B.Sc. 2nd year Microbiology

Assistant Professor Shubham Shukla
Department of Microbiology

Enzymes

Introduction

• Enzymes are biological macromolecules (mostly proteins) that act as catalysts in living organisms.
• They speed up biochemical reactions without being consumed in the process.
• Each enzyme is highly specific for a particular substrate or type of reaction, ensuring that the complex network of life processes occurs efficiently and in an organized manner.
• Enzymes are essential for various metabolic activities such as digestion, respiration, DNA replication, and energy production.
• Structurally, enzymes are composed of amino acids and may include non‑protein components such as:
• Cofