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In this article, we will discuss the mode of action of an enzyme in detail. But before proceeding to discuss the mode of action of enzymes, first, let us see what are enzymes.
What are Enzymes?
Enzymes refer to the biological catalysts that speed up (catalyze) the rate of chemical reactions that occur inside our bodies. Enzymes are considered biological because they function inside the living systems. They are considered catalysts because they speed up chemical reactions. It should be kept in mind that the enzymes remain unchanged at the end of the reaction. It means that they do not get used up by the reaction which means that they can be reused.
Classification of Enzymes
Enzymes can be either intracellular or extracellular.
- Intracellular enzymes: These enzymes speed up the reactions that occur inside the cells. The example of these enzymes include catalase
- The byproduct of several metabolic reactions is hydrogen peroxide.
- Since it is harmful to the cells, hence the body must convert it into useful elements.
- The catalase enzyme transforms hydrogen peroxide into water and oxygen, thus preventing potential damage to the cells and tissues.
- Extracellular enzymes: These enzymes speed up the reactions that occur outside the cells. Examples of these enzymes include amylase.
- Extracellular enzymes usually carry out the process of digestion.
- This is because the digested macromolecules are extremely large to enter the cell.
- Amylase enzyme takes part in the digestion of carbohydrates as it hydrolyses the starch into simple sugar.
- Salivary glands and pancreas secrete this enzyme to digest the starch in the mouth and small intestine.
Enzymes are Globular Proteins
All enzymes are globular proteins and contain regions known as active sites. The active site of an enzyme has a certain shape and enables the substrate to bind. Other enzymes may contain regulatory regions where an inhibitor can bind which is known as the allosteric site.
Enzymes control metabolic pathways in a biochemical cascade of reactions. Enzymes catalyze every metabolic reaction within living organisms. Hence, enzymes are critical for the existence of life.
In the next section of the article, we will discuss the mode of action of the enzyme.
Mode of Enzyme Action
- Enzymes contain an active site where certain substrates bind creating an enzyme-substrate complex.
- The specific shape of an active site of an enzyme allows it to fit a specific substrate.
- Extreme heat and pH can alter the shape of an active site and thus prevent the binding of substrate. This is referred to as denaturation.
- The collision of substrates with the active site of enzymes should occur at the right orientation and speed for the reaction to take place.
- The complementary nature between the shape of the active site on the enzyme and its substrates results in the specificity of an enzyme.
- The complex tertiary structure of the protein that makes up an enzyme determines the shape of the active site as well as the specificity of the enzyme.
- Chains of amino acids create the proteins. Peptide bonds hold together these chains of amino acids.
- The shape of an enzyme is determined by the order of amino acids.
- If the order is changed, the resulting three-dimensional shape alters
- When an enzyme and its substrate join with each other, an enzyme-substrate complex is created
- The formation of the enzyme-substrate complex is temporary, before the catalyzation of reaction by enzymes and release of products.
- The reactions of enzymes can be either catabolic or anabolic
- In catabolic reactions, the complex molecules are broken down into simpler products. This occurs when a substrate is drawn into the active site and then it is divided into two or more distinct molecules. Cellular respiration and hydrolysis reactions are examples of catabolic reactions
- In anabolic reactions, two or more complex molecules are built from simpler ones when two or more substrates are drawn into the active site, to create bonds between them and release one product. Protein synthesis and photosynthesis are examples of anabolic reactions.
Lowering the Activation Energy of a Reaction
All chemical reactions are connected with energy changes. There must be sufficient activation energy for the reaction to move forward. Enzymes work on the mechanism of minimizing the activation energy of the reaction. Activation energy refers to the minimum amount of energy required for a reaction to occur. In the absence of enzymes, the reactions that occur inside our body will not be possible at normal body temperature.
Enzymes catalyze chemical reactions because they affect the stability of bonds in the reactants. Substrate becomes more reactive due to the destabilization of bonds.
In the next section, we will discuss how enzymes work.
Models Explaining the Action of Enzymes
To explain how enzymes work, scientists have proposed the following two ideas:
- The lock and key model
- The induced-fit model
The above two models are best-accepted theories and are based on the available evidence.
Now, we will explain these two models one by one.
The Lock and Key Hypothesis
According to this hypothesis:
- Enzymes are globular proteins. This implies that the complex tertiary structure of the protein that makes up the enzyme determines their shape as well as the shape of their active site.
- The first model of enzyme activity was illustrated by Emil Fischer in 1890.
- According to him, both enzymes and substrates are rigid structures that are locked into each other as accurately as the key goes into the lock. This is referred to as the lock and key hypothesis.
- Later, this idea was altered and adapted to our present understanding of enzyme activity.
Now, let us see what is induced fit hypothesis.
The Induced-fit Hypothesis
- The modified model describing the activity of an enzyme is referred to as the induced fit hypothesis.
- This model resembles the lock and key hypothesis closely.
- According to this model, the enzyme and its substrate interact with each other.
- When the substrate molecule enters the enzyme, the enzyme and its active site can alter the shape slightly.
- These alterations in shape are referred to as conformational changes.
- It ensures to give an ideal binding arrangement between the enzyme and substrate
- It also maximizes an enzyme’s capability to speed up the reaction.
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