What are enzymes?

Right from waking up in the morning to going to bed at night, we use enzymes in our everyday life. They are versatile in nature and functions, and thus have a wide range of applications. Enzymes help us in preparation of foods such as bread, digestion of food, biosynthesis of various essential molecules in the body, etc.

They are also used in various industries such as food, textile, fuel, cleaning products, animal nutrition, and so on.

So, what are these enzymes?

Enzymes are proteins which catalyse chemical reactions in cells.

Side note
Catalysts are substances which accelerate the rate of chemical reactions but do not change the equilibrium.

As enzymes are synthesised by living cells, they are known as biocatalysts. 

Enzymes basically help in modifying the reactants to produce products. 

Now, before we jump to understand how enzymes work, let us first go through certain terms which will help us understand enzymes better.


These are the reactant molecules on which enzymes act. E.g. proteins or lipids.

Enzymes are specific to the substrates that they act on, i.e. enzymes which breakdown proteins will not act on lipids. 

This specificity can be towards a family of substrates.

E.g. amylase enzyme is present in the saliva and carries out hydrolysis (breakdown) of starch.


As enzymes have such a powerful role in breaking down the proteins, it increases the risk of ill effects like digestion of our cells in the absence of substrate.

E.g. stomach ulcers 

To avoid such ill effects, enzymes must work only in the presence of substrate. Such regulatory checks on enzymes are provided by various modes.

‘Holo’ stands for complete. Holoenzymes are complete and active enzymes.

These are formed by attachment of coenzyme or cofactor to the incomplete/apoenzyme.

Coenzymes are organic attachments essential for the functioning of enzymes, whereas cofactors are inorganic molecules like metal ions.

E.g. glucose 6-phosphate is inactive unless its cofactor magnesium binds. 

Active site

It is the location/cleft in the enzyme at which substrate binds and is converted to the products.

It contains functional groups which are responsible for the reaction.

An active site contains a substrate-binding site and catalytic site. In some enzymes, they can be the same.

The enzyme action

Chemical reactions occur when the reacting molecule gains the required minimum amount of energy. This is called the energy of activation.

Enzymes lower this energy of activation thereby accelerating the reaction.

The enzyme forms an enzyme-substrate complex.

Binding of the enzyme to the substrate causes a conformational change in the enzyme.

This allows interactions between the substrate molecules and the enzyme functional groups.

The activated substrates and the enzyme form a transition-state complex, (unstable high energy complex with a strained electronic configuration).

The transition-state complex decomposes to form products, which dissociate from the enzyme.

The enzyme returns to its original form.

The free enzyme then binds another set of substrates and repeats the process.

Many theories and hypotheses have been put forth to explain the mechanism of enzyme actions. Each of them tries to explain a particular aspect of the action.

The most important ones are:

  • Lock and key model
  • Induced-fit model

Lock and key model

Emil Fischer model of lock and key states that the three-dimensional structure of the active site of the enzyme is complementary to the substrate.

The active site by itself provides a rigid, pre-shaped template fitting with the size and shape of the substrate molecule.

Induced-fit model

Daniel Koshland postulated the induced-fit model of enzyme action. 

It states that the shapes and the active sites of enzymes are complementary to that of the substrate, only after the substrate is bound to the enzyme.

Proper alignment between the enzyme and the substrate is essential for reaction to occur. Thus, the theory is known as the induced-fit model.

Hand glove analogy is used to understand the induced-fit model.

At first, the glove is in a partially folded position, but as the hand enters it, the glove further opens up to accommodate the structure of the hand in it.

Similarly, conformational changes occur prior to the substrate completely fitting inside the enzyme.

The induced-fit model explains various matters related to enzyme actions such as inactivation of the enzyme on denaturation, saturation kinetics, allosteric modulation, and competitive inhibition.

Lock and key model of enzyme action fail in explaining these points and thus the induced-fit model is more acceptable.

These are so many more interesting facts about enzymes like enzyme naming & classification, enzyme inhibition, how enzymes are used in the detection of diseases, etc. But let us discuss them next time. Visit Akshara website for the next article on enzymes.

-Madhura Naik

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