positions 1 and 15 of the C24 compounds), and there can be one to six chlorine atoms in various positions. They constitute approximately 15% of the total lipids, but 90% of the polar lipids of the flagella. Eight chlorosulfolipids have now been characterized from this organism, and some representative examples are illustrated. Of these 'danicalipin A' (the last illustrated) is the major component, and the stereochemistry of the substituents has been determined.

Structures of some chlorosulfolipids of Ochromonas danica

Many aspects of the biosynthesis of these unusual lipids remain to be confirmed, but it is believed that docosanoic acid is hydroxylated at C14, before reduction to the 1,14-diol and sulfation. Chlorine atoms are presumed to be inserted into the saturated alkyl chain of the 1,14-diol sulfate in a stepwise fashion by a free radical process involving chlorinases, which have yet to be characterized. The two end points of the biosynthetic process are 2,2,11,13,15,16-hexachloro-1,14-docosanediol disulfate and 2,2,12,14,16,17-hexachloro-1,15-tetracosanediol disulfate. O. danica is unable to remove the sulfate groups, so the lipids are remarkably inert metabolically. When this organism is grown with excess bromide ions, bromosulfolipids are produced with the same positional and stereochemical selectivity as in danicalipin A.

Such a high proportion of chlorosulfolipids (and an absence of phospholipids) in the flagella of O. danica implies that they must be the major constituents of the membranes of this organelle, which must be differentiated in some manner from the contiguous exterior surface membranes. At first glance, it is not easy to understand how such lipids, which are highly soluble in water and carry a polar substituent in the centre of the hydrocarbon chain, can form a membrane bilayer. This can only be possible if there are some positively charged ions buried deep in the hydrocarbon layer that shield the negative sulfate groups. Haines has postulated that an as yet unidentified molecule, possibly a divalent metal ion or protein bearing charged residues, offsets the negative charge of the sulfate group at the physiological pH. The anionic lipid head groups may serve as a proton-conducting pathway along the surface of membranes.

Since the initial studies, a range of further chloro- and bromosulfolipids have been found in algae and other organisms, and as toxins affecting shellfish. Many of the chlorosulfolipids found in fresh-water algae are the same as those of O. danica, but others have some distinctive features, including a chlorovinylsulfate group. Complex undecachlorosulfolipids, isolated from the digestive glands of toxic mussels, are causative agents of diarrhetic shellfish poisonings, which tend to be associated with marine algal blooms.


2.   Taurine-containing Lipids (Taurolipids)

A number of lipids have been found that are conjugated to taurine (ethanolaminesulfonic acid), of which the best known are certain bile acids, which are discussed elsewhere on this site. Taurine itself is synthesised from cysteine via oxidation and decarboxylation reactions. It is very rare in plants but is plentiful in animal tissues, especially the brain. Aside from the bile acids, the first taurolipids to be recognized were novel C18 hydroxy acids (3, 4 or 5 hydroxyl groups) with an amide link to taurine, which were isolated from the ciliated protozoan Tetrahymena. The hydroxyl on carbon 3 is acylated with normal fatty acids (approx. 30% 16:0), and in one variant, carbon 7 is similarly acylated. The deacylated backbone has been termed ‘lipotaurine’. Biosynthesis is believed to involve conjugation of stearic acid with taurine, with subsequent sequential insertion of hydroxyl groups.

Formula of a taurolipid
Taurolipid R1 R2 R3 R4
 
Taurolipid A OH OH H H
7-Acyltaurolipid A CH3(CH2)14COO OH H H
Taurolipid B OH OH OH H
Taurolipid C OH OH OH OH
 

A range of fatty N-acyltaurines have been isolated from both the central nervous system and peripheral tissues of animals. In the former, the fatty acyl groups are largely long-chain saturated, but in liver and kidney, arachidonoyl and docosahexaenoyl species predominate. They are believed to be produced through enzyme-dependent conjugation of fatty acyl-CoA esters with taurine via the action of the peroxisomal acyltransferase, acyl-CoA amino acid N-acyltransferase-1. Like the other biologically active amides in animals, the levels of these metabolites are controlled by the activity of the fatty acid amide hydrolase (FAAH).

