Types of Enzyme Inhibition and Inhibitors

Inhibitors are the compounds that block the enzyme activity either temporarily or permanently. They interact with certain groups in the enzymes and reduce or stop their activity.

In order to learn the types of inhibition, understanding the working of enzymes is necessary.

Working of Enzyme

Enzymes are very specific in their action. This specificity is provided by the active site. The active site is a pocket-like structure that binds the specific substrate and converts it into the product. Before this conversion enzyme is attached to the substrate and forms an enzyme-substrate complex. The general equation for the mechanism of enzyme action is given below:

Enzyme draws substrate toward its active site. The substrate binds to the active site of the enzyme and forms an enzyme-substrate complex. Then the catalytic chemical reaction takes place. This reaction leads to the formation of the product. After the formation of the product, the enzyme is again available for a new reaction.

What is Enzyme Inhibition?

Under some circumstances, the active site or the enzyme itself gets occupied with some unwanted compounds. These compounds may block active sites temporarily or permanently. This process is known as enzyme inhibition. When an inhibitor binds to the enzyme, it stops the substrate from entering the enzyme. It also hinders the catalyzing reaction. Due to this inhibition, enzymes will not be able to perform their function properly.

Types of Inhibition

Inhibition is mainly of two types:

• Irreversible Inhibition
• Reversible inhibition

Irreversible inhibition

In irreversible inhibition, the inhibitor inactivates the enzyme permanently. They are covalently bonded to the active site of the enzymes. The effect of irreversible inhibition, however, cannot be reversed by increasing substrate concentration. They have some reactive functional groups. These groups react with amino acids in the active site and block the active site permanently.

Irreversible inhibitors do not necessarily destroy the structure of enzymes. They may alter the active site of its target. So the active site will no longer be available for further action. They are very useful in some cases i.e they may block the enzymes needed for the growth of cancer cells.

Irreversible inhibitors are further divided into three types:

• Group-specific reagent
• Reactive Substrate Analogs
• Suicide Inhibitors

Group-specific reagent

They are covalently bonded to a particular amino acid on the enzyme molecule. They not only inhibit but also predict the importance of a particular amino acid residue in an enzyme for catalysis.

For example, chymotrypsin is a digestive enzyme secreted by the pancreas that can be used for the digestion of protein. The -OH group of a seriene residue on this enzyme gets modified by di-isopropylphosphpfloridate (DIPF), an irreversible inhibitor, which blocks the enzyme activity permanently. Hence by comparing the turnover rate before and after inhibition one can predict the importance of a particular residue in an enzyme for catalysis.

Reactive Substrate Analogs

Reactive substrate analogs are similar to that of the actual substrate. They also bind covalently with enzymes leading to irreversible inhibition. Due to their higher reactivity and specificity, they initially bind with the active site and modify it irreversibly. They can even react with the substrate. They can be used to test the efficiency of enzymes if a substrate analog binds to them.

Suicide Inhibitors

They are the inert sort of molecules that gets activated only after binding to the active site of the enzyme. Suicide inhibitors usually are the substrate analogs that bind with the enzyme active site and blocks it permanently by making covalent bonds during normal catalysis reaction. For example, 5- fluorouracil acts as a suicide inhibitor of thymidylate synthase during the synthesis of thymine from uridine.

Reversible Inhibition

In reversible inhibition, enzymes form a loose, non-covalent bond with inhibitors. This results in blocking the enzyme function temporarily. When such inhibitors are removed, enzymes can function normally. In simple words, they do not have a permanent impact on enzyme action. Also, they do not alter the enzyme structure. Reversible inhibition is classified into four subtypes:

1. Competitive Inhibition
2. Non-competitive inhibition
3. Uncompetitive Inhibition
4. Mixed Inhibition

Competitive Inhibition

Competitive inhibitors, as their name suggests, compete with the substrate for the active site of the enzyme. They have a similar shape to substrates. Once they approach the enzyme, the enzyme binds them considering them as a substrate. Hence, no enzyme-substrate complex will form. It is a reversible reaction. So the active site can be restored by increasing the concentration of substrate.

Non-competitive inhibition

Non-competitive inhibitors do not compete for the active site. They bind to an enzyme at a region other than the active site called an allosteric site. They alter the structure of the enzyme in such a way that even a genuine substrate wouldn’t be able to bind with the enzyme due to differences in structure.

This is a specific type of enzyme inhibition. It differs from the other types of inhibition. These inhibitors can bind to the enzymes as well with the enzyme-substrate complex. So the inhibitors and the substrate can bind enzymes at the same time. This reduces the rate of a chemical reaction without affecting the binding affinity of the substrate.

Uncompetitive Inhibition

Uncompetitive inhibitors do not bind to the free enzyme but always attack the enzyme-substrate complex. This type of inhibition is known as anti-competitive inhibition. The inhibitors block the substrate binding by changing the shape of the active site. This type of inhibition occurs when there are two or more substrates or products.

Uncompetitive inhibitors also bind to the allosteric site of the enzyme-substrate complex. This binding leads to a decrease in the Vmax but Km will not be changed as the substrate was bound already.

Here

• Km is the half-saturation constant and it refers to the substate concentration at which half of the active sites are saturated with substrate.
• V_max is the maximum rate of enzyme catalyzed reaction. It refers to the reaction rate when all the enzymes are saturated with substrate. Km and Vmax are constant at a given temperature and pressure.

Mixed Inhibition

This type of inhibitor binds to the enzyme irrespective of its attachment to the substrate. It is named so because it involves both types of inhibition i.e. competitive and uncompetitive. It depends upon whether they are binding to an enzyme with the substrate or without the substrate. Similarly, they also bind the allosteric site rather than the enzyme’s active site.

In some cases, mixed inhibition may lead to an increase in Km whereas, in some cases, it may decrease the Km.

Similarities between uncompetitive inhibition and non-competitive inhibition

• Increasing substrate concentration has no effect on both types of inhibition.
• They do not compete with the substrate for binding.
• Both types do not allow product formation.
• They do not interfere with substrate binding.
• In both cases, V_max will be lowered.
• Both affect enzyme efficacy.
• Both these types of inhibitors bind to the allosteric site rather than the active site.
• Uncompetitive inhibition and non-competitive inhibition are types of reversible inhibition.

Differences between uncompetitive inhibition and non-competitive inhibition

Uncompetitive inhibitors bind only to the enzyme-substrate complex whereas, noncompetitive inhibitors can bind equally with enzyme as well as enzyme-substrate complex.

Purpose of Inhibition

• Enzyme inhibitors reduce the rate or sometimes even stop the enzyme-catalyzed reactions.
• They can act as a drug and stop a particular enzymatic reaction through a negative feedback mechanism.
• Viral infections can also be cured using enzyme inhibitors as these inhibitor molecules stop the formation of protein coats.
• Inhibitors can also act as herbicides inhibiting the enzymes needed for photosynthesis as well as the production of carotenoids and lipids.
• They also have the ability to stop enzymes required for the growth of cancer cells.

Concepts Berg

What is meant by enzyme inhibition?

Inhibitors are the compounds or agents that block the enzyme either temporarily or permanently by making interactions with certain groups in the enzymes. This process is known as enzyme inhibition.

What are the different types of inhibitors?

The two major types of inhibitors are:

• Irreversible Inhibitors
• Reversible inhibitors

How do you know if an inhibitor is competitive or non-competitive?

Competitive and noncompetitive inhibitors can be differentiated depending upon the site to the enzyme they are attached with. If an inhibitor aims for an active site only, it is a competitive inhibitor whereas, a non-competitive inhibitor, however, binds to a site other than the active site.

Is non-competitive inhibition the same as mixed inhibition?

Mixed Inhibition involves both competitive and non-competitive inhibition. The mixed inhibitor has a different affinity for free enzyme and enzyme-substrate complex. Non-competitive inhibition, however, doesn’t depend on substrate association.

How can you distinguish between competitive and non-competitive inhibition in an isolated system?

A competitive inhibitor resembles the shape of the enzyme and competes for the active site. It binds to free enzymes only. Non-competitive inhibitors, however, can bind to free as well as bound enzymes. Similarly, it does not compete with the substrate.

Can inhibition be useful?

Inhibition is used to:

• Monitors the enzyme activity.
• Limit the enzyme activity back to normal values.
• Help to stop unwanted growth.
• Cure viral infections