![]() This enzymatic regulation may be indirect, in the case of an enzyme being regulated by some cell signalling mechanism (like phosphorylation), 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.Ī result that may seem at first counter intuitive, is that regulated steps tends to have small flux control coefficients. As a result, the movement of molecules through a metabolic network is governed by simple chemical equilibria (at reversible steps), with specific key enzymes that are subject to regulation (at irreversible steps). Reversible reactions have no or very small free energy change. Irreversible reactions at regulated steps of a pathway have a negative free energy change, thereby promoting spontaneous reactions in one direction only. The movement of molecules through reversible steps is generally unregulated by enzymes, but rather regulated by the concentration of products and reactants. In addition to this, sharing of some metabolites between the different networks further tightens the connections between the different networks.Įxisting metabolic networks control the movement of molecules through their enzymatic steps by regulating enzymes that catalyze irreversible reactions. This connection is provided by usage of common cofactors such as ATP, ADP, NADH and NADPH. To allow these networks to interact, a tight connection between them is necessary. These networks can then be used to study metabolism within cells. These reactions form metabolic networks within cells. A flux control coefficient can only be measured in the intact system and cannot for example be determined by inspection of an isolated enzyme in vitro.Ĭellular metabolism is represented by a large number of metabolic reactions involving the conversion of the carbon source (usually glucose) into the building blocks needed for macromolecular biosynthesis.A step in a linear chain with a flux control coefficient of one means that that particular step has complete control over the steady-state flux.A step with a flux control coefficient of zero means that, that particular step, has no influence over the steady-state flux.In a linear chain of reactions, the flux control coefficient will have values between zero and one.The control of flux is measured by the flux control coefficient. ![]() The control of flux is a systemic property, that is it depends, to varying degrees, on all interactions in the system.The change in flux that occurs due to the above requirement being communicated to the rest of the metabolic pathway in order to maintain a steady-state.The degree to which metabolic steps determine the metabolic flux varies based on the organisms' metabolic needs.The degree of influence is measured by the flux control coefficient.Ĭontrol of flux through a metabolic pathway requires that Furthermore, it is observed that throughout a steady-state pathway, the flux is determined to varying degrees by all steps in the pathway. J = V f − V r Īt equilibrium, there is no flux. The flux of the metabolites through each reaction (J) is the rate of the forward reaction (V f), less that of the reverse reaction (V r): It is easiest to describe the flux of metabolites through a pathway by considering the reaction steps individually. In this manner, flux is the movement of matter through metabolic networks that are connected by metabolites and cofactors, and is therefore a way of describing the activity of the metabolic network as a whole using a single characteristic. Flux is therefore of great interest in metabolic network modelling, where it is analysed via flux balance analysis and metabolic control analysis. Within cells, regulation of flux is vital for all metabolic pathways to regulate the pathway's activity under different conditions. Flux is regulated by the enzymes involved in a pathway. For other uses of "Flux", see Flux (disambiguation).įlux, or metabolic flux is the rate of turnover of molecules through a metabolic pathway.
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