Iron-sulphur proteins are ancient and drive fundamental processes in cells, notably electron transfer and CO 2 fixation. Ironsulphur minerals with equivalent structures could have played a key role in the origin of life. However, the 'iron-sulphur world' hypothesis has had a mixed reception, with questions raised especially about the feasibility of a pyrites-pulled reverse Krebs cycle. Phylogenetics suggests that the earliest cells drove carbon and energy metabolism via the acetyl CoA pathway, which is also replete in Fe(Ni)S proteins. Deep differences between bacteria and archaea in this pathway obscure the ancestral state. These differences make sense if early cells depended on natural proton gradients in alkaline hydrothermal vents. If so, the acetyl CoA pathway diverged with the origins of active ion pumping, and ancestral CO 2 fixation might have been equivalent to methanogens, which depend on a membrane-bound NiFe hydrogenase, energy converting hydrogenase. This uses the proton-motive force to reduce ferredoxin, thence CO 2 . The mechanism suggests that pH could modulate reduction potential at the active site of the enzyme, facilitating the difficult reduction of CO 2 by H 2 . This mechanism could be generalised under abiotic conditions so that steep pH differences across semi-conducting Fe(Ni)S barriers drives not just the first steps of CO 2 fixation to C1 and C2 organics such as CO, CH 3 SH and CH 3 COSH, but a series of similar carbonylation and hydrogenation reactions to form longer chain carboxylic acids such as pyruvate, oxaloacetate and a-ketoglutarate, as in the incomplete reverse Krebs cycle found in methanogens. We suggest that the closure of a complete reverse Krebs cycle, by regenerating acetyl CoA directly, displaced the acetyl CoA pathway from many modern groups. A later reliance on acetyl CoA and ATP eliminated the need for the proton-motive force to drive most steps of the reverse Krebs cycle.

Coronavirus (COVID-19) disease is an emerging situation that brought challenges to all sectors, including academia and research. Undergraduate and postgraduate students in biochemistry and molecular biology have been affected significantly due to the recent laboratory closures. Experiments have been suspended for long causing extreme stress to the students. Virtual laboratory is a powerful educational tool that enables students to conduct experiments at the comfort of their home. An excellent opportunity to engage students with technology and in parallel to avoid unforeseen disruptions, as happened recently due to pandemic.

Methanogens are putatively ancestral autotrophs that reduce CO 2 with H 2 to form biomass using a membrane-bound, proton-motive Fe(Ni)S protein called the energy-converting hydrogenase (Ech). At the origin of life, geologically sustained H + gradients across inorganic barriers containing Fe(Ni)S minerals could theoretically have driven CO 2 reduction by H 2 through vectorial chemistry in a similar way to Ech. pH modulation of the redox potentials of H 2 , CO 2 and Fe(Ni)S minerals could in principle enable an otherwise endergonic reaction. Here, we analyse whether vectorial electrochemistry can facilitate the reduction of CO 2 by H 2 under alkaline hydrothermal conditions using a microfluidic reactor. We present pilot data showing that steep pH gradients of approximately 5 pH units can be sustained over greater than 5 h across Fe(Ni)S barriers, with H + -flux across the barrier about two million-fold faster than OH – -flux. This high flux produces a calculated 3-pH unit-gradient (equating to 180 mV) across single approximately 25-nm Fe(Ni)S nanocrystals, which is close to that required to reduce CO 2 . However, the poor solubility of H 2 at atmospheric pressure limits CO 2 reduction by H 2 , explaining why organic synthesis has so far proved elusive in our reactor. Higher H 2 concentration will be needed in future to facilitate CO 2 reduction through prebiotic vectorial electrochemistry.

Iron-sulfur (FeS) proteins are ancient and fundamental to life, being involved in electron transfer and CO2 fixation. FeS clusters have structures similar to the unit-cell of FeS minerals such as greigite, found in hydrothermal systems linked with the origin of life. However, the prebiotic pathway from mineral surfaces to biological clusters is unknown. Here we show that FeS clusters form spontaneously through interactions of inorganic Fe2+/Fe3+ and S2− with micromolar concentrations of the amino acid cysteine in water at alkaline pH. Bicarbonate ions stabilize the clusters and even promote cluster formation alone at concentrations >10 mM, probably through salting-out effects. We demonstrate robust, concentration-dependent formation of [4Fe4S], [2Fe2S] and mononuclear iron clusters using UV-Vis spectroscopy, 57Fe-Mössbauer spectroscopy and 1H-NMR. Cyclic voltammetry shows that the clusters are redox-active. Our findings reveal that the structures responsible for biological electron transfer and CO2 reduction could have formed spontaneously from monomers at the origin of life.