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In biochemistry, steady state refers to the maintenance of constant internal concentrations of molecules and ions in the cells and organs of living systems. [1] Living organisms remain at a dynamic steady state where their internal composition at both cellular and gross levels are relatively constant, but different from equilibrium concentrations. [1]
Many, but not all, biochemical pathways evolve to stable, steady states. As a result, the steady state represents an important reference state to study. This is also related to the concept of homeostasis, however, in biochemistry, a steady state can be stable or unstable such as in the case of sustained oscillations or bistable behavior.
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. [2] Control of flux in a metabolic pathways: The control of flux is a systemic property, that is it depends, to varying degrees, on all interactions in the system.
In biochemistry, control coefficients [1] are used to describe how much influence a given reaction step has on the flux or concentration of the species at steady state.This can be accomplished experimentally by changing the expression level of a given enzyme and measuring the resulting changes in flux and metabolite levels.
The steady state approximation, [1] occasionally called the stationary-state approximation or Bodenstein's quasi-steady state approximation, involves setting the rate of change of a reaction intermediate in a reaction mechanism equal to zero so that the kinetic equations can be simplified by setting the rate of formation of the intermediate equal to the rate of its destruction.
Another way to understand the properties of a linear pathway is to take a more analytical approach. Analytical solutions can be derived for the steady-state if simple mass-action kinetics are assumed. [2] [3] [4] Analytical solutions for the steady-state when assuming Michaelis-Menten kinetics can be obtained [5] [6] but are quite often avoided ...
Let the pathway be at steady-state and imagine increasing the concentration of enzyme, , catalyzing the first step, , by an amount, . The effect of this is to increase the steady-state levels of S and flux, J. Let us now increase the level of by such that the change in S is restored to the original value it had at steady-state.
A physiologic interpretation of clearance (at steady-state) is that clearance is a ratio of the mass generation and blood (or plasma) concentration. Its definition follows from the differential equation that describes exponential decay and is used to model kidney function and hemodialysis machine function: