Beyond classical overflow metabolism: Toward a unified mechanistic framework

Overflow metabolism is traditionally defined as the production of low-energy-yield metabolites despite the availability of more energy-efficient pathways, exemplified by acetate secretion in Escherichia coli, ethanol production in yeast, and lactate formation in mammalian cells. Despite extensive study, the mechanisms underlying overflow metabolism remain incompletely understood. Increasing evidence further demonstrates that overflow-like metabolism is not restricted to carbon excess but also occurs under nutrient limitation, environmental stress, recombinant protein expression, and within microbial communities, indicating that its physiological significance extends far beyond the classical paradigm. Here, we review these diverse manifestations, evaluate the underlying mechanisms of overflow metabolism, and propose adopting a separable, reversible Michaelis-Menten formulation as a unifying conceptual framework. This framework integrates catalytic capacity, substrate saturation, product inhibition, and thermodynamic driving forces into a cohesive explanation. Within this framework, overflow metabolism emerges when substrate influx exceeds the processing capacity of more energy-efficient pathways, resulting in flux redistribution toward alternative routes with distinct energetic, kinetic, and/or proteome-allocation characteristics. By bridging enzyme kinetics, resource allocation, thermodynamics, and microbial ecology, this framework may unify diverse forms of overflow metabolism and provide a predictive foundation for engineering this metabolic behavior in microbial systems.