Figure 1.

Models of positive feedbacks in protocells. Each panel depicts protocell models of autotrophic CO₂ fixation driven by H₂ (not shown) and catalysed by membrane-bound iron-sulfur clusters chelated by amino acids (yellow squares with green circles) under hydrothermal-type conditions. Fixed CO₂ initially forms a simple two-carbon activated acetate, equivalent to acetyl CoA (C₂), which acts as the primary substrate for a branching protometabolism based on the universally conserved core of biochemistry. (a) The null model considers the base-case in the absence of nucleotide formation in which the products are fatty acids (FA), amino acids, half of which (AA₁) feedback on CO₂ fixation, sugars (S) and energy (E). The subsequent models consider the catalytic function of nucleotides (N) produced from sugars (S), energy (E) and the other half of amino acids (AA₂). In (b), nucleotides catalyse CO₂ fixation ($K_C^N$) alone; in (c), nucleotides catalyse individual synthetic pathways ($K_i^N$, where i ∈ FA, AA₁, AA₂, S, E) in addition to CO₂ fixation and in (d), nucleotides perform autocatalysis ($K_N^N$) in addition to CO₂ fixation. Solid arrows represent synthesis reactions and dotted arrows represent catalysis and are shown in red for the variant models. Note that the energy currency E (equivalent to acetyl phosphate) is consumed by nucleotide synthesis, hence is equivalent to a substrate and is shown by a solid line.