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Differences Between Irreversible and Reversible Enzyme Inhibitors
Enzymes are proteins that catalyze specific biochemical reactions in living organisms. Enzyme inhibitors are molecules that reduce or eliminate the activity of enzymes. They can be classified into two types: irreversible enzyme inhibitors and reversible enzyme inhibitors. The main difference between the two types is their ability to bind to the enzyme and the nature of the inhibition.
What are Irreversible Enzyme Inhibitors?
Irreversible inhibitors bind tightly to the enzyme, thus dissociating very slowly from it. They can form either covalent or non-covalent bonds with their target.
Many important drugs, such as penicillin, are irreversible enzyme inhibitors. Penicillin is an antibiotic capable of killing bacteria by covalently binding to the enzyme transpeptidase, therefore preventing the synthesis of the bacterial cell wall.
Another example is aspirin, which forms covalent bonds with the enzyme cyclooxygenase, resulting in the reduction of the inflammatory processes.
Irreversible inhibitors can be classified into three categories: group-specific reagents, substrate analogs, and suicide inhibitors.
Group-specific reagents can bind to a particular amino-acid residue of the enzyme and irreversibly modify it. They are, therefore, less specific, being able to interact with many enzymes.
Substrate analogs present a similar structure to the substrate of the enzyme and can covalently modify its active site residue.
Suicide inhibitors are the most specific enzyme inhibitors. They bind as a substrate to the enzyme and are processed through the catalytic reaction. The catalysis then generates an intermediate that covalently inactivates the enzyme.
What are Reversible Enzyme Inhibitors?
Reversible inhibitors form non-covalent bonds with the enzyme. They are characterized by a rapid dissociation from their target. Reversible inhibitors can be classified into two main categories, competitive and non-competitive inhibitors.
A reversible inhibitor is competitive when the enzyme can bind with its active site, either to the inhibitor forming an enzyme-inhibitor complex (EI), or to the substrate forming an enzyme- substrate (ES) complex
In case of competitive inhibition, the binding of the enzyme to the substrate or to the inhibitor are mutually exclusive: the enzyme can never bind to the inhibitor and to the substrate at the same time.
The reduction of the catalytic activity of the enzyme is achieved by the reduction of the proportion of the enzyme-substrate complex.
Increasing the substrate concentration can relieve the inhibition of the enzyme.
In case of reversible non-competitive inhibition, the substrate and the inhibitor bind simultaneously to different sites of the enzyme, rendering it inactive. This inhibition cannot be overcome by increasing the substrate concentration.
Differences: Irreversible and Reversible Enzyme Inhibitors
Irreversible enzyme inhibitors bind covalently to the enzyme's active site or another important functional group, rendering the enzyme permanently inactive. The binding is strong and irreversible, meaning that the inhibitor cannot be removed from the enzyme without denaturing it. This type of inhibition is usually used for therapeutic purposes, such as the inhibition of bacterial enzymes, or for research purposes, such as in the study of enzyme kinetics. Examples of irreversible enzyme inhibitors include aspirin, which inhibits the cyclooxygenase enzyme, and penicillin, which inhibits bacterial cell wall synthesis.
In contrast, reversible enzyme inhibitors bind non-covalently to the enzyme, and the inhibition can be reversed by removing the inhibitor. Reversible inhibitors can be further divided into two subtypes: competitive inhibitors and non-competitive inhibitors.
Competitive inhibitors bind to the enzyme's active site, competing with the substrate for binding. This type of inhibition can be overcome by increasing the concentration of the substrate, which will outcompete the inhibitor. Competitive inhibitors do not alter the maximum velocity (Vmax) of the reaction, but they do increase the apparent Michaelis constant (Km). This means that a higher substrate concentration is required to achieve half-maximal velocity in the presence of the inhibitor. Examples of competitive inhibitors include statins, which inhibit the enzyme HMG- CoA reductase, and methotrexate, which inhibits the enzyme dihydrofolate reductase.
Non-competitive inhibitors bind to an allosteric site on the enzyme, causing a conformational change that alters the enzyme's activity. This type of inhibition cannot be overcome by increasing the concentration of the substrate. Non-competitive inhibitors decrease the maximum velocity (Vmax) of the reaction, but they do not alter the apparent Michaelis constant (Km). This means that the substrate can still bind to the enzyme with the same affinity, but the rate of the reaction will be reduced. Examples of non-competitive inhibitors include cyanide, which inhibits the enzyme cytochrome c oxidase, and azide, which inhibits the enzyme catalase.
The following table highlights the major differences between Irreversible and Reversible Enzyme Inhibitors −
Characteristics |
Irreversible Enzyme Inhibitors |
Reversible Enzyme Inhibitors |
|---|---|---|
Symbolization |
Irreversible enzyme inhibitors and reversible enzyme inhibitors are capable of binding to enzymes and reducing their catalytic activity. |
Dengue is a weakness causing viral disease of the tropics, caused by mosquitoes, and results in sudden fever and acute pains in the joints. |
Characteristics |
Irreversible inhibitors bind tightly to the target enzyme, and the dissociation of the enzyme- inhibitor complex is very slow. |
The inhibition effect is irreversible. Reversible inhibitors, on the other side, are characterized with a rapid dissociation of the enzyme-inhibitor complex. The inhibition effect is reversible. |
Incidence |
Irreversible inhibitors can be classified into three categories: group-specific reagents, substrate analogs, and suicide inhibitors. |
Reversible inhibitors are classified into two groups: competitive and non-competitive inhibitors. |
Types |
Irreversible substrate analogs and reversible competitive inhibitors act similarly by imitating the enzyme-specific substrate. |
Substrate analogs irreversibly modify the active site of the enzyme, while the competitive inhibition can be reversed by increasing substrate concentration. |
Conclusion
In conclusion, irreversible enzyme inhibitors bind covalently to the enzyme's active site or another important functional group, rendering the enzyme permanently inactive, while reversible enzyme inhibitors bind non-covalently to the enzyme, and the inhibition can be reversed by removing the inhibitor.
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