Flux, or metabolic flux is the rate of turnover of molecules through a metabolic pathway. Flux is regulated by the enzymes involved in a pathway. Within cells, regulation of flux is vital for all metabolic pathways to regulate the metabolic pathway's activity under different conditions. Flux is therefore of great interest in metabolic network modelling, where it is analysed via flux balance analysis.
Existing metabolic networks regulate the movement of chemicals through their enzymatic steps by mostly regulating enzymes that catalyze irreversible reactions, while the movement of chemicals through reversible steps is generally unregulated directly. As a result, the movement of precursors/products through a metabolic network is governed by simple chemical equilibria, with specific key enzymes that are subject to regulation. This regulation may be indirect, in the case of an enzyme being regulated by some cell signalling mechanism, or it may be direct, as in the case of allosteric regulation, where metabolites from a different portion of a metabolic network bind directly to and affect the catalytic function of other enzymes in order to maintain homeostasis.
In this manner, flux is the movement of matter through metabolic networks that are connected by chemical equilibria, and is therefore a way of describing the activity of the metabolic network as a whole using a single characteristic.