Steady state
A steady state or dynamic equilibrium is a stationary process in which substances, particles or energy continue to flow into a system and flow out again to the same extent - or z. B. leave the system in a different form as a result of a reaction - so that their amount in the system remains constant over time. The net difference between inflow and outflow is almost constant over time, so large or small amounts can be converted in steady state conditions as long as the sum is zero. Part of the essence of the steady state is that it is an open system and that transport processes determine the equilibrium concentrations of the individual substances in the space being observed (e.g. in each cell). After disorders (heterostasis) there is a tendency to return to the status quo ( homeostasis ). Disturbances in the status quo show up in different amounts of the substances involved (too much or too little). In the German-speaking area, a distinction is made between dynamic equilibrium, chemical equilibrium and homeostasis. A system in steady state changes into thermodynamic equilibrium when the flows between the system parts dry up.
The term steady state goes back to the Austrian-Canadian biologist Ludwig von Bertalanffy , among others .
properties
Living cells can maintain a steady state of substrates over longer periods of time because typical enzymatic conversions are part of a chain of reactions. In one of these, the handled substrate is replenished by the upstream enzyme or by transport processes and the resulting product is diverted by the downstream enzyme.
These conditions are clearly not given in the classic enzyme kinetic measurements , but it is precisely in this area that the concept of steady states has become common. In a typical arrangement, there is namely a single point at which defines the substrate concentration or is known and therefore the reaction rate can be assigned: the start of the reaction . However, this assignment is only possible through extrapolation (keyword: “tangent to the origin”), as shown below. Therefore, not a steady state in the strict sense, but the enzyme kinetics in a closed system is described. In thermodynamics , especially in non-linear thermodynamics, an equilibrium with entropy production is called a steady state .
Enzyme kinetics
According to the Michaelis-Menten theory , the existence of the enzyme-substrate complex, ES, is the central phenomenon for understanding enzyme kinetic measurements . According to the following general reaction equation
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the enzyme E first enters into a reversible bond with its substrate S, whereby ES is formed. In a slower second step, which includes a chemical conversion, the enzyme-product complex, EP, is formed, from which the product is released through dissociation. Under the conditions of enzyme kinetics, the following simplification has become established
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(2)
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with the following reason:
- Compared to the substrate-product conversion (ES to EP), the dissociation process (EP to E + P) proceeds very quickly, so it has a relatively high reaction speed, so that the step with k 3 can be neglected compared to the rate-determining step with k 2 .
- At the beginning of the measurable reaction, the concentration of the free substrate is defined; it corresponds to the concentration used. In addition, a condition [E] ≪ [S] is usually observed, according to which the concentration of the catalyst (enzyme) is far below that of the substrate and the substrate content bound in ES is negligible;
- at the start of the reaction there is still no reverse reaction, i. H. Implementation of P via EP and ES to S.
These are exactly the conditions for typical enzyme kinetic measurements: the initial enzymatic velocity v 0 is measured , that is, the turnover rate directly after the combination of all the necessary components. Experimentally, the tangent is placed at the origin of the recorded time-conversion curve and its slope is determined, v 0 .
Shortly after the enzyme has been mixed with the substrate, there is an initial phase (" pre-steady-state ") in which the ES complex is built up. Tracking this phase requires special measuring methods ( " stopped flow method " ) and is outside the standard repertoire of conventional enzyme kinetics. The reaction quickly reaches the described “quasi-stationary” state, during which the reaction speed v 0 reflects the substrate concentration. For the sake of simplicity, the term “steady state” is used here, although v 0 is restricted to the initial phase of the reaction and is therefore only accessible through the aforementioned extrapolation. Only under saturation conditions ([S] >> [E]), that is, the conditions of an activity measurement , the linear phase at the beginning of the reaction is very extended. Under these conditions, however, no information about the affinity parameter (Michaelis constant K m ) or the catalytic efficiency (k cat / K m , called “ kcat-over-Km ” in the jargon ) can be obtained.
Literary description
A steady state is described very clearly in the poem The Roman Fountain by Conrad Ferdinand Meyer . In the 4th version of the poem from 1866 it says: "... The waters rise / In the second bowl in the middle / And this is full again / They flood into the third: / One taking and one giving / And all stay rich / and all floods live / and yet rest at the same time . "
See also
literature
- Detlef Doenecke, Peter Karlson: Karlsons Biochemie and Pathobiochemie. Georg Thieme, 2005. ISBN 9783133578158 .
- Peter Schopfer, Axel Brennicke: Plant Physiology. Springer, 2010. ISBN 9783827423528 .
Web links
- Entry on steady state . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.S05961 .
Individual evidence
- ↑ dynamic equilibrium. In: Lexicon of Biology. Spektrum Akademischer Verlag, accessed on October 22, 2016 .
- ↑ a b steady state and thermodynamic equilibrium . In: Basic knowledge school . Chemistry high school. Duden, 2015, ISBN 978-3-411-04594-5 .
- ^ Horace Robert Horton (et al.): Biochemistry . Pearson Studies, 2008. ISBN 9783827373120 . P. 187ff.