Enzyme Inhibitors

Enzymes are proteins that help speed up metabolic activities or chemical reactions in the body. The human body undergoes many chemical reactions every day that are fastened by enzymes. These enzymes are obtained from proteins formed by linking amino acids in different combinations in the form of long-chain compounds.

All living organisms have enzymes, and they are produced in the body.

Enzymes are also called catalysts. Catalysts are those chemical substances that increase the reaction’s rate but do not take part in the reaction. Amino acids combine to form proteins, and proteins act as enzymes or catalysts and are categorised as biomolecules. Amino acids can be mainly of two types – essential and nonessential. The essential ones are not produced in the body and have to be taken in our diet as a supplement, but the nonessential amino acids are produced in the body.

When combined in various forms, 20 amino acids form many enzymes whose sequence is specific for a particular protein.

Characteristics of enzymes

Enzymes have characteristic features like

1. Enzymes have a catalytic nature
2. A small amount of enzyme added to a reaction also can speed up the process
3. They speed up the process of reaction and do not undergo any chemical change during the complete reaction
4. Enzymes are affected by temperature, which means they can degenerate at high or low temperatures and work efficiently only at optimum temperatures
5. Enzymes are also pH sensitive, which means highly acidic or highly basic nature is not suitable for the enzymes to survive and perform efficiently
6. Inhibitors are another factor that affects enzyme activity
7. Enzymes work specifically for a chemical reaction
8. Enzymes cannot change the free energy, which does not change whether the reaction is absorbing energy or releasing overall.

Enzymes can usually be categorised in two ways: positive and negative catalysts. Reactions can be sometimes useful or harmful, or not useful. So, according to the requirement, we use various catalysts.

• If the reaction is useful and slow, then a positive catalyst is used, increasing the rate of reaction.
• If the reaction is not useful and is either slow or fast, then a negative catalyst is used, reducing the reaction rate or stopping the reaction.

Positive catalysts are otherwise called promoters, and negative catalysts are otherwise called inhibitors. 

Promoters and inhibitors: Promoters are the chemical substances that help increase enzymes’ activity.

Inhibitors are chemical substances that help in decreasing the activity of enzymes.

Enzymes usually perform the task of reducing the activation energy of reactants to cross the energy barrier and convert into products. Activation energy is the energy required to perform a reaction. Enzymes bind with the reactant particles and help in the chemical bond breaking of existing bonds into chemical bonds. Reactants are the reacting species called substrate molecules, and enzymes are the catalysing agents.

Reactants have certain energy with which the reaction is initiated, but not all reactant molecules have such energy to cross the energy barrier and convert to products. In such a case, products formed would be low without any external aid. Increasing the temperature of the reaction by adding a catalyst can help in increasing the yield. If the reaction is exothermic, or the reactants get denatured on exposure to high temperatures, then using a catalyst can help in getting the best results.

Substrate-enzyme complex: Enzymes have active sites of various shapes. The enzyme binds with the substrate at the active sites to catalyse a reaction specific to one or more substrate molecules. The part where the substrate binds the enzyme is the active site.

A substrate enzyme complex is like a lock and key model, as a particular key can only open that lock the same way a specific enzyme can only bind with a specific substrate and perform the action. 

Enzyme inhibitors

Enzyme inhibitors reduce the reaction rate by influencing the binding of substrate to the enzyme. Inhibitors can be organic or inorganic, like drugs, antibiotics, toxins, antimetabolites, etc.

• Inhibitors can be usually reversible, irreversible, and allosteric
• Reversible inhibitors can be competitive, non-competitive, and uncompetitive
• Irreversible inhibitors are suicide inhibitors

Irreversible inhibitors inactivate the enzyme by covalent bonds binding to a particular group at the active site. This inhibitor enzyme bonding is so strong that it cannot be reversed even by adding excess substrate. The peptidases trypsin and chymotrypsin contain serine groups at the active site and are inhibited by DIFP (Diisopropyl fluorophosphate).

Reversible inhibitors bind to the active site by non-covalent bonds that can be reversed easily, including competitive and non-competitive.

Competitive inhibitors: Competitive inhibitors resemble the substrate particles and compete with the substrate to bind at the active site of an enzyme. The inhibitor does not act on the enzyme but prevents the approach of the substrate to the enzyme. 

Non-competitive inhibitors

Non-competitive inhibitors bind at the allosteric sites of an enzyme. Allosteric sites are locations other than the active sites of an enzyme. All allosteric enzymes do not act as non-competitive inhibitors.

They might act as competitive as well as non-competitive inhibitors.

Competitive inhibition: In competitive inhibition, a molecule involves other than the normal substrate molecule binds to the active site of an enzyme. The structure and nature of the inhibitor are similar to the substrate, allowing the inhibitor to bind the active site easily. Thus, it blocks the active site and prevents the substrate from binding. 

Non-competitive inhibition: In the case of non-competitive inhibition, a molecule involves a substrate that binds to a site other than the active site of an enzyme. So, due to the binding of inhibitors to the allosteric site, conformational changes arise to the active site, thus not allowing the substrate to bind. Since the inhibitor is not directly competing for the active site, this is said to be non-competitive inhibition. 

Conclusion

An enzyme inhibitor can act in several ways. The inhibitor can bind and prevent a substrate from reaching the enzyme’s active site. It can also stop the enzyme from catalysing its reaction.

The action can be both reversible and irreversible. Inhibitors are playing a huge role in the research on drugs and diseases. They are increasingly used to create new medicines or enhance existing ones, leading to new hope in treating diseases like HIV, cancer, and neurological disorders.