Universal Motifs and the Diversity of Autocatalytic Systems

Significance Autocatalysis, the ability of chemical systems to make more of themselves, is a hallmark of living systems, as it underlies metabolism, reproduction, and evolution. Here, we present a unified theory of autocatalysis based on stoichiometry. This allows us to identify essential motifs of autocatalytic networks, namely, autocatalytic cores, which come in five categories. In these networks, internal catalytic cycles are found to favor growth. The stoichiometry approach furthermore reveals that diverse autocatalytic networks can be formed with multiple compartments. Overall, these findings suggest that autocatalysis is a richer and more abundant phenomenon than previously thought. Autocatalysis is essential for the origin of life and chemical evolution. However, the lack of a unified framework so far prevents a systematic study of autocatalysis. Here, we derive, from basic principles, general stoichiometric conditions for catalysis and autocatalysis in chemical reaction networks. This allows for a classification of minimal autocatalytic motifs called cores. While all known autocatalytic systems indeed contain minimal motifs, the classification also reveals hitherto unidentified motifs. We further examine conditions for kinetic viability of such networks, which depends on the autocatalytic motifs they contain and is notably increased by internal catalytic cycles. Finally, we show how this framework extends the range of conceivable autocatalytic systems, by applying our stoichiometric and kinetic analysis to autocatalysis emerging from coupled compartments. The unified approach to autocatalysis presented in this work lays a foundation toward the building of a systems-level theory of chemical evolution.