Entering the labyrinth: A hypothesis about the emergence of metabolism from protobiotic routes.

The occurrence of organized chemical transformations defined as metabolism is one of the most important characteristics of life. Surprisingly though, there is not a consensus about how those transformations were originated in the origin of life. RNA world advocates suggest that biochemical pathways started with ribozymes that were further substituted by enzymes. However, most of the biosynthetic routes of ribozymes described do not overlap with the enzymatic routes, and there is not a clear theory about how this transition happened. An important step to solve this dilemma has been elucidated in the last decade when researchers found that some complex routes of chemical transformations, such as the glycolytic and the citric acid pathways, already existed in prebiotic Earth due to physicochemical forces alone. Defined here as protobiotic pathways, we propose that those metabolic exchanges working without the aiding of any biological catalysts were the ones that guided the origin of metabolism. Under this scenario, some quasi-randomly encoded peptides at the origins of translation systems would be capable to bind metabolites in protobiotic routes. When those bounds facilitated or accelerated the conversion of metabolites along the protobiotic path and the products were beneficial, then natural molecular selection acted to preserve the system. Thus, we propose that the origin of metabolism happened when peptides started to bind metabolites in protobiotic routes without disturbing (and possibly aiding) their chemical transformation paths. This should have been the entry point to the metabolic labyrinth, the key step that allowed peptides to come into the path of chemical transformations and further evolve into the enzymes that coordinate nowadays the biochemical pathways.