Ether lipids of the organic world: formation and ...

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Ether Lipids of the Organic World leh. FORMATION AND BIOTRANSFORMATION OF ETHER LIPIDS. The formation of ethylene (I - number of chemicRl iun~tion ...
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Ether lipids of the organic world: formation and biotransfonnation V.M. Dembitsky - Department of artificial ecosystems. Institute of Ecology of the Volga River Basin, USSR Academy of Sciences, Togliatti, 445003, USSR.

ABSTRACT

A hypothesis of a probable chemical synthesis of ~ther lipids at. the early stages of the Earth's evolution ~nd the ways of their biotransformation in different kingdoms of the organic world are dis­ cussed. The original "biological membranes" are suggested to contain primarily ether lipids, whereas in the-process of originating and improving the photosynthetic mechanism, they gradually became oxidi7.ed, under the effect of photosynthesi7.ing org~nisms, into plasmalogens and later into diacyl lipids. Ether lipids composition of five kingdoms of the organic world is discussed. INTRODUCfION·

The last two d ecad es have been character; zed by a attention of chemists, biochemists and membranologists on problems of biomembrane ether lipids. According to numerous authors, ether lipids are fairly 1I',idespread in nature /I-9/. We surmise it to be rather fundamental to es­ t~blish the WRy of formation ~nd biotr~nsform~tion of ether ~ipids of the organic world in the process of chemical ~nd biological evolution. p~rticul~r

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Ether Lipids of the Organic World

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FORMATION AND BIOTRANSFORMATION OF ETHER LIPIDS

The formation of ethylene (I - number of chemicRl iun~tion on Fig.I), isoprene (2), propylene, penta­ dien (3) and some other nonsaturated hydrocarbons occurs from methane, ethane and propane. In their turn, these compounds (1,2,3) are found to be pre­ cursors for synthesis of linear hydrocarbons (4-9) and polyisoprenoids (10-12). When the condensation occurs in the presence of excessive hydrogen, it gives rise to a large series of hydrocarbons (4-12). When the reaction takes place in water, the respec­ tive fatty alcohols (13-15,18) or diols (16,17,19, 20) are produced as demonstrated by several authors /10,11/. Opinions vary as to the pH value of the anci­ ent isolated gutter of the ocean. According to some scientists, it was substantially alkaline pH /12, 13/, but most of them keep to the opinion that it Wqs acidic pH and contained a fairly strong solu­ tion of hydrochloric and boric acids /8, 14,15/. In Addition to these, phosphoric, hydrosulphuric, fluohydric, sulfuric and other mineral acids were also present in it /8/. The presence of boric acid in the Rcirlic medium promotes the form~tion of higher fatty alcohols due to the forma­ tion of alkylborates which become gradually hydro­ lyzed /11/. This is not typical, however, for fatty acids. Availability of acidic pH, ca, IOOoC temperRture conditions, higher aliphatic (13-16), isoprenoid (18-20), and mixed (17) alcohols as we1l AS low-molecu~ar polyols (ethylene glycol (21), g'ycerol (22). butantetraol (23) and others) pro­ moted the formation of mono-, di-, tri-, tetra­ ethers of polyols and other similar chemical com­ pounds. They also included the compounds which now~days are rather wirlely represented in biomem­ branes of qll kingdoms of the organic world, vi~.: monoalky~ethylene glycol (24), I,2-di-0-alkylgly­ cerols (27), 2,3-di-0-phytanylglycerols (38) an~ I,2-di-0-phytanyl-butantetraol (34) in some archae­ bacteria. One of the initial lipid membranes was likely to form from such ether lipids. The possibility of formation of fatty acid esters and lower polyols (21-23) remainsalterna­ tive. This problem was considered in detail in works of Oro /94-96/ and Deamer /97-99/. This pos­ sibility is not excluded a1together, but at acidic pH and high temperatures, the water solution eqUilibrium normally becomes shifted to the side of the ester bond hydrolysis. In case of the initial ocean (exactly isolated gutters) with alkaline pH, the possibility of ester bonds formation is'practi­

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Formation and Transformation

