Enzymes are biological substances that speed up the rate of a chemical reaction by decreasing activation energy and are not consumed at the end of the reaction. In a living system, many substances can influence the working of the enzyme.
What are enzyme inhibitors?
A chemical substance that can attach itself with the enzyme in place of the
substrate and hinder its activity temporarily or permanently is called an
inhibitor. Inhibitors block the active site of enzymes temporarily or
permanently and as a result, substrates cannot be converted into products.
You can study normal enzyme reaction
here.
Video lesson
Types of enzyme inhibitors
There are two main types of inhibitors
- Reversible inhibitors
- Irreversible inhibitors
1. Reversible inhibitors
They temporarily hinder the activity of the enzyme by forming weak linkages with enzymes. As the name indicate, their effect can be reversed or neutralized partly or completely by increasing the substrate concentration.
The reversible inhibitors are further divided into two main types.
- Competitive inhibitors
- Non-competitive inhibitors
A. Competitive inhibitors
These inhibitors have structural similarity with the substrate so they are picked by the binding sites of enzymes to form enzyme-inhibitor complex instead of the enzyme-substrate complex. However, they are not converted into products by the catalytic sites, thus blocking the active site and inhibiting the activity of the enzyme.
For example, methotrexate which is an antineoplastic drug has a structure analogous to vitamin folic acid. It inhibits dihydrofolate reductase by binging to it ate active site. Another example of a competitive inhibitor is the inhibition of Succinic dehydrogenase by malonic acid. Malonic acid has the structural similarity of succinate.
B. Non-competitive inhibitors
They do not compete with the substrate for binding with the enzyme. Instead, they bind at a place other than the active site to form an enzyme-inhibitor complex and changes the shape of the enzyme. Due to alteration in the structure of enzyme and active site, the substrate cannot bind to the enzyme, and enzyme catalysis is hindered.
|
| Diagram showing competitve inhibition. E=enzyme, S=substrate and I=Inhibitor. |
For example, the amino acid alanine is a non-competitive inhibitor of
the enzyme pyruvate kinase.
2. Irreversible inhibitors
These inhibitors permanently inhibit the enzyme catalysis. They bind at the active site by forming a covalent bond or destroy the globular structure of the enzyme as well as block the active site physically. Their effect can not be reversed by increasing the substrate concentration.
An example of an irreversible inhibitor is Penicillin, which inhibits the enzyme transpeptidase and inhibits the synthesis of the bacterial cell wall. Diisopropyl fluorophosphate which is found in nerve gas is another example of an irreversible inhibitor. It stops the transmission of nerve impulses by binding to the enzymes.
Examples of enzyme inhibitors
The examples of enzyme inhibitors are combined in the table below.
|
Types of inhibitors |
Inhibitor |
Enzyme |
Substrate |
|
Competitive inhibitors
|
Methotrexate |
Dihydrofolate reductase |
Folic acid |
|
Malonic acid |
Succinic dehydrogenase |
Succinate |
|
|
Cyanide |
cytochrome oxidase |
Electron transport chain |
|
|
Non-competitive inhibitors
|
Alanine |
Pyruvate kinase |
Phosphoenolpyruvate |
|
Glucose-6-phosphate |
Hexokinase |
Glucose |
|
|
Irreversible inhibitors
|
Penicillin |
Transpeptidase |
Peptidoglycan |
|
Diisopropyl fluorophosphate |
Cholinesterase |
Acetylcholine |
|
|
|
Aspirin |
Cyclooxygenase |
Arachidonate |
References
- Warshel, A. (1978). Energetics of enzyme catalysis. Proceedings of the National Academy of Sciences, 75(11), 5250-5254.
- Bjelaković, G., Stojanović, I., Bjelaković, G. B., Pavlović, D., Kocić, G., & Daković-Milić, A. (2002). Competitive inhibitors of enzymes and their therapeutic application. Med Biol, 9, 201-206.
- González‐Bello, C. (2016). Designing irreversible inhibitors—worth the effort?. ChemMedChem, 11(1), 22-30.
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