Autocatalytical chemical cycle

Cycle 1 an autocatalytic chemical cycle which provides material for the other two cycles. 

The chemoton scheme is illustrated in Figure 8.10. It consists of three subsystems a metabohc autocatalytic network, a bilayer membrane, and arephca-ble information carrier molecule, or template. Here, as in autopoiesis, the membrane is an important player. However, what happens within the membrane is depicted in more detail. In particular, looking at Figure 8.10, note that the entire system is fed by nutrient X which hrst feeds the metabolic network with the various A molecules. This is an autocatalytic chemical cycle, autocatalytic because two Ai molecules are produced from the original single Ai molecule. 

Figure 8.10 Ganti s chemoton (redrawn from Maynard-Smith and Szathmary, 1995, based on the original by Ganti, 1984). The metabolic subsystem, with intermediates Ai -> A2 ->. . . A5, is an autocatalytic chemical cycle, consuming X as nutrient and producing Y as waste material pV is a polymer of n molecules of V which undergoes template replication R is a condensation byproduct of this replication, needed to turn into T, the membranogenic molecule the symbol Tm represents a bilayer membrane composed of m units made of T molecules.

Figure 8 Schematic representation of the minimal autocatalytic reaction cycle for the self-replicating a-helical peptide. The electrophilic and nucleophilic peptide fragments E and N are recognized by template T through interhelical hydrophobic interactions to form catalytic complex [T E N], Subsequent chemical ligation produces an identical copy of the template that remains bound in the product duplex. Dissociation relies on the two templates that can then undergo further catal3rtic cycles. The inset depicts the mechanism of the amide bond formation that includes a transthioesterification between the activated C-terminus of electrophile E and the N-terminal cysteine side chain of nucleophile N as a first step. Rapid rearrangement of the intermediate thioester gives rise to the final amide bond.

The encapsulation results in a chance collection of molecules that then form an autocatalytic cycle and a primitive metabolism but intrinsically only an isolated system of chemical reactions. There is no requirement for the reactions to reach equilibrium because they are no longer under standard conditions and the extent of reaction, f, will be composition limited (Section 8.2). Suddenly, a protocell looks promising but the encapsulation process poses lots of questions. How many molecules are required to form an organism How big does the micelle or liposome have to be How are molecules transported from outside to inside Can the system replicate Consider a simple spherical protocell of diameter 100 nm with an enclosed volume of a mere 125 fL. There is room within the cell for something like 5 billion molecules, assuming that they all have a density similar to that of water. This is a surprisingly small number and is a reasonable first guess for the number of molecules within a bacterium. 

The chloroplast must operate its carbon cycle as an autocatalytic breeder reaction, but it must export elaborated carbon and chemical energy to its cellular environment. In order to export, it must produce more than it uses, but can only do this by returning newly synthesized intermediates to the cycle. In order to satisfy the needs of the cell, it must release newly made products to the cytoplasm. These competing processes can be accomplished... 

Autocatalysis is a distinctive phenomenon while in ordinary catalysis the catalyst re-appears from the reaction apparently untouched, additional amounts of catalyst are actively produced in an autocatalytic cycle. As atoms are not interconverted during chemical reactions, this requires (all) the (elementary or otherwise essential) components of autocatalysts to be extracted from some external reservoir. After all this matter was extracted, some share of it is not introduced in and released as a product but rather retained, thereafter supporting and speeding up the reaction(s) steadily as amounts and possibly also concentrations of autocatalysts increase. At first glance, such a system may appear doomed to undergo runaway dynamics ( explosion ), but, apart from the limited speeds and rates of autocatalyst resupply from the environment there are also other mechanisms which usually limit kinetics even though non-linear behavior (bistability, oscillations) may not be precluded ... 

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