Rammu, Hanadi; (2023) Building the prebiotic machinery for autotrophic CO₂ fixation at the origin of life. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Phylogenetics and thermodynamics point to an autotrophic origin of life, which is supported by the ubiquity of membrane bioenergetics across the domains of life. In autotrophic bacteria and archaea, transmembrane vectorial proton gradients are utilised by membrane-bound machinery to transform the disequilibria into carbon and energy metabolism. The acetyl-CoA pathway is the most ancient of the six known CO₂ fixation pathways, filled with transition metal clusters such as the ancient iron-sulfur proteins ferredoxin (Fd) and energy-converting hydrogenase (Ech). It is hypothesized that the first organisms could have used a process akin to this pathway to live off the redox potential between dissolved H₂ and CO₂ in geochemical hydrothermal vents of the Hadean. This thesis explores how such a system might arise; could prebiotic versions of Ech and Fd operate on natural proton gradients to drive protocell growth in alkaline hydrothermal vents? I show that lipid vesicles with bilayer membranes can self-assemble from complex mixtures of 6-12 single chain amphiphiles. Vesicle stability across alkaline pH, in the presence of salts, their ability to encapsulate solutes makes them promising candidates for protocellular structures. I investigate the redox properties of FeS clusters spontaneously coordinated by L-Cysteine (Cys) at alkaline pH. Cys-FeS clusters show similar redox properties to [4Fe-4S] maquettes and Fd. The catalytic ability of these ‘prebiotic’ FeS clusters are briefly explored for CO₂ fixation and is so far unsuccessful. Given the importance of Cys in co-ordinating FeS clusters, the thesis also begins to explore a route for the prebiotic synthesis of Cys from L-serine using the essential cofactor pyridoxal. Despite some analytical challenges, Cys synthesis can be achieved from the intermediate O-phospho-L-serine. My research lays a solid foundation for the ongoing experimental investigation of chemiosmotic and autotrophic CO₂ fixation at the origin of life.

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