Enzymes: Structure, Classification and Function

Enzymes are complex protein molecules that are responsible for biochemical reactions involved in respiration, digestion, circulation, and excretion. They act as catalysts as they help in increasing or decreasing rate of the chemical reactions occurring in a cell. Different types of enzymes are present in a cell, each specific to a particular reaction. 

• Excess or deficiency of enzymes are associated with several health problems in human. 
• The binding of enzymes takes place at the active site of a specific molecule by recognising the correct substrate.
• Once after the formation of enzyme-substrate complex, shape of enzyme changes. 

Enzyme is an important topic covered under CBSE Class 12 Chemistry Unit 14 Biomolecules. Units X-XVI carry a combined weightage of 28 marks in CBSE Class 12 Board Exams. 

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What are Enzymes?

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Enzymes are called biological catalysts that perform all the biochemical reactions as well as metabolic processes that are essential to sustain life. The most prominent characteristics of these biomolecules are their catalytic proficiency and specificity. These catalysts selectively bind to a particular molecule called substrate and convert it into a distinct molecule called the product.

• Due to the absence of enzymes, most reactions in the biological system will not occur at perceptible rates.
• The complex food molecules are broken down into smaller components by the action of Digestive enzymes.
• These catalysts help in DNA replication, cell regulation, and hormone production.
• They are widely used in the food industry, pharmaceutical industry, textile industry, biochemical industry, etc.
• Examples of enzymes include amylases, proteases, lipases, peptidases, etc. 

Structure of Enzymes

The structure of enzymes comprises a group of linear amino acid chains. It is a three-dimensional structure that connects with the substrate binding site or the active site to initiate a reaction or a process. These biocatalysts contain about 100 to 1000 amino acids in a single chain. The active site constitutes the binding and catalytic site. Ribozymes are enzymes of ribonucleic acids, that are present to perform various RNA functions.

Structure of Enzymes

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Classification of Enzymes

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The International Union of Biochemists (IUB) has classified enzymes into six major classes based on their functional specificity. They are as follows:

1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases

The table below highlights the enzyme class, the type of reaction catalyzed, and some examples:

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Co-Factors and Coenzymes

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Co-factors are non-protein components found in enzymes that affect the catalytic activity of enzymes. An enzyme without its cofactor is referred to as an apoenzyme and the enzyme along with the cofactor is a holoenzyme. Cofactors are subdivided into three groups namely:

• Prosthetic groups: The non-protein molecule that is either loosely bonded or tightly bonded with the enzymes.
• Metal Ions: The metal ions accept or donate the electrons to perform catalytic reactions.
• Coenzymes: A type of cofactor that interacts with an enzyme to enhance the catalytic activity. Examples of coenzymes include FAD, heme compounds, and NAD⁺.

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Mechanism of Enzyme Action

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The primary objective of an enzyme is to first form an enzyme-substrate complex. This happens when the substrate binds to the active site of the enzyme. The mechanism of enzyme action comprises three basic stages: 

• The Enzyme (E) first recognizes and binds the Substrate (S) to its active or binding site. 
• Formation of the activated enzyme-substrate complex (ES) takes place. 
• Then finally the enzyme acts upon the specific bonds within the substrate that in turn alters the substrate resulting in the formation of the product (P).

E + S ⇔ ES ⇔ EP ⇔ E + P

Important Hypotheses about Enzymes

Throughout the process, the enzyme ultimately lowers the energy barrier and allows the organisms to carry out reactions faster. Enzyme specificity for the substrate also plays an important role in the mechanism of enzyme action. There are two distinct theories explaining the mechanism of enzymes:

1. Lock-and-Key Hypothesis

The lock and key concept was first introduced by Emil Fisher in 1984. It is commonly known as the Fisher Theory.

• In this hypothesis, the ‘lock’ is referred to as the ‘active site’ of the enzyme, whereas, the ‘key’ is the ‘substrate’.
• Just like a specific key type can fit the keyhole similarly, a specific substrate can bind to the enzyme active site to form a complex.

2. Induced-Fit Hypothesis

Induced-Fit hypothesis was first introduced by Daniel Koshlan in 1958. It is one of the most common methods known.

• In this theory, the enzyme active site does not have a firm shape resulting in the incapability of the substrate to bind to the site.
• When the substrate binds with the enzyme, the active site changes its structure, thus becoming complementary to the shape of the substrate.

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Characteristics of Enzymes

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Following are some of the important characteristics of enzymes that are listed below:

• All enzymes are Proteins, except for ribozymes.
• Enzymes bind to only substrates, which indicates their specificity. 
• They can increase the rate of a biochemical reaction by effectively lowering the activation energy.
• They do not change the equilibrium state of the biochemical reaction on which it is acting, rather they change the rate at which the equilibrium is achieved.
• These biocatalysts remain unaffected at the end of the reaction.
• Denaturation of enzymes occurs after heating. 
• These biocatalysts can be reused after a chemical reaction is complete.

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Factors Affecting Enzyme Catalysis

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The activity of an enzyme is affected by various factors, all of which ultimately contribute to the change in the structure of the biocatalyst.

Temperature and pH

Every enzyme functions the best at a narrow range of pH and temperature or what is called an optimum PH and optimum temperature.

• Any alterations in the pH levels will lead to interruptions in the enzyme-substrate bonding and the three-dimensional structure of the enzyme.
• An increase in temperature results in the denaturation of the catalyst. 

Enzyme Concentration

When the enzyme concentration is less than the substrate, the reaction rate directly depends on the enzyme concentration. An increase in catalyst concentration increases the reaction.

Substrate Concentration

 An Increase in substrate concentration results in higher reaction rates. If the concentration of the substrate is too high, saturation of enzyme occurs which will not increase the rate of reaction. 

Allosteric regulation

Some enzymes have an extra binding site known as an allosteric site or regulatory site. When an inhibitor or an activator molecule binds to the enzymes’s regulatory site, some conformational changes are induced. These changes either enhance or reduce the enzyme activity.     

Enzyme inhibition

The activity of some specific enzymes is seen to be greatly influenced by certain inhibitor molecules. These molecules bind to the enzymes either covalently or non-covalently. In both cases, the catalytic activity of the enzyme is decreased.

Salt Concentration

If the concentration of salt is too high or too low then this could lead to breakage in amino acid interactions. It can also disrupt the secondary and tertiary structures.

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Enzyme Kinetics

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The rate of an enzyme-catalyzed reaction and its product formation is determined by enzyme kinetics. Michaelis-Menten equation is applied which is depicted through the following equation and the graph.

V = velocity of reaction

[S] = substrate concentration

V_max = maximum velocity 

K_m = Michaelis constant

The initial velocity of the reaction increases with the substrate concentration [S] till it reaches the maximum velocity (V_max) and then V becomes independent of S. Michaelis constant i.e., K_m here represents the substrate concentration when the reaction velocity is half of the maximum value. 

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Things to Remember

• Enzymes are biochemical catalysts that work on lock-and-key mechanism.
• Denaturation of biocatalysts occurs at a temperature of 40ºC.
• Eduard Buchner, a German chemist is known as the ‘father of enzymes’.
• An increase in the rate of biochemical reactions is called enzyme catalysis.
• The heme group in hemoglobin is an example of a prosthetic group.