Metabolism before enzymes: Forging links between geochemistry and biochemistry
Life transcended at some instance during the transition from geochemistry to biochemistry. Although fossil record of this incident is hard to come by, phylogenetic analysis could shed some light about this event. Studies reveal that metabolism of early life forms may have comprised of two extant autotrophic carbon fixation pathways, the Wood-Ljungdahl pathway and the reverse Tricarboxylic acid cycle (rTCA, aka reverse Kreb's cycle). We demonstrate that under a geological setting consisting of high CO2 concentrations, a variety of native metals as well as certain metal ions promote chemistries observed in these pathways. The distribution of these intermediates depends on reaction parameters such as pH, temperature and pressure. The intermediates are transient in nature giving rise to a chemical flux, which we hypothesize to be driven by redox disequilibrium between the metallic species and its less reduced environment. The same chemical environment also promotes the reductive carboxylation of acetate to pyruvate, linking the non-enzymatic chemistries of the W-L pathway and the rTCA cycle. The plausibility of this chemistry in a completely abiotic environment supports the idea that "enzymes do not perform feats of magic, they just accelerate and add specificity to reactions that tend to occur anyway".