Autocatalytic cycle

In the bromate-iron clock reaction, there is an autocatalytic cycle involvmg the species intennediate species HBrO. This cycle is comprised of the following non-elementary steps ... 

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. 

Crutzen and co-workers (Sander and Crutzen, 1996 Vogt et al., 1996) have developed a model for chemistry in the marine boundary layer at midlatitudes, in which autocatalytic cycles involving sea salt particles generate photochemically active gases such as BrCl, Br2, and Cl2. It is likely that such chemistry also occurs in the Arctic as well. In these cycles, reactions (125) and (126) in the condensed phase,... 

In the hydrogen-oxygen and B-Z reactions considered above, the autocatalytic cycles correspond to a value for n of unity. The resulting rate law, rate = klab, involves the product of two concentrations and is known as quadratic autocatalysis . In the reaction between iodate and iodide ions, I is produced through an autocatalytic cycle which, at its simplest, corresponds... 

The question is therefore, what are the principal requirements of an autotrophic carbon-fixation mechanism An organic molecule serves as a C02 acceptor molecule, which becomes carboxylated by a carboxylase enzyme. This C02 acceptor molecule needs to be regenerated in a reductive autocatalytic cycle. The product that can be drained off from such a metabolic cycle should be a central cellular metabolite, from which all cellular building blocks for polymers can be derived examples of such central metabolites are acetyl-CoA, pyruvate, oxaloacetate, 2-oxoghitarate, phosphoe-nolpyruvate, and 3-phosphoglycerate. Importantly, the intermediates should not be toxic to the cell. The irreversible steps of the pathway are driven by ATP hydrolysis, while the reduction steps are driven by low-potential reduced coenzymes. 

How specific must the reactions of an autocatalytic cycle be for it to grow Imagine a cycle with n constituents, and m other active substances in the medium (which also include the reagents). Considering all possible reactions between the constituents and the other substances, King found that the cycle grows exponentially only if... 

How probable is it that a randomly generated autocatalytic cycle of size n will persist Assuming randomly assigned rate coefficients and concentrations, King defines a kinetic complexity to a cycle (Y = n(m - 1)), where n is the number of constituents and m is the number of the active substances in the medium including reagents, and calculates the probability that a cycle of... 

In conclusion, selection of rate coefficients and concentrations of reagents are needed to make an autocatalytic cycle that persists with more than a very small number of constituents. Random search will not do for anything but the smallest cycles. 

One experimental approach would be to generate heterogeneous vesicles in broths containing the constituents of the formose reaction, initially limiting the concentration of formaldehyde. After leaving the system for a while, formaldehyde would be added to the solution. Those vesicles that grew and divided fastest would be selected for. There may be rare conditions under which the constituents of the formose reaction are permeable to the membrane. They enter the vesicle and become trapped as soon as those conditions change. Formaldehyde could then enter the vesicles and allow the autocatalytic cycle to run. 

A catalytic amount of TBAF is effective in the allylation of aromatic imines with 10 in refluxing THF." Similar to the TBAF-catalyzed carbonyl allylation, the imine allylation is promoted by a fluoride-triggered autocatalytic cycle. 

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.

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 ... 

For biological reproduction the construction of autocatalytic cycles is required which reduces the options to use broadly diverse metals (elements in general) in biological systems. 

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