Prebiotic membranes

Phospholipids are the main constituents of most biological membranes, and are produced in modern cells by complex enzymatic processes. As such, they cannot be considered prebiotic compounds. 

A different approach is proposed by Ourisson and his group. They start from the consideration that the amphiphilic molecules in primitive membranes must have been very different from the modem eucaryotic ones (Ourisson and Nakatani, 1994), and they argue that simple polyprenyl or dipolyprenyl phosphates satisfy 

More convincing evidence has been obtained for the synthesis of simpler compounds, such as straight-chain fatty acids. This observation is important because, as we have already seen in the chapter on self-organization and self-reproduction, these compounds form stable vesicles. Prebiotic synthesis of these compounds was reported for example by Nooner et al. (1976). More recently monocarboxylic acids have been observed from a spark discharge synthesis (Yuen et al, 1981) and from a Fischer-Tropf type of reaction (McCollom et al, 1999 Rushdi and Simoneit, 

University of Santa Cruz, California Prebiotic membranes 

It is not yet understood how life began on Earth nearly four billion years ago, but it is certain that at some point very early in evolutionary history life became cellular. All cell membranes today are composed of complex amphiphilic molecules called phospholipids. It was discovered in 1965 that if phospholipids are isolated from cell membranes by extraction with an organic solvent, then exposed to water, they self-assemble into microscopic cell-sized vesicles called liposomes. It is now known that the membranes of the vesicles are composed of bimolecular layers of phospholipid, and the problem is that such complex molecules could not have been available at the time of life s beginning. Phospholipids are the result of a long evolutionary process, and their synthesis requires enzymatically catalyzed reactions that were not available for the first forms of cellular life. 

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Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

Some carboxylic acids have been isolated from meteorites, the sodium salts of which have produced vesicle-like stmctures in water (Deamer, 1986). Their aromatic stmctures are, however, very different from those of any known constituent of biomembranes, and an import from the cosmos by way of meteorites does not look like a reasonable hypothesis of the origin of the phospholipids of biomembranes. Zhang s contribution to the present volume (Chapter 20) may well offer a major step forward in positing plausible prebiotic membranes. 

D. W. Deamer and J. P. Dworkin have reported in detail on the contribution of chemistry and physics to the formation of the first primitive membranes during the emergence of precursors to life the authors discussion ranges from sources of amphiphilic compounds, growth processes in protocells, self-organisation mechanisms in mixtures of prebiotic organic compounds (e.g., from extracts of the Murchison meteorite) all the way to model systems for primitive cells (Deamer and Dworkin, 2005). 

Buick R, Thornett JR, McNaughton NJ, Smith JB, Barley ME, Savage M (1995) Nature 375 574 Bungenberg de Jong H, Decker WA, Swan OS (1930) Biochem Z 221 392 Deamer DW (1998) Membrane Compartments in Prebiotic Evolution. In Brack A (Ed.) The Molecular Origins of Life. Cambridge University Press, p 189 Deamer DW, Dworkin JP (2005) Chemistry and Physics of Primitive Membranes. In Walde P (Ed.)... 

Micelles are capable of self-replication if an appropriate chemical reaction occurs within the micelle itself that produces more of the same amphiphile that forms the micelle. Such self-replication has been demonstrated for both ordinary micelles in an aqueous medium [139] as well as for reverse micelles, [140] which are spherules of water stabilized by an amphiphile in an organic solvent. Some of the prebiotic potentialities of replicating membranous vesicles have been investigated, [141] and they have been characterized as "minimum protocells. [142]... 

The question as to the potential availability of the requisite amphiphilic precursors in the prebiotic environment has been addressed experimentally by Deamer and coworkers, [143,145] who looked into the uncontaminated Murchison chondrite for the presence of such amphiphilic constituents. Samples of the meteorite were extracted with chloroform-methanol and the extracts were fractionated by thin-layer chromatography, with the finding that some of the fractions afforded components that formed monomolecular films at air-water interfaces, and that were also able to self-assemble into membranous vesicles able to encapsulate polar solutes. These observations dearly demonstrated that amphiphiles plausibly available on the primitive Earth by meteoritic infall have the ability to self-assemble into the membranous vesides of minimum protocells. ... 

Prior sequestration of the prebiotic reactions within the micropores of weathered feldspars or other porous rock matrices also avoids many of the other problems of catalysis and dilution encountered by models of chemical biogenesis. That is, this mechanism attains viable evolutionary chemical selection among spatially discrete systems without the need to assume an unlikely capture-and-enclosure event involving a pre-existing lipid membrane. [192] Thus autocatalysis of chiral molecules could evolve before the actual appearance of free-floating lipid vesicles. 

The ability to catalyse the evolution or oxidation of H2 may have been exploited by the earliest life forms as H2 would have been present in the early prebiotic environments. The origins of the proton-dependent chemiosmotic mechanism for ATP synthesis may also reflect the formation of proton gradients created by hydrogenases on either side of the cytoplasmic membrane. In addition, it has been speculated that the coupling of H2 and S metabolisms was also of fundamental importance in the origin of life. These two processes seem intimately coupled in the bifunctional sulfhydrogenase found in Pyrococcus furiosus (a combination of subunits for hydrogenase and sulfite reductase) which can dispose of excess reductant either by the reduction of protons to H2 or S° to H2S (Ma et al. 1993 Pedroni et al. 1995). 

Also membrane-forming compounds may come from space (Fawless and Yuen, 1979 Deamer, 1985 Deamer et al, 1994 Deamer and Pashley, 1989 McCollom et al, 1999). The observation that membranogenic compounds can also be of prebiotic origin is of particular interest, especially in view of the membrane-first hypothesis, or more generally for making the point that membranes and vesicles were present very early in the prebiotic scene (Deamer et al, 1994). All this will be discussed further in Chapters 10 and 11. 

The conclusion is that membranous vesicles readily form a variety of amphiphilic molecules that would have been available in the early Earth environment, along with hundreds of other organic species. It is likely that during the chemical evolution leading to the first catalytic and replicating molecules, the ancestors of today s proteins and nucleic acids, membranous vesicles were available in the prebiotic environment, and ready to provide a home for the first forms of cellular life. 

The other lipid commonly present in eukaryotic membranes is cholesterol, a polycyclic structure produced from isoprene by a complex biosynthetic pathway. It is interesting to ask whether it is conceivable that prebiotically plausible reactions might also produce complex amphiphiles. The earliest investigations aiming to answer this question were carried out by Hargreaves et al. [36], Oro and coworkers [37,38], and, more recently, Ourisson et al. [39] and Conde-Frieboes and Blochliger [40]. In all such reactions,... 

Although it is clear that complex lipids can be synthesized under laboratory simulations using pure reagents, the list of required ingredients does not seem plausible under prebiotic conditions. Therefore, it is unlikely that early membranes were composed of complex lipids such as phospholipids and cholesterol. Instead, there must have been a source of simpler amphiphilic molecules capable of self-assembly into membranes. One possibility is lipidlike fatty acids and fatty alcohols, which are products of FTT simulations of prebiotic geochemistry [12] and are also present in carbonaceous meteorites. Furthermore, as will be discussed later, these compounds form reasonably stable lipid bilayer membranes by self-assembly from mixtures (Fig. 4a). 

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