Cellular origins of metabolism. (A) AHVs could have harbored earliest cellular structures capable of sustaining carbon-fixing cycles on the early Earth. C2 organic compounds, such as acetate and glyoxylate, could have been synthesized in deep seabeds from inorganic C1 carbon sources, such as CO2 and HCN. Acetate and glyoxylate could have been produced nonenzymatically through the Wood-Ljungdahl pathway [7, 8, 19] and HCN-oligomerization reactions [20] under prebiotic conditions. Because these pathways are linear and simple, they could have sponteneously arisen in oceanic crusts in unconfined environments. However, autocatalytic carbon-fixing pathways would likely have required a controlled environment to operate and self-amplify, necessitating a transition to cellular metabolism. Thin-walled vent micropores made of iron sulfides could have furnished a suitable environment for first carbon-fixing cycles to emerge, providing a barrier to facilitate primitive forms of energy coupling and contain the organic products of the foregoing cycles. (B) The protometabolic network in Fig. 1C is driven by inorganic catalysts and reducing agents, such as FeS and Fe0 . Under hydrothermal conditions, mixtures of FeS and Fe0 can be synthesized through the interaction of Fe2+ and HS− [26]. As a result of these interactions in the protocell model, assemblages of FeSFe0 precipitate on lime minerals, which would have been present in hydrothermal-vent environments. The resulting mineral mixtures are distributed inside the protocell either as a continuous matrix (a) or dispersed particles (b). (C) Ferrous iron that participates in metabolic reactions and mineral synthesis enters the protocell from outside by passive diffusion. The dissolution of the iron-sulfide membrane on the acidic side also yields ferrous iron, which contributes to its import flux. The dissolution rate of iron-sulfide membrane and the membrane transport rate of ferrous iron are denoted J0 Fe2+ and JFe2+ , respectively. (D) Protocell model, simulating the operation of first carbon-fixing cycles in AHVs micropores.
biorxiv
Metabolism constitutes the core chemistry of life. How it began on the early Earth and whether it had a cellular origin is still uncertain.
A leading hypothesis for life’s origins postulates that metabolism arose from geochemical CO2-fixing pathways, driven by inorganic catalysts and energy sources, long before enzymes or genes existed.
The acetyl-CoA pathway and the reductive tricarboxylic acid cycle are considered ancient reaction networks that hold relics of early carbon-fixing pathways. Although transition metals can promote many steps of these pathways, whether they form a functional metabolic network in abiotic cells has not been shown.
Here, we formulate a nonenzymatic carbonfixing network from these pathways and determine its functional feasibility in abiotic cells by imposing the fundamental physico-chemical constraints of the early Earth.
Using first principles, we show that abiotic cells could have sustainable steady carbon-fixing cycles that perform a systemic function over a relatively narrow range of conditions. Furthermore, we find that in all feasible steady states, the operation of the cycle elevates the osmotic pressure, leading to volume expansion.
These results suggest that achieving homeostatic metabolic states under prebiotic conditions was possible, but challenging, and volume growth was a fundamental property of early metabolism.
Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻