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cally mullified because of its hydrolysis, whereas the formation of ether lipids under the same condi­ tions in the presence of catalysis is quite reaso­ nable. Moreover, membranes consisting of acyl-lipids (26,3I,39) could not ensure the survival of primi­ tive "living" organisms, as they would merely melt /4/. The present-dayarchaebacteria can be conside­ red to live under similar conditions as the primi­ tive protocells: they are able to survive at tempe­ ratures near IOOoo /I6,I7,I8/ and pH values equal to I /4,I9/. The only aspect common to them is the presence of ether lipids both isoprenoid and likely with iso-, anteiso-, and aliphatic chains in their membranes. Appearance of the protein synthesis controlled by RNAs became one of the critical events in the process of formation of the very first cell: another event of a no lesser importance was surely the for­ mation of a lipid membrane. Proteins which synthesis occured under the con­ trol of RNA could not ensure the reproduction of these particular RNA molecules, if their habitat were not confined by the external membrane. Thus, At the early stages of primitive "living" cells ori­ gin, their membranes consisted mainly of ether li­ pids (24,27,33-37) which were subsequently attached in a genetic way too. In the absence of oxygen a broad development and spreading had to take place in anaerobic cells (bacteria) and particularly in archaebacteria containing ether lipids as the major membranes of living cell existed for a fairly long time, at least until the pH value reached the neut­ rf:ll level and· the ocean· temperature lowered up to 70 0 0. Photosynthesizing bacteria should have appea­ red by that time (some 3.5 to 3.0 billion years ago) A grqdual splitting of primitive living organisms was likely to start at this time. Those which adap­ ted themselves to high temperatures and low pH va­ lues formed a branch that nowadays encompasses a subkingdom of archaebacteria. These organisms were distinguished by the presence of isoprenoid alco­ hols (I8-20) and glycerol-based (22) di-, and tet­ raethers (35-38). Archaebacteria are believed to insignificantly change the lipid components of their membranes during the elapsed evolution time. The membrAnes of another branch of these initial cells consisted of di-O-alkylglycerols (27) and, probably, of mono-O-alkylethylene alcohols (24). They probably contained fatty alcohols (I3) as well ::IS iso-·and anteisoalcohols lI4,I5). A gradual pro­ cess of membrane dialkylglycerols oxidation should have started in cert::lin cells due to the perfection of the photosynthesizing mechanism in the process

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of evolution. It occured in several stages, viz.: form~tion of plasmalogens with subseouent introduc­ tion of the double chain in the d,fi-position of al­ kyl radicals (25,28,29), oxidation of the double ch~in resulting in the formation of acyl lipids (26. 30-32). The p~cking and mobility of the membrane lipid ch~ins ~re greatly influenced by steroids and spe­ cific~lly by cholesterol /20,21/. In principle, cholesterol and other steroids could possibly be synthesized in accord with the following scheme: isoprene condensation giving rise to squalane (40), dehydr~tion of the latter causing the ,formation of squ~lene (41) which, in i~s turn, is a major pre­ cursor for synthesizing practically all known ste­ roids through lanosterol (42) to cholesterol (43). Cholesterol can form either ether bonds with fat­ ty alcohol (13) or plasmalogen and 'es.ter bonds, ,the latter being widespread in n~ture. The replacement of an ether bond by a vinyl ~nd then by ~n esther one in biomembrane lipids fa­ cilitates splitting the fatty chains by enzyme hvdrolAses /22,2;/. Many of these have been alrea­ dy detected ~nd fairly well studied, vi?.: phospho­ lipases AI ~nd A2, ~ip~ses hydrolyzing ester bonds /24/ as well ~s pl~smalogen~ses hydrolyzing vinyl bonds /25,26/. There are no, however, valid data testifying to the existence of enzymes hydrolyzing ether bonds, although they were found in Tetrahy­ mena pyriformis /271. Thus the second branch has actually origina­ ted four trends of evolution, i.e. eubacteria, plants, fungi and animals (Fig.2). We shall consider now the distribution of alk­ oxylipids (ether lipids) in all. the five kingdoms of the org~nic world; this will bring to a better understanding of our hypothesis based on the origin ~nd biotransformation of these compounds. KINGDOM OF PROKARYOTES

Bacteria ~re represented nowadays by two subking­ noms th~t are only remotedly reJated to each other. These are eubacteria and archaebacteria. Alkyl-gly6erols (;2) and plasmalogens (30) are founn exclusivel-y in obligate anaerobes in which ' They comprise from 12 to 80% of the'totalpolar lipids. This problem has been well det~iled in mt.l.ny reviews /3,28-31/. AlkyL ethers are present in small amounts and do not exceed 4% of total phosphol ipids- (PL) in eubBcteria /3, ;1/. An unusual ether lipid was found in C.butyricum; its structu­ ralformula was established to be-the following;

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Fonnation and Transfonnation

Fig.I. A hypothetic scheme of synthesis and bio­ tr~nsform~tion of ether lipids in the process of evolution.

