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Enzymes may be defined as biological proteins capable of catalysing various biochemical reactions taking place in the human body. Enzymes play a significant role in regular body functions such as digestion, respiration, body growth, and so on. Almost all body tissues contain enzymes.

The enzyme is basically a linear chain of amino acids, which together form a 3D structure. The number of amino acid residues present in an enzyme ranges from an average of 62 to 2500. The enzyme contains an active site and functional groups to which the reactant binds. The shape of the active site usually depends on the shape of the respective substrate.

Lock and key hypothesis

To understand how an enzyme catalyses a particular reaction, it is essential to understand the concept of the lock and key hypothesis. The lock and key hypothesis states that the interaction between an enzyme and substrate is specific.

The 3D enzyme consists of an active site to which the substrate binds. The shape of the active site is specific and matches the molecular shape of the substrate. The active site is, in turn, composed of the binding site and catalytic site.

Mechanism of enzyme catalysis

The enzyme catalysis of a biochemical reaction begins when the substrate binds to the enzyme's active site.

  • The substrate molecule undergoes a few collisions and attaches itself to the active site strongly. The bond between the substrate and enzyme may be reversible or irreversible based on the type of bond formed.
  • An enzyme-substrate complex is formed. The molecules present in the catalytic site of the active site catalyse the substrate. /li>
  • Once the reaction completes, the resultant products are released from the enzyme. However, the enzyme remains unchanged and can be reused for other biochemical reactions.

The reaction is indicated as follows:
[Substrate molecule] + [Enzyme] → [Enzyme Substrate complex] → [Enzyme] + [Products]
[S] + [E] → [ES] → [E] + [P]
The enzyme-substrate complex formation is temporary. Thus, the enzyme remains unchanged by the end of catalysis.

Classification of enzymes

Based on the type of reaction catalysed, the enzymes are classified into the following six types.

1. Oxidoreductases
An oxidoreductase enzyme catalyses oxidation and reduction reactions. The reaction in which oxidation and reduction take place simultaneously is called the Redox reaction. In a redox reaction, one molecule undergoes oxidation by losing electrons, whereas another molecule undergoes reduction by gaining electrons.
Example: Pyruvate dehydrogenase is an oxidoreductase that catalyses the oxidation of pyruvate molecules into acetyl-CoA.

2. Transferases
The function of the transferase enzyme is to catalyse the transportation reactions taking place in the body. These enzymes usually catalyse the transfer of a functional group from the donor molecule to the acceptor molecule.
Example: Transaminase is a transferase responsible for transferring amino acids from a donor molecule to another.

3. Lyases
Lyases are enzymes that break or make double bonds. While breaking a double bond, the lyase adds molecules such as water or ammonia to the molecule. While creating a double bond, the lyase eliminates small molecules such as water, ammonia, or carbon dioxide from the molecule.
Lyases also catalyse reactions involving the addition of a new ring structure to the substrate.

Example: Aldolase catalyses the breakdown of fructose-1, 6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This splitting reaction of fructose-1, 6-bisphosphate is a part of glycolysis.
Dehydratases and decarboxylases also fall under the category of lyases.

4. Hydrolases
Hydrolases are enzymes catalysing the hydrolysis reaction of a substrate. Hydrolysis is a reaction where a molecule undergoes cleavage in the presence of OH- and H+ ions.
Example: Pepsin is an enzyme hydrolysing the peptide bonds present in protein molecules.

5. Ligases
The primary function of ligases is to speed up the ligation process. Ligation is a reaction where two molecules are ligated or joined together. New bonds such as C--N, C--O, C--C, or C--S may be formed in this reaction. The breakdown of ATP also takes place, thereby giving ADP and the free phosphate ion. Thus, the phosphate ion is utilised in the ligation process.
Example: The best example of ligase enzymes is the DNA Ligase. It is the enzyme that catalyses the ligation of two DNA fragments by the formation of a phosphodiester bond.

6. Isomerases
Isomerases are another class of enzymes that catalyse isomerisation reactions. Isomerisation is a reaction where the isomer of the substrate is formed, thereby changing the shape of the molecule. Isomers of a compound have the same molecular formula but differ in structure or spatial arrangement.
Example: Phosphoglucomutase catalyses the isomerisation of glucose-1-phosphate into glucose-6-phosphate. In this reaction, the phosphate group is transferred from one position to another within the same compound.

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