In kidney, N-acyltaurines have been shown to activate receptors that control calcium channels, and in pancreatic β-cells this occurs also with triggering of insulin secretion. N‑Arachidonoyl- and N‑oleoyltaurine have been shown to induce a significant inhibition of a cancer cell line in vitro, but the 20:0 and 24:0 fatty acyl analogues are produced endogenously in vivo and regulate the healing of skin wounds in mice. Arachidonoyltaurine has been shown to be an excellent substrate for lipoxygenases, but the functions of the resulting hydroxyeicosatetraenoyltaurines have yet to be determined.

Formulae of miscellaneous taurolipids

Marine invertebrates are a rich source of unusual lipids. For example, a biologically active taurine-containing lipid, termed 'irciniasulfonic acid B', was isolated from a marine sponge, Ircinia sp., and comprised 3-methyl-8-hydroxy-dec-2-enoic acid conjugated to taurine, with various unusual fatty acids linked to the hydroxyl group. Carteriosulfonic acids and taurospongin A are related lipids found in sponges. In addition, compounds termed copepodamides and consisting of taurine connected by an amide linkage to isoprenoid fatty acids, which are conjugated in turn to polyunsaturated fatty acids, were shown to be produced by marine copepods (zooplankton). At minute concentrations, these act as a signal to bloom-forming dinoflagellates (phytoplankton) and induce production of paralytic shellfish toxins, presumably as a defense response. An unusual N-acyltaurine, linked to a dihydroxy acid, has been found in a sea urchin. Cerilipin is a bacterial analogue of the ornithine lipids and is discussed in that context.

A tauroglycolipid, 1,2-diacyl-3-glucuronopyranosyl-sn-glycerol taurineamide, was isolated from a seawater bacterium Hyphomonas jannaschiana, which has the further unusual feature of an absence of phospholipids. The main fatty acyl chains are saturated and monoenoic (C16 to C20).

Formula of 1,2-diacyl-3-glucuronopyranosyl-sn-glycerol taurineamide

An unusual ganglioside, taurine-conjugated GM2, has been isolated from brain samples from patients with Tay-Sachs disease, a well-known glycosphingolipid (GSL) storage disease. In this unusual lipid, the carboxyl group of N-acetylneuraminic acid is amidated by taurine. As this lipid is not present in normal brains, it seems probable that it is associated with the pathogenesis of the disease, possibly as a means of removing the excess of GM2 from the tissue.


3.   Other Lipid Sulfates

The first sulfated fatty acids to be identified were the 'caeliferins', which were found in the oral secretions of a species of grasshopper. They are believed to elicit the release of volatile organic compounds as a defence response when the insects graze upon plants. Sulfate esters of oleate, linoleate and linolenate have now been found in some fungal species, where they may have antifungal activity.

Structure of caeliferin A

The outer membranes of the cell walls of virulent strains of Mycobacterium tuberculosis contain a number of trehalose-containing glycolipids. Some of these are sulfated and consist of a sulfated trehalose moiety to which up to four fatty acids are linked, including palmitate or stearate and two or three of the very-long-chain multi-branched acids that are characteristic of Mycobacteria (phthioceranic acids). There are two main forms; sulfolipid-I comprises a family of homologous 2-palmitoyl(stearoyl)-3-phthioceranoyl-6,6'-bis(hydroxyphthioceranoyl)trehalose 2'-sulfates, while sulfolipid-II constitutes a family of homologous 2-stearoyl(palmitoyl)-3,6,6'-tris(hydroxyphthioceranoyl)trehalose 2'-sulfates (see also our web page on mycolic acids).

Sulfolipid from mycobacterium tuberculosis

Biosynthesis is initiated by a sulfotransferase that converts trehalose into trehalose-2-sulfate. As trehalose sulfates increase the virulence of M. tuberculosis in human cells in vitro, but apparently not in other animal models, inhibition of their biosynthesis is an important pharmacological target. These lipids do not appear to be present in other species of Mycobacteria.


4.   Other Sulfolipids

In 1978, the marine diatom, Nitzschia alba, was found to contain a number of interesting sulfonolipids as membrane constituents, i.e. 1‑deoxyceramide-1-sulfonate and phosphatidylsulfocholine (a sulfonium analogue of phosphatidylcholine), in addition to sulfoquinovosyldiacylglycerol. The last is present in an amount comparable to that in higher plants, although the organism is not photosynthetic. 24-Methylene-cholesterol sulfate was detected also.1-Deoxyceramide-1-sulfonate was first isolated from the Bryozoa Watersipora cucullata and consists of a long chain-base, analogous to sphingosine but with a sulfonate moiety attached to carbon 1. The predominant fatty acid (64%) in the Nitzschia lipid is trans-3-hexadecenoic acid, which is normally associated with the phosphatidylglycerol of plant chloroplasts.

Formulae of 1-deoxyceramide-1-sulfonate and phosphatidylsulfocholine

Phosphatidylsulfocholine, with two methyl groups attached to the sulfur atom as opposed to three attached to nitrogen, completely replaces phosphatidylcholine in Nitzschia alba. However, it has subsequently been found in other marine diatoms and algae that also contain phosphatidylcholine. Experiments with isotopically labelled substrates in Nitzschia alba confirmed that both methyl groups and the sulfur atom were derived from methionine.

A sulfonic acid derivative of ceramide, N-fatty acyl capnine or capnoid, is recognized as a major lipid component of gliding bacteria of the genera Cytophaga, Capnocytophaga, Sporocytophaga, and Flexibacter. These are organisms that are able to move over solid surfaces, but not through liquids, although they do not appear to have flagella or other organs of propulsion. Capnine is 2-amino-3-hydroxy-15-methylhexadecane-1-sulfonic acid and occurs in the organisms both in the free form and as N-acylated derivatives, though up to 20% of other homologues can occur, depending on species. The N-acyl fatty acids are much more heterogeneous and vary from C14 to C16 in chain-length, a high proportion with iso- or anteiso-methyl branches and hydroxyl groups in positions 2 and 3. Related compounds, termed sulfobacins A and B, i.e. (2R,3R)-3-hydroxy-2-[(R)-3-hydroxy-15-methylhexadecanamido]-15-methylhexadecanesulfonic and (2R,3R)-3-hydroxy-15-methyl-2-[13-methyltetradecanamido]-hexadecanesulfonic acids, respectively, were found in Chryseobacterium sp. Similar lipids were reported from the Gram-negative sea-water bacterium Cyclobacterium marinus.

Capnine and halocapnine - formulae

Recently, a new sulfonolipid with some structural affinity to the capnoids was isolated from the halophilic bacteria Salinibacter ruber and Salisaeta longa. It has the structure 2-carboxy-2-amino-3,4-hydroxy-17-methyloctadec-5-ene-1-sulfonic acid for which the trivial name halocapnine is suggested. As its 3-O-acyl derivative (and not N-acyl), it represents about 10% of the total cellular lipids of the former.

The experimental evidence suggests that biosynthesis of capnine occurs by the condensation of 13-methylmyristoyl-coenzyme A with cysteic acid, in a manner analogous to the condensation of palmitoyl-coenzyme A with serine during the biosynthesis of sphingoid bases. The function of capnoids is obscure, but there are suggestions that they may have a role in the motility of the organisms.


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Lipid listings Credits/disclaimer Updated: August 4th, 2017 Author: William W. Christie LipidWeb icon