Curated by RSF Research Staff
The origin of metabolism
A big question in science is how life emerged from ostensibly abiotic environments. What demarcates the transition from prebiotic matter to living systems? What environments could have fostered such complex chemical circuitry? Life is supported on three primary pillars: (1) replication – a molecular system capable of encoding information, most importantly its own reproduction; (2) Synthesis – the molecular machinery to read and execute encoded information to assemble new parts and replicate; and (3) Metabolism – the ability to extract energy from the environment to drive far-from equilibrium processes including chemical synthesis of molecular “building blocks”.
A popular explanation for the emergence of the first chemical replicators is known as the ‘RNA world hypothesis’. Ribonucleic acid is a polymer that is capable of encoding information in its nucleic acid sequence, and performing enzymatic catalysis – it therefore could have potentially served as a single link between replication and synthesis processes. This is appealing because one needs the information molecule to faithfully produce the nanomachinery of metabolism, particularly the enzyme that performs replication, yet an enzyme is needed in turn to produce the information molecule. The RNA molecule could serve as the link between these two domains – replication and synthesis.
However, there are problems with the RNA world hypothesis that seem to indicate it does not adequately explain the first steps in the emergence of living systems. One major issue with the hypothesis is the generation of ribonucleotides to begin with, and their polymerization into a covalently bonded macromolecule. These are both high energy chemical processes, and it is difficult to imagine how they would occur without an abundant and replenishing source of ribonucleotides and a process driving their polymerization, i.e. a primordial metabolism.
This is known as the ‘metabolism first’ hypothesis. Before the information molecule and the replicator, there was a prebiotic metabolism capable of generating the building blocks needed for these systems as well as the high-energy molecules to drive the non-equilibrium reactions. Dr. Gerald Pollack, renowned for his work on structured water, has described processes by which early organic molecules could have been sufficiently concentrated to permit sustained synthesis and replication – the first cells. Structured water could have formed gel-like microscopic structures that functioned with the dual roles of an ancient pre-cytoplasm and a boundary layer to the higher-entropy external environment.
Pollack’s hypothesis explains how the sequestration and concentration of ancient organic molecules was achieved, but we still need an explanation for how the prebiotic central metabolites required for a primordial metabolism were generated, and a new report has experimental evidence for just such a mechanism. A team of scientists from the University of Kentucky and the Massachusetts Institute of Technology (MIT) in the United Sates, and McGill University in Canada have published a paper describing a connection between ZnS prebiotic photosynthesis and clay replication. The paper has related how prebiotic metabolites available from simple sunlight promoted reactions can catalyze the synthesis of clay minerals (i.e., a zinc clay called sauconite). The work shows that central metabolites such as succinate and malate can enable the nucleation process for clay formation. These prebiotic metabolites have been generated by photocatalysis with ZnS, and this work demonstrates how they can catalyze the synthesis of clays.
This describes how primordial metabolic networks and clay mineral catalysis coevolved, supporting and feeding-back into each other to drive the formation of organic molecules needed for life. The mechanism of semiconductor promoted photochemistry would then have played a major role in promoting reactions that otherwise are not favored, providing the foundation for present day complex metabolism.
The living system plays an integral role in the feedback operations that inform the greater system how to organize and develop. In this way, life and sentience are functional aspects of the universe. Understanding how life emerges in the universe is therefore a key aspect of a unified, fully coherent theory of the everything.
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