Origin of the Reductive Tricarboxylic Acid (rTCA) Cycle-Type CO(2) Fixation: A Perspective

The reductive tricarboxylic acid (rTCA) cycle is among the most plausible candidates for the first autotrophic metabolism in the earliest life. Extant enzymes fixing CO(2) in this cycle contain cofactors at the catalytic centers, but it is unlikely that the protein/cofactor system emerged at once in a prebiotic process. Here, we discuss the feasibility of non-enzymatic cofactor-assisted drive of the rTCA reactions in the primitive Earth environments, particularly focusing on the acetyl-CoA conversion to pyruvate. Based on the energetic and mechanistic aspects of this reaction, we propose that the deep-sea hydrothermal vent environments with active electricity generation in the presence of various sulfide catalysts are a promising setting for it to progress. Our view supports the theory of an autotrophic origin of life from primordial carbon assimilation within a sulfide-rich hydrothermal vent.