What is inhibitor enzyme

There are a number of molecules (inhibitors, I) which, due to their structure, resemble a substrate molecule. They are therefore equally bound to the active center, but are usually not implemented because of their chemical properties. They compete (compete) for the binding site with the actual substrate molecules. Often their affinity for the enzyme is even much higher than that of the substrate, so that it no longer has a chance to react with the enzyme. The enzyme molecules are largely inactivated by the inhibitor binding. This competitive inhibition can be read off from the following reaction kinetics.

It can be seen from it that the kM.-Value has increased, because essentially the EI complex is formed instead of the ES complex. The effect of the inhibitor thus depends directly on its concentration and the [substrate] / [inhibitor] ratio.

left picture: Reduction of the turnover rate of an enzyme-catalyzed reaction by a competitive inhibitor (competitive inhibition) (blue curve) - red: Control = not inhibited, right picture: Competitive inhibition as shown by Lineweaver and Burk.

A second type of inhibitor leads to the non-competitive inhibition. It is not bound to the active center, but to any other point on the enzyme surface. As a result, the shape of the enzyme and thus also the substrate binding site is deformed, so that the substrate molecule now fits less well. It is therefore implemented far less. The kM.-Value remains unchanged, the substrate concentration has no influence on the inhibitory effect.

left picture: Reduction of the conversion rate of an enzyme-catalyzed reaction by a non-competitive inhibitor. Non-competitive inhibition: vMax is reduced, the kM.Value remains unchanged. right picture: Non-competitive escapement as represented by Lineweaver and Burk.

There are inhibitors whose effect is reversible, i.e. the old state is restored after washing out. But there are also those whose effects cannot be repaired because they irreversibly impaired the structure of the enzyme (denature).

Inactivation can be achieved on the one hand by molecules (chemically), but on the other hand also by physical factors (temperature, short-wave radiation, etc.). This is the basis for the fact that every enzyme has an optimum temperature, because turnover rate and enzyme inactivation are temperature-dependent processes that are opposite to one another.

An increase in temperature initially leads to an increase in enzyme activities; as the temperature rises more sharply, signs of denaturation (thermal denaturation) begin to predominate. Optimum curves are rarely built symmetrically. Accelerated decline in activity is common.

If an enzymatically catalyzed reaction is allowed to proceed at different temperatures, a family of curves is obtained from kinetics of different shapes.

With increasing temperature, the conversion rate increases, but at the same time the thermal inactivation rate of the enzyme increases. There is therefore an optimal temperature range for enzyme activity (the value is specific to each enzyme). Yellow curve: Reactions at 40 ºC, orange: 50 ºC, Light Blue: 30 ºC, red: 60 ºC, dark blue: 20 ºC.

From the corresponding graph it can be seen that the deflection of the time conversion curve is more pronounced the higher the temperature. This means that at the beginning of the reaction all the enzyme molecules in solution are still fully active and are characterized by increased conversion rates (compared to values ​​that were measured at low temperatures). As time progresses, more and more molecules precipitate through thermal denaturation, which means that the temperature optimum is lower, the longer the reaction time. At a very high temperature (e.g. already at 37 ºC) an enzyme can achieve a lot in a short time, but has to be constantly replaced by newly synthesized enzyme molecules in order to meet the requirements of the cell (e.g. in a warm-blooded animal or in a plant cell which exposed to direct, intense solar radiation). As can be seen from the following figure, a number of parameters can be inferred from reaction kinetics, which are useful for understanding turnover rates.

The following conclusions can be drawn from the form of the kinetics: VioletCurve: linear course = constant reaction speed, yellow and green curve: Continuous delay in the reaction: An inactivation process runs counter to the course of the reaction, or the reaction slows down due to substrate depletion. red curve: Disruption of the reaction due to opposite implementation. orange curve: autocatalytic curve progression (positive feedback, see following section).