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Ether Upids of the Organic World

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I-(I'-glycero~lkyl)-2-acyl-sn-glycero-3-phospho­

ethanolAmine ~nd its N-methyl-derivative; they ma­ ne up 20 to 30% of PL /,2",/, P1Asmalogens (30) ~re not found in micrOAero­ phlles, aerobes and facultative Anaerobes. A number of them are found in Clostridium butyricum, 4~~ being mainly in phosphatidylethanolamine (PE), N­ methyl-PE and phosphatidylglycerol (PG). Phospha­ tidylserine (PS) is fairly rare in bacteria, but ,?% of totAl PL found in Me~asphRera e!sdenii was PS, Pl~smRlogens comprise 72% in PS ~nd 87% in PE. C~rrliolipin (DPG) simil.!'lr to PE !'lnd PG contains pl~smalogen forms in Sphaerophorus ridiculosis /37/ PL of Anl1eropl~sma Rbactoetostaum make up ,3% of plAsmalogen PG /38/. In Butyrivibrio all glycero­ PL Are represented exclusively by plasmalogen PG /39/. Plasmalogen phosphatidylcholine (PC) was founo in anaerobic spirochetes Treponema phagede­ nis /40/ and T.hyonysenteriae' /41/;-''Plasmalogen­ forms making up 60.6% in DPG, 74.8% in PG, 88.3% in MGDG (monogalactosyldiacylglycerol), 96.4% in the non-identified galactolipid and 6.,% in PC. Bacterial plasmalogens are substantially important as regulators of the membrane liquid-crystal sta­ te. All other eubacteria contain only diacyl gly­ cero~ipirls (GL) (31) /3/. Archaebacteria vary from eubacteria primarily in !'l gre!'lter variety of membrane lipids. While eubActeriA are mainly characterized by the presen­ ~e of diacyl-GL (31), archaebacteria are distin­ ~uished by ether lipids with isoprenoid chains (34-,8) /4,5.42,43/. Optical activity for the mid­ dle (second) carbon atom of glycerol ih archaebac­ teria is opposite to the one occuring in other kl.·ngdoms /4/. Halophiles contain only di-ethers (,8) /4,9/: the same refers to Haloalkalophiles too. Methanogens contain approximately equal amo­ unt of ~iethers (38) /4,9/ and tetraethers (37). Thermophilic meth~nogens, Thermoacidoliles and Thermophi~ic anaerobes contain primari'y tetra­ ethers (,?) /4,5,42/: archaebacteria contain other chemical structur~s as well (35,36.,8) /4.5,42/. KINGDOM OF PROTOcrlSTA

Ether-linkeo lipids (32) are winely represented in Proto7.oa ''Inn pqrtI cul Jirl y in Cil iophora. Membrane lipid composition is most comprehensivel~ studied in Tetrahymena pyriformis. 29% of PL in strain NT-I are represented by alkyl glycerols (32) inclu­ ding 66% in PC and 20% in I-0-Al~yl-2-acyl-3­ glycero-( 2-aminoethyl) phosphonate (CAEP) :/44/ .. Strain W in T.pyriformis contains 75% of CAEP./45/.

Kingdom of Protoctista

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P LAN T

FUNGI

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ANIMAL

Mycophycophyta Chordata (31)1 . (24-27,29-32, Basidiomycota 39,43) (3I)V kTunicata Ascomycota 1(30,31,32) (3~) , \ 30,32). Arthropoda (30,31,32,39) AnnelidaJ . . (30,31.32) . Zygomycota ~Echinodermata (31) (~Y>32,3I,24-26) Cnidaria Mollusca (30,32,31) (30,31,32)

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Porifera (32,31,30,27.39)

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