Asymmetric co-substrate usage at a metabolic branch point can drive overflow metabolism

Metabolic behaviours of cells, such as metabolic overflow, involve regulation of metabolic fluxes across different pathways. Two well-established concepts in flux regulation are transcriptional regulation, involving changes in enzyme levels through gene regulation, and allosteric regulation, involving changes in enzyme catalytic activities through metabolite binding. Here, we describe an additional mechanism arising through co-substrate usage around metabolic branch points. Using models of different branch-point motifs, we find that asymmetric usage of a given co-substrate by reactions at and around a branch point induces flux switching at that branch point as influx increases. This provides cells with an inherent ‘self-regulation’ embedded in the architecture of metabolic pathways. We then develop a specific model of the yeast pyruvate branch point involving NADH usage. Using this model, we show that co-substrate based flux switching can explain experimental data on yeast metabolic overflow at high glucose influx, as well as how altering NADH dynamics shifts the initiation point of metabolic overflow. We conclude that asymmetric co-substrate utilisation around branch points provides an inherent regulatory mechanism that could explain observed relations between cosubstrate dynamics and metabolic behaviours. Additionally, the presented theory and its extensions will provide a model-based approach to manipulating co-substrate dynamics for metabolic engineering.