Emergence of protometabolisms and the self-organization of non-equilibrium reaction networks
by
Raphaël Plasson(Nordita)
→
Europe/Stockholm
122:026 (House 12)
122:026
House 12
Description
The debate between “Metabolism first” and “Replication first” theories is shaping the discussion about how life originated (Pross, 2004), emphasizing either the necessity of a structured reaction network to maintain information, or the necessity of information to shape the reaction network. In order to solve this apparent paradox, a general approach comes down to understanding how protometabolisms can lead to the emergence of the first template replicators (Shapiro, 2006; de Duve, 2007), from which open-ended evolutive systems can develop (Ruiz-Mirazo, 2008).
On the one hand, replication systems must maintain their informational integrity, characterized by a specific topology of the reaction network, implying the necessity of a continuous consumption and use of energy. On the other hand, the presence of a source of free energy should have lead to the self-organization of reaction networks (Plasson and Bersini, submitted), that is to the emergence and maintenance of protometabolisms. Such reservoirs of energy (originating from several external energy sources, like sun light, reduced material from Earth crust, meteorites entering the atmosphere, etc.) generate both linear fluxes of reaction and reaction loops, as attractors of the network (Plasson, et al. submitted). This implies the spontaneous generation of network catalysis and autocatalysis, which introduces positive and negative feedbacks inside the system. In such dynamical reaction networks, bifurcation mechanisms leads to the extinction of some pathways, and to the advantage of the persistence of other. The topology of the reaction network is subjected to a spontaneous evolution, driven by free energy transfers. Rather than the increase of complexity, this process can be better described as a change in the nature of the complexity, from horizontal complexity (i.e. a large number of simple molecules reacting non-selectively with each other) to vertical complexity (i.e. a large number of complex molecules, built on a limited number of building blocks, engaged in autocatalytic cycles). Such self-organization phenomenon can be linked to the notion of “logical depth” developed by Bennett (1986). A simple model of dynamic polymerization of amino acids will be described as a simple model example of such self-organization of reaction network by bifurcation mechanisms (Plasson, et al. 2007).
In this scope, the gap separating prebiotic systems from the first reproductive systems can be described as evolutive protometabolisms. The bifurcations, driven by the fighting mechanisms between the network sub-elements, are sources of topological changes inside the reaction networks, from randomness to structures organized around some central compounds. This may have constituted the first replicators, not as template replicators of similar molecules, bu as network replicators of similar reaction cycles, competing with each others.
References
Pross, A. (2004). Causation and the origin of life. Metabolism or replication first? Origins of Life and Evolution of the Biosphere, 34:307–421.
Shapiro, R. (2006). Small molecule interactions were central to the origin of life. The Quarterly Review of Biology, 81(2):105–125.
de Duve, C. (2007). Chemistry and selection. Chemistry & Biodiversity, 4:574–583.
Ruiz-Mirazo, K., Umerez, J. and Moreno, A. Enabling conditions for “open-ended evolution” (2008). Biology and Philosophy, 23:67–85.
Plasson, R. and Bersini, H. (submitted). Energetic and entropic analysis of mirror symmetry breaking process in recycled microreversible chemical system. Submitted to the Journal of Physical Chemistry B. http://arxiv.org/abs/0804.4834.
Plasson, R., Bersini, H. and Brandenburg, A. (submitted). Decomposition of Complex Reaction Networks into Reactons. Submitted to Biophysical Journal. http://arxiv.org/abs/0803.1385
Bennett, C. H. (1986). On the nature and origin of complexity in discrete, homogeneous, locally interacting systems. Foundations of Physics, 16:585-592.
Plasson, R., Kondepudi, D. K., Bersini, H., Commeyras, A. and Asakura, K. (2007). Emergence of homochirality in far-from-equilibrium systems: mechanisms and role in prebiotic chemistry. Chirality, 19:589–600.