Introduction:

Enzymes are biomolecules that catalyze specific chemical reactions in living organisms. They are essential for the proper functioning of cells and helps in a wide range of biological processes, including metabolism, DNA replication, and cell signaling.

Structure of Enzymes:

• Enzyme consists of one or more polypeptide chains, which are made up of amino acids.
• The primary structure of an enzyme is the linear sequence of amino acids that make up the enzyme.
• The secondary structure of an enzyme refers to the local, regular arrangements of amino acids that are stabilized by hydrogen bonds. The two most common secondary structures are the alpha helix and beta pleated sheet.
• The tertiary structure of an enzyme is the three-dimensional structure formed by the interactions between amino acids and the folding of the polypeptide chain.
• The quaternary structure of an enzyme refers to the interactions between multiple polypeptide chains to form a functional enzyme.

Classification of Enzymes:

• Enzyme can be classified based on the type of reaction they catalyzes, such as metabolic enzymes, digestive enzymes, nucleic acid enzymes, and regulatory enzymes.
• Enzymes can also be classified based on the type of mechanism they use to catalyze reactions, such as enzyme that uses covalent catalysis, acid-base catalysis, and metal ion catalysis.

Classification based on the type of reaction they catalyze:

Enzymes can also be classified based on the International Union of Biochemistry and Molecular Biology (IUBMB) Enzyme Commission (EC) system, which assigns a unique numerical identifier to enzymes based on their substrate and the type of reaction they catalyze.

• Metabolic enzyme: These enzymes catalyze the chemical reactions that occur in cells as part of metabolism. These reactions include those involved in the breakdown of nutrients for energy production and the synthesis of molecules needed for growth and repair. Examples include lactase, which breaks down lactose in milk, and catalase, which breaks down hydrogen peroxide in cells.
• Digestive enzyme: Digestive system produces this enzyme to break down food into smaller molecules that can be absorbed and used by the body. Examples include pepsin, which breaks down proteins in the stomach, and amylase, which breaks down carbohydrates in the small intestine.
• Nucleic acid enzyme: These enzymes help in the synthesis, repair, and regulation of DNA and RNA. Examples include helicases, which unwind double-stranded DNA, and polymerases, which catalyze the synthesis of DNA and RNA.
• Regulatory enzyme: These enzymes control the activity of other enzymes. They can act as enzymes, transport molecules, or structural components. Examples include kinases, which transfer phosphate groups to other molecules to regulate their activity, and transcription factors, which bind to specific regions of DNA to control the expression of genes.

Classification of enzyme based on the type of mechanism they use to catalyze reactions:

Enzymes can be classified based on the type of mechanism they use to catalyze reactions. The three main types of mechanisms are:

• Covalent catalysis: These enzymes catalyze reactions by temporarily forming a covalent bond with the substrate. This can involve the formation of a covalent intermediate, or a transition state intermediate, between the enzyme and substrate. Examples of enzyme that uses covalent catalysis include serine proteases such as trypsin and chymotrypsin, which catalyze the hydrolysis of peptide bonds.
• Acid-base catalysis: These enzymes catalyze reactions by using the ionization state of specific amino acid residues to activate the substrate. These residues act as catalytic acids or bases, which can donate or accept protons to stabilize the transition state of the reaction. Examples of enzyme that uses acid-base catalysis include the enzymes involved in the citric acid cycle, such as isocitrate dehydrogenase, which catalyze the oxidation of isocitrate.
• Metal ion catalysis: These enzymes catalyze reactions by using metal ions, such as zinc or manganese, to activate the substrate. The metal ions can act as Lewis acids, which can coordinate to the electron-deficient atoms of the substrate and stabilize the transition state. Examples of enzyme that uses metal ion catalysis include ribonucleotide reductase, which catalyzes the reduction of ribonucleotides to deoxyribonucleotides, and carbonic anhydrase, which catalyzes the reversible hydration of carbon dioxide.

Classification of enzyme based on the International Union of Biochemistry and Molecular Biology (IUBMB) 

The International Union of Biochemistry and Molecular Biology (IUBMB) Enzyme Commission (EC) system is a widely popular method of classifying enzymes based on the type of reaction they catalyze and the substrate they act on. In this system describes 6 classes –

•  1: Oxidoreductases – enzyme that catalyzes oxidation-reduction reactions
•  2: Transferases – enzyme that transfers groups from one molecule to another
•  3: Hydrolases – enzyme that catalyzes the hydrolysis of various types of bonds
•  4: Lyases – enzyme that catalyzes the cleavage of various types of bonds by other means than hydrolysis and oxidation
•  5: Isomerases – enzyme that catalyzes the isomerization of various types of molecules
•  6: Ligases – enzyme that catalyzes the formation of various types of bonds by joining two molecules together, typically accompanied by the hydrolysis of a high-energy phosphate bond.