Autocatalytic self-replication

A particularly interesting type of micellar catalysis is the autocatalytic self-replication of micelles [58]. Various examples have been described, but a particularly interesting case is the biphasic self-reproduction of aqueous caprylate micelles [59]. In this system ethyl caprylate undergoes hydroxyl catalysed hydrolysis to produce the free carboxylate anion, caprylate. Caprylate micelles then fonn. As these micelles fonn, they solubilize ethylcaprylate and catalyse further production of caprylate anion and caprylate micelles. 

In a self-reproducing, catalytic hypercycle (second order, because of its double function of protein and RNA synthesis) the polynucleotides Ni contained not only the information necessary for their own autocatalytic self-replication but also that required for the synthesis of the proteins Ei. The hypercycle is closed only when the last enzyme in the cycle catalyses the formation of the first polynucleotide. Hypercycles can be described mathematically by a system of non-linear differential equations. In spite of all its scientific elegance and general acceptance (with certain limitations), the hypercycle does not seem to be relevant for the question of the origin of life, since there is no answer to the question how did the first hypercycle emerge in the first place (Lahav, 1999). 

Shreion Lifson (1997) utilized a nice arithmetic to illustrate the power of the autocatalytic self-replication. He took the example of a normal hetero-catalytic process that makes one molecule of B from A at the rate of one per second. Then, it would require 6 x 10 s to make one mole of B. If instead there is an autocatalytic process by which B gives rises to 2B, and 2B give rise to 4B, and 4B to 8B, and so on, it requires only 79 s to make one mole of B. 

The hexamer formation proceeds via the ter-molecular complex M, and the proximity between A and B in this complex facihtates their covalent linkage. Thus, once the complex D is dissociated, two T molecules are formed, and the autocatalytic self-replication process can start with the progression described above two give four, four give eight, eight give sixteen, and so on. 

Bachmann, P. A., Luisi, P. L., and Lang, J. (1992). Autocatalytic self-replication of micelles as models for prebiotic structures. Nature, 357, 57-9. 

Frank proposed a mechanism for the autocatalytic self-replicating process in which a chemical substance catalyzes its own production and acts as an anticatalyst for the production of the enantiomer without mentioning any actual compound or actual reaction [17]. hi this kinetic model, it is possible to obtain an enantiomerically enriched compound from an ex-... 

Bachman PA, Luisi PL, Lang J (1992) Autocatalytic self-replicating micelles as models for prebiotic structures. Nature 357 57-59... 

Figure A3.14.17. Self-replicating spots in the FIS reaction in a CFUR, comparing an experimental time sequence with numerical simulation based on a simple autocatalytic scheme. (Reprmted with pennission from Lee etal [M], Macmillan Magazines Ltd. 1994.)...

The scientific world was amazed to hear that David Lee, from the laboratory of Reza Ghadiri (Scripps Research Institute, La Jolla, California), had found a self-replicating peptide (Lee et al., 1996) there are analogies to the experiments with oligonucleotides (see Sect. 6.4). Lee was able to show that a certain peptide, containing 32 amino acids, can both function as a matrix and also support its own synthesis autocatalytically. The information transfer is clearly more complex than that involved in nucleic acid replication. In the case of this particular peptide, both the... 

In accordance with the autocatalytic process, matrices are again formed. It is surprising that the autocatalysis decreases when only 1 of the 15 building blocks of the peptide has the opposite handedness, e.g., when the N-peptide fragment contains one D-amino acid as well as the 14 L-amino acids. These experimental results show that such a system is able to form homochiral products via self-replication. It can be assumed that similar mechanisms influenced the origin of homochirality on Earth (Saghatelian et al., 2001 Siegel, 2001). 

Fig. 6.5 Scheme necessary for a minimal self-replicating system with an autocatalytic reaction cycle (von Kiedrowski, 1999)... 

How can negative fluctuations in entropy production occur or be triggered As Manfred Eigen shows in his evolution theory, fluctuations in entropy production can be caused by the coming into being of a self-replicating molecular species which is capable of selection. Autocatalytically active mutants can also have the same effect. Looked at this way, the phenomenon of evolution consists of a continuous series of instabilities, i.e., collapses of stationary states. 

The point is also made [134] that the very high surface areas and the richly interconnected three-dimensional networks of these micron-sized spaces, coupled with periods of desiccation, could together have produced microenvironments rich in cat-alytically produced complex chemicals and possibly membrane-endosed vesides of bacterial size. These processes would provide the proximate concatenation of lipid vesicular precursors with the complex chemicals that would ultimately produce the autocatalytic and self-replicating chiral systems. A 2.5 km2 granite reef is estimated to contain possibly 1018 catalytic microreactors, open by diffusion to the dynamic reservoir of organic molecules. .. but protected from the dispersive effects of flow and convection [134] as well as protected from the high flux of ultraviolet radiation impinging on the early Earth. [123,137]... 

At the point where amphiphiles were recruited to provide the precursors to cell membranes, stable lipid vesicles could have evolved [141] to enclose autocatalytic chiral hypercycles. Credible models for the subsequent evolution of vesicles containing self-replicating chiral molecules have appeared in the literature. [193,194] These vesicles could then emerge from the feldspar spaces [134,192] as micron-sized self-reproducing, energy-metabolizing vesicular systems protobacteria ready to face the hydrothermal world on their own terms. 

The autocatalytic hypothesis was backed by the reaction s pH sensitivity. Addition of CO2 to the micelles increased acidity resulting in vesicles as the pH dropped below 7. Overall the experiment showed that vesicles could form from a reaction that generates amphiphilic molecules that in turn form autocatalytic micelles. Upon addition of a gas, common in planetary atmospheres, the micelles form vesicles that could function as protocells. The protocells are self-replicating so this process can be considered to be autopoietic, from the Greek for self-forming, and therefore fulfils an essential step in the chemical evolution of life. 

Scheme 5. Schematic representation of the minimal au-tocatalytic reaction cycle of a self-replication a-helical peptide. The peptide fragments E and N are preorientated at the template through hydrophobic, interhelical interactions. The amide bond formation leads to an identical copy of the template, which further accelerates the peptide ligation in the autocatalytic cycle.

The ability of nucleic acids to act as templates for self-replication is a fundamental process in Nature s chemistry. The Rebek group have employed both templat-ing and recognition effects for the production of assembled "systems that promote replication of the templating molecule. An important feature of this work is the fact that the presence of the usual weak intermolecular forces allowed the corresponding host-guest complexes to form and dissipate rapidly. The resulting dynamic behaviour provides an environment for an efficient autocatalytic replication process to occur. 

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