ScienceWeek

ORIGIN OF LIFE: MODELS OF PRIMITIVE CELLULAR COMPARTMENTS

The following points are made by M.M. Hanczyc et al (Science 2003 302:618):

1) The bilayer membranes that surround all present-day cells and act as boundaries are thought to have originated in the spontaneous self-assembly of amphiphilic molecules into membrane vesicles (1–5). Simple amphiphilic molecules have been found in meteorites and have been generated under a wide variety of conditions in the laboratory, ranging from simulated ultraviolet irradiation of interstellar ice particles to hydrothermal processing under simulated early Earth conditions.

2) Molecules such as simple fatty acids can form membranes when the pH is close to the pK[sub-a] (K[sub-a] is the acid dissociation equilibrium constant) of the fatty acid carboxylate group in the membrane (3). Hydrogen bonding between protonated and ionized carboxylates may confer some of the properties of more complex lipids with two acyl chains, thus allowing the formation of a stable bilayer phase. Fatty acid vesicles may be further stabilized (to a wider range of pH and even to the presence of divalent cations) by the admixture of other simple amphiphiles such as fatty alcohols and fatty acid glycerol esters. Recent studies have shown that saturated fatty acid/fatty alcohol mixtures with carbon chain lengths as short as 9 can form vesicles capable of retaining ionic fluorescent dyes, DNA, and proteins (4).

3) Vesicles consisting of simple amphiphilic molecules could have existed under plausible prebiotic conditions on the early Earth, where they may have produced distinct chemical micro-environments that could retain and protect primitive oligonucleotides while potentially allowing small molecules such as activated mononucleotides to diffuse in and out of the vesicle. Furthermore, compartmentalization of replicating nucleic acids (or some other form of localization) is required to enable Darwinian evolution by preventing the random mixing of genetic polymers, thus coupling genotype and phenotype. If primordial nucleic acids assembled on mineral surfaces, the question arises as to how they eventually came to reside within membrane vesicles. Although dissociation from the mineral surface followed by encapsulation within newly forming vesicles (perhaps in a different location under different environmental conditions) is certainly a possibility, a direct route would be more satisfying and perhaps more efficient.

4) In summary: The clay montmorillonite is known to catalyze the polymerization of RNA from activated ribonucleotides. The authors report that montmorillonite accelerates the spontaneous conversion of fatty acid micelles into vesicles. Clay particles often become encapsulated in these vesicles, thus providing a pathway for the prebiotic encapsulation of catalytically active surfaces within membrane vesicles. In addition, RNA adsorbed to clay can be encapsulated within vesicles. Once formed, such vesicles can grow by incorporating fatty acid supplied as micelles and can divide without dilution of their contents by extrusion through small pores. These processes mediate vesicle replication through cycles of growth and division. The authors suggest the formation, growth, and division of the earliest cells may have occurred in response to similar interactions with mineral particles and inputs of material and energy.

References (abridged):

1. J. M. Gebicki, M. Hicks, Nature 243, 232 (1973)

2. J. M. Gebicki, M. Hicks, Chem. Phys. Lipids 16, 142 (1976)

3. W. R. Hargreaves, D. W. Deamer, Biochemistry 17, 3759 (1978)

4. C. L. Apel, D. W. Deamer, M. N. Mautner, Biochim. Biophys. Acta 1559, 1 (2002)

5. P.-A. Monnard, C. L. Apel, A. Kanavarioti, D. W. Deamer, Astrobiology 2, 139 (2002)

Science http://www.sciencemag.org

--------------------------------

ON CLAYS AND THE ORIGIN OF LIFE

The following points are made by David W. Wolfe (citation below):

1) Among the types of molecules that can be attracted to and held by the charged surfaces of clays are many organic compounds, including some forms of amino acids and nucleotides. The hypothesis that clays served as the template for the sequencing of simple proteins or genes is based on this fact. In addition to their charged surface, clay crystals have a very convoluted shape with many nooks and crannies, which could serve to bring pairs of amino acids or nucleotides of particular sizes or shapes into the correct orientation to facilitate synthesis. This is similar to the mechanism by which the large folded protein-enzymes of modern cells catalyze the biosynthesis of macromolecules.

2) The speculation that clays might have served this catalytic role in the origin of life is half a century old, having been first advocated by the British physical chemist John Desmond Bernal (1901-1971) in the early 1950s. Experiments in the 1970s demonstrated that a common type of clay known as montmorillonite (named for the French town of Montmorillon where it was first quarried) could catalyze the sequencing of specially prepared amino acids. Some of the protein-like molecular chains synthesized in this way were up to 60 amino acids long.

3) In the 1980s, James Lawless, of NASA's Ames Research Center in California, and others found that nucleotides could be made to bind to various clays when certain metals, such as zinc or copper, were also present in minute quantities in solution. Then, during the 1990s, J.P. Ferris and his colleagues managed to attach nucleotides together into long chains using montmorillonite clays as catalysts. The procedure required some preparatory treatment of the nucleotides, but the results nonetheless provided support for the idea that clays could have catalyzed the first simple RNA genes.

Adapted from: David W. Wolfe: Tales from the Underground: A Natural History of Subterranean Life. Perseus Publishing 2001, p.28. http://www.amazon.com/exec/obidos/ASIN/0738201286/scienceweek

ScienceWeek http://www.scienceweek.com