Autocatalytic network

Cousins, G. R. L. Poulsen, S.-A. Sanders, J.K.M. Molecular evolution Dynamic combinatorial libraries, autocatalytic networks and the quest for molecular function. Curr. Opin. Chem. Biol. 2000,4, 270-279. 

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. 

It cannot be stressed too strongly that without exception, all known cellular life possesses an autocatalytic metabolism, even if the cells are het-erotrophic. Thus for the autocatalytic nature of the whole metabolic network it is not necessary to be able to identify a smaller autocatalytic core as the reductive citric acid cycle or the Calvin cycle. Imagine the following thought experiment. Take away all metabolites from a cell but leave all the water and the informational macromolecules in place. Can the network be recreated from the food materials only, or not Let us be generous and provide enough ATP also for the supposed kick-start. The fact is that no contemporary cell could resume its activity in this experiment. Consequently, all cells today possess a distributive autocatalytic network that cannot be seeded from outside, because some of its seed components cannot be taken up from medium. 

In order to have a metabolic and enzymatic system, it would be necessary to have proteinaceous enzymes which can catalyse the synthesis of other enzymes, and for this they should be capable of making peptide bonds. Such systems have not been found in nature, so far, but according to Stuart Kauffman (1986) they could have existed in the past, and in primitive compartments could have produced autocatalytic networks which had the potential to jump from chaos to order. Even if we admit that those enzymes existed, however, we still have the problem of accounting for the origin of the complex autocatalytic networks that housed them. 

Coussins GRL, Poulsen S-A, Sanders JKM (2000) Molecular evolution dynamiuc combinatorial libraries, autocatalytic networks and the guest for molecular function. Curr Opin Chem Biol 4 270-279... 

All of these observations combined lead us to believe that the development of modem life as a steady-state relaxation process in fact took place through the sequential emergence of two separate charmels. The first in time, and the simple, was the emergence of reductive metabolism through autocatalytic networks either identical or similar to the rTCA cycle. All its reagents are small molecules that are selected by simple kinetic and physical properties from the complete set of CHO molecules of comparable size [65], and the reaction networks involving them are relatively densely sampled, either within the cycle or in the side-reactions that generate biomass from it. 

To summarize, thisprovides one vision of a crucial bridge between the physicists claim of a fine-tuned universe (one predisposed to the production of water, carbon, and nitrogen) and the reality of life on earth. The existence of stable nanotube structures demonstrates how biochemical molecular fine-tuning could give rise to complex entities, presumably through a process of prebiotic molecular selection applied to primitive, quasi-living autocatalytic networks. 

Autocatalytic networks the transition from molecular self-replication to molecular ecosystems, D. H. Lee, K. Severin and M. R. Ghaderi, Curr. Opin. Chem. Biol., 1997,1, 491. 

Physical processes Order-disorder structures, ordered-phase transitions, symmetry breaking, spontaneous magnetization, non-equilibrium crystallization phenomena, percolation, electrodeposition, formation of dissipative structures, turbulence and instabilities in fluid dynamics, and diffusion-limited aggregation process. Biological processes Excitation in muscles, pulsation of heart, calcium waves, natural fold-up of protein molecules, deposition of lipid bilayers, auto-regulation of homeostasis morphogenesis, hyper-cycles and autocatalytic networks, etc. 

Any set of reactions with a cyclic arrangement that additionally includes a downstream process that reproduces one of the metabolites involved in the loop collectively behaves as an autocatalytic network in which all the components of the loop are reproduced when the cycle is running [23, 24]. 

Vasas V, Szathmary E, Santos M (2010) Lack of evolvability in self-sustaining autocatalytic networks constraints metabolism-first scenarios for the origin of life. Proc Natl Acad Sci USA 107 1470-1475... 

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