Cell growth relies on anabolism synthesizing precursors for macromolecule biosynthesis from nutrients and from energy equivalents (synthesized from nutrients by catabolism). Since anabolism involves catabolic pathways – i.e. anabolic precursors (e.g. pyruvate) are intermediates of catabolism – catabolism and anabolism are severely intertwined and not readily disentangled. The fact that energy equivalents are then also synthesized by anabolism (e.g. pyruvate synthesis from glucose yields 2 ATP and 2 NADH) obscures the exact contribution of energy supply by catabolism for cell growth. For instance, how much of the ATP needed to make 1 gram biomass is actually supplied by catabolism? When is catabolism providing NAD(P)H in addition to ATP for anabolism? Since genome-scale stoichiometric models (GSSMs) study catabolism and anabolism as entangled processes, we need to develop a method first for separating catabolism and anabolism.

To address this, we developed a computational method, which is general, unbiased and enables us to understand the role of catabolism across diverse microbes. For this contribution, we analysed GSMMs of seven microbial species across 50 growth conditions, utilizing various organic and inorganic carbon, electron, and nitrogen sources. We found that the amount of ATP produced in anabolism varies from none at all to all, and that redox cofactors, such as NAD(P)H, are exchanged between catabolism and anabolism in certain cases.

Given these results, we reasoned that catabolism is “driven” (determined) by anabolism. Accordingly, we expect that the stoichiometry of the net anabolic reaction can be determined prior to that of the catabolic reactions and its energy need (ATP and/or NAD(P)H) subsequently dictates the net stoichiometry of the catabolic reaction. Therefore, we aimed to identify the factors that determine the amount of ATP produced during anabolism. Typically, donating electrons to a terminal electron acceptor yields ATP. Accordingly, the presence of an electron acceptor in the anabolic overall reaction correlates positively with the fraction of ATP produced in anabolism. The need for reduced redox cofactors in anabolism depends on the electron balance within this process. Anabolism converts a carbon substrate into biomass and potentially also into carbon-containing byproducts, with the electron balance of these components dictating the use of electron acceptors or donors. If an electron donor supplied by catabolism and an external electron acceptor cannot exchange electrons, both are consumed in anabolism. The energy carriers that are consumed in anabolism, must be supplied by catabolism using the same carbon and/or energy source.

To conclude, the fraction of energy supplied by catabolism for cell growth varies depending on the nature of the involved energy sources and is determined by the pathway stoichiometry of anabolism.