Competitive inhibition of enzymes is one of three modes of inhibition. Enzymes must not have a permanent effect in the organism. Otherwise the biochemical reactions would take place at the same time with (relatively) high speed. When not needed, they substrates are often inhibited by inhibitors. Enzymes can be irreversibly or reversibly inhibited. Reversible can proceed by competitive or non-competitive mechanisms.
Competitive inhibition – definition
The name of competitive inhibition is derived from the competition between substrate and inhibitor and active site. That is why competitive inhibition is also called competing inhibition. Another term would be competitive antagonism. The competitive inhibition is reversiblei.e. reversible.
The word “competitive” comes from the Latin word “competitor”, which means competitor.
Competitive inhibition – enzymes
To better understand competitive inhibition and how it works, it is worth taking a general look at enzymes.
Under enzymes complex protein molecules are understood. These proteins enable and speed up biochemical reactions in the body. That is why enzymes are also called biocatalysts. Since enzymes are central drivers for biochemical metabolic processes in organisms, life is not possible without them.
If you would like to know more about enzymes, please have a look at the appropriate explanation!
Structure of enzymes
There are two different types of enzyme structure: Most enzymes consist of a single protein or polypeptide chain. They are therefore monomers. However, there is also the possibility that enzymes consist of several protein chains. These are oligomers.
Furthermore, enzymes are differentiated according to whether they have a cofactor or not. The cofactor is a kind of umbrella term for inorganic substances or organic molecules, which are also called coenzymes.
As a coenzyme, a non-proteinaceous called a component of enzymes. The coenzyme helps enzymes speed up (catalyze) reactions. This is done by briefly binding the coenzyme to the enzyme.
In biochemistry, the terms monomer, dimer, trimer, etc. describe a large number of identical subunits lined up next to one another. However, there is also the term polymer, which refers to the fact that similar subunits are lined up.
Competitive inhibition – substrate concentration
The substrate concentration and the inhibitor concentration are essential in the context of competitive inhibition. Because the competitive inhibitor binds to the active site, it prevents the substrate from docking to the active site. This inhibits the enzyme. But to what extent do substrate concentration and inhibitor concentration play a role here?
Importance of Inhibitor Concentration
If the inhibitor concentration is greater than the substrate concentration, then the inhibitory effect of the reaction is greater. So there are many more inhibitor molecules present. The substrates then have to compete for docking to the active site, which becomes more difficult for them as the concentration of inhibitor increases.
Importance of substrate concentration
If the substrate concentration is greater than the inhibitor concentration, many more substrates can dock to the active center in comparison. The substrates occupy the enzymes and one speaks of an enzyme-substrate complex. The substrates can then be converted.
Even if the inhibitor is structurally very similar to the substrate, the enzyme recognizes the difference between substrate and inhibitor, which is why the inhibitor is not converted. This means that competitive inhibition can be reversed by an increased substrate concentration compared to the inhibitor concentration. The inhibitor is then again displaced by the enzyme.
Competitive inhibition model
Another aspect that also needs to be considered in competitive inhibition is the rate of reaction.
The maximum speed of an enzyme with an inhibitor is the same as without an inhibitor. However, if an inhibitor is present, a higher substrate concentration, i.e. more substrate, is required to reach the maximum rate, since a part of the enzyme molecules is constantly blocked by the inhibitor.
Michaelis-Menten equation
The Michaelis-Menten equation describes the dependence of the reaction rate on the substrate concentration in an enzymatic reaction.
Competitive inhibition – example
Here is an example of competitive inhibition:
Alcohol dehydrogenase (ADG) is an enzyme responsible for breaking down alcohols. This creates pyruvate, which can then be used in the metabolism, for example to generate energy.
As long as ethanol is broken down, there is no problem. But it often happens that alcohol consists not only of ethanol, but also of methanol, which then binds to ADG. Methanol is first converted to formaldehyde and then to formic acid. Poisoning occurs in the body. Therefore, ethanol (the actual substrate) must intervene against and displace the competitive inhibitor.
Competitive Inhibition – The Most Important
- In competitive inhibition, an inhibitor binds to the active site of the enzyme.
- The inhibitor resembles the substrate in its spatial structure and competes with it for the binding site.
- Competitive inhibition is reversible.
- The maximum reaction speed is reached despite inhibition. This only requires a higher substrate concentration to displace the inhibitors.
proof
- Spektrum.de: competitive inhibition (09/08/2022)
- chemeurope.de: Enzyme (08.09.2022)
- netdoktor.de: Enzyme structure (08.09.2022)
- Spektrum.de: Michaelis menten equation (08.09.2022)