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Enzymes – Definition, Structure, Classification, Examples

  • Last Updated : 22 Dec, 2021

In living organisms, a biomolecule is a chemical substance. Chemicals mostly made up of carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus fall within this category. Biomolecules are the fundamental building elements of life and play critical roles in living creatures. Amino acids, lipids, carbohydrates, proteins, polysaccharides, and nucleic acids are examples of biomolecules.


Enzymes are nitrogenous organic molecules produced by living organisms such as plants and animals. A long chain of one or more amino acids is connected together using amide or peptide bonds to make them.

They are high-molecular-mass proteins that catalyse natural processes in the bodies of animals and plants. They are also known as polypeptides. Enzymes are categorised into distinct categories based on their structure and properties. Enzymes have a specific method of action (Lock-and-Key mechanism and Enzyme Fit Hypothesis).

Structure of Enzyme

  • Enzymes are proteins that are made up of several polypeptide chains, also known as amino acids, that have been folded and coiled numerous times.
  • They have linear chains of amino acids in three-dimensional structures.
  • The enzyme’s catalytic activity is determined by the amino acid sequence. Only a small portion of an enzyme’s structure participates in catalysis and is located around the binding sites.
  • They have separate sites; the active site of an enzyme is made up of the catalytic and binding sites.

Classification of Enzymes

The International Union of Biochemists divides enzymes into six types based on the sort of reaction they catalyse (I U B). Oxidoreductases, transferases, hydrolases, lyases, ligases, and isomerases are the six types of enzymes. The following are their functions:

  • Oxidoreductases: Oxidoreductase is an enzyme that catalyses the oxidation and reduction reactions in which electrons are transferred from one form of a molecule (electron donor) to the other (electron acceptor). Consider the enzyme pyruvate dehydrogenase. Cofactors for oxidoreductase enzymes are commonly NADP+ or NAD+.


  • Transferases: These catalyse the transfer of a chemical group (functional group) from one compound (referred to as the donor) to another compound (referred to as the recipient) (called the acceptor). A transaminase, for example, is an enzyme that transfers an amino group from one molecule to another.


  • Hydrolases: They are hydrolytic enzymes that catalyse the hydrolysis reaction by cleaving the bond and hydrolyzing it with water molecules, i.e. they catalyse the hydrolysis of a bond. Pepsin, for example, breaks down peptide connections in proteins.


  • Lyases: They are enzymes that catalyse bodywork by creating a double bond or adding a group to a double bond without involving hydrolysis or oxidation. Aldolase (a glycolysis enzyme) catalyses the conversion of fructose-1, 6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, for example.


  • Isomerases: They’re an enzyme family that converts a chemical from one isomer to another. Isomerases aid intramolecular rearrangements by breaking as well as forming bonds. In glycogenolysis, for example, phosphoglucomutase catalyses the conversion of glucose-1-phosphate to glucose-6-phosphate (the phosphate group is moved from one position to another in the same substance). For energy to be released fast, glycogen is converted to glucose.


  • Ligases: Ligase is a catalytic enzyme that catalyses the ligation or connecting of two big molecules by establishing a new chemical link between them. DNA ligase, for example, catalyses the formation of a phosphodiester bond between two DNA fragments.


Enzyme Cofactor

Cofactors are chemical substances that are not proteins and are found in enzymes. A cofactor affects the action of an enzyme by acting as a catalyst. Apoenzymes are enzymes that do not require a cofactor. The holoenzyme is made up of an enzyme and its cofactor.

Three Kinds of Cofactors Present in Enzymes:

  1. Prosthetic groups: These are cofactors that are always covalently or permanently linked to an enzyme. Many enzymes have a FAD (Flavin Adenine Dinucleotide) prosthetic group.
  2. Coenzyme: A coenzyme is a non-protein organic molecule that only interacts to an enzyme during catalysis. It is separated from the enzyme at all other times. NAD+ is a widely used coenzyme.
  3. Metal ions: Certain enzymes require a metal ion in the active site to establish coordinate bonds during catalysis. A number of enzymes use the metal ion cofactor Zn2+.

Mechanism of Enzyme Action

The active site of an enzyme draws substrates and catalyses the chemical process that produces products. Allows the products to disassociate or detach from the enzyme’s surface after product production. The enzyme-substrate complex is the combination of an enzyme and its substrates.

The reaction requires the collision of any two molecules, as well as the correct orientation and a sufficient quantity of energy. This energy must be transferred between these molecules in order to overcome the reaction’s Activation Energy barrier. Without any catalysts, the substrate and enzyme produce an intermediate reaction with low activation energy.

Two of the most well-known mechanisms of enzyme function are the Induced Fit Hypothesis and the Lock and Key Mechanism.

  • Induced Fit Hypothesis: In 1958, Daniel Koshland proposed the induced fit model. One of the most common models for characterising the enzyme-substrate interaction is this one. The active site of the enzyme, according to the idea, does not have a firm shape. As a result, the substrate does not completely fit into the enzyme’s active site. As a result, when the enzyme binds to the substrate, the active site changes form, becoming complementary to the substrate’s shape. Because of the flexibility of the protein, this conformational shift is possible.
  • Lock and Key Mechanism: Emil Fischer proposed the lock and key concept in 1894, and it is now known as Fisher’s theory, which describes the enzyme-substrate interaction. Emil Fischer proposed the lock and key model in 1894. As a result, it’s sometimes referred to as Fisher’s theory. The enzyme-substrate interaction is described by the second model.
  1. The enzyme’s active site functions as the ‘lock,’ while its substrate serves as the ‘key,’ according to the lock and key concept. As a result, the form of the enzyme’s active site complements the shape of the substrate. By generating an useless intermediate product, the enzyme-substrate complex, the active site of the enzyme can hold the substrate closer to the enzyme.

Enzymes as Biochemical Catalysts

Biochemical catalysts are also known as enzymes, and the phenomenon is known as biochemical catalysis. Enzymes are widely used to enhance or expedite the efficient preparation and effect of beverages, chocolates, curd, predigested infant food, washing powders, and other products.

Examples of Enzyme Catalysis

  • Cane sugar inversion: Cane sugar is converted to glucose and fructose by the enzyme invertase.

C12H22O11(aq)+H2O(1) → C6H12O6(aq) + C6H12O6(aq)

  • Conversion of milk to curd: The enzyme lactase, which is released by lactobacilli, is responsible for turning milk into curd.
  • Conversion of glucose into ethyl alcohol: Glucose is converted to ethyl alcohol and carbon dioxide by the zymase enzyme.

C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(aq)

  • Conversion of starch into maltose: Starch is converted to maltose by the diastase enzyme.

Factors Affecting Enzyme Catalysis

  1. Concentration of Substrate: In the presence of an enzyme, the rate of a chemical reaction increases as the substrate concentration rises until a limiting rate is achieved, after which additional increases in the substrate concentration have no effect on the reaction. The enzyme molecules are saturated with the substrate at this point. The extra substrate molecules are unable to react until the substrate that has already been bound to the enzymes has reacted and been released.
  2. Concentration of Enzyme: When the enzyme concentration is much lower than the substrate concentration, the rate of an enzyme-catalyzed reaction is proportional to the enzyme concentration. This is true for any catalyst; when the catalyst concentration rises, the reaction rate rises as well.
  3. Temperature: For most chemical reactions, a temperature increase of 10°C about doubles the reaction rate, according to a well-known rule of thumb. This rule applies to all enzymatic reactions to some extent. Even a slight increase in temperature, after a certain threshold, induces denaturation of the protein structure and disruption of the active site, resulting in a drop in reaction rate.
  4. Hydrogen Ion Concentration (pH): Most enzymes are proteins, and they are sensitive to variations in pH or hydrogen ion concentration. The degree of ionisation of an enzyme’s acidic and basic side groups, as well as the substrate components, is affected by changes in pH. The catalytic activity of an enzyme is altered when one of these charges is neutralised. Over a narrow pH range, an enzyme’s activity is at its peak. The enzyme’s optimal pH is determined by the median value of this pH range.
  5. Inhibition of Enzymes: Enzymes must occasionally be slowed to aid and ensure that our bodies’ systems operate appropriately and efficiently. For example, if an enzyme produces too much of a product, it must be possible to reduce or stop production. Inhibitors are required in such situations.

Enzymes Inhibition: A molecule blocks the active site, causing the substrate to compete with the inhibitor for binding to the enzyme. Non-competitive inhibitors bind to an enzyme in a location other than the active site, reducing its effectiveness. Inhibitors that bind to the enzyme-substrate complex are known as noncompetitive inhibitors. The products exit the active site with less ease, slowing the reaction. Irreversible inhibitors bind to an enzyme and render it inactive for the rest of its life.

Drug Action of Enzymes

Drugs that act on the active sites of enzymes can control, i.e. inhibit or stimulate, enzyme function. The majority of medications that act on enzymes are inhibitors, and the majority of them are competitive inhibitors, meaning they compete with the enzyme’s substrate for binding. The bulk of the original (first generation) kinase inhibitors, for example, bind to the enzyme’s ATP pocket.

Examples of Enzymes

  1. Lipases are a group of enzymes that aid in the digestion of lipids in the intestine.
  2. Amylase is a protein that aids in the conversion of carbohydrates to sugars. Saliva contains this enzyme.
  3. Maltase is a sugar that breaks down maltose into glucose and is found in saliva. Maltose can be found in a variety of foods, including potatoes, pasta, and beer.
  4. Trypsin is an enzyme that breaks down proteins into amino acids and is located in the small intestine.
  5. Lactase is an enzyme present in the small intestine that aids in the breakdown of lactose, a sugar found in milk, into glucose and galactose.
  6. Helicase is a DNA unravelling enzyme.
  7. DNA Polymerase is a type of enzyme that makes DNA from deoxyribonucleotides.

Sample Questions

Question 1: What is the function of all enzymes?


The basic role of all enzymes is to assist support life by speeding up the rate of a chemical process. Without enzymes, all biochemical reactions in our bodies would be extremely slow, making living difficult.

Question 2: How do enzymes work?


Enzymes work by attaching to reactant molecules and keeping them in a position that allows chemical bond-breaking and bond-forming to occur more quickly. Enzymes perform the function of biological catalysts. They lower the Activation Energy of a reaction coordinate, allowing the reaction to proceed at a faster rate.

Question 3: What are factors affecting enzyme catalysis?


The factors affecting enzyme catalysis are:

  1. Concentration of Substrate
  2. Concentration of Enzyme
  3. Temperature
  4. Hydrogen Ion Concentration (pH)
  5. Inhibition of Enzymes

Question 4: What is enzyme cofactor?


Cofactors are non-protein chemical compounds that are found in enzymes. By serving as a catalyst, a cofactor influences the action of an enzyme. Apoenzymes are enzymes that don’t need a cofactor to function. An enzyme plus its cofactor make constitute a holoenzyme.

Question 5: What is Lock and Key Mechanism?


The lock and key notion was proposed by Emil Fischer in 1894, and it is now known as Fisher’s theory, which explains the enzyme-substrate relationship. In 1894, Emil Fischer proposed the lock and key paradigm. As a result, Fisher’s theory is sometimes referred to. The second model describes the enzyme-substrate interaction.

According to the lock and key idea, the enzyme’s active site serves as the ‘lock,’ while its substrate serves as the ‘key.’ As a result, the active site of the enzyme matches the shape of the substrate. The enzyme’s active site can retain the substrate closer to the enzyme by forming an ineffective intermediate product, the enzyme-substrate complex.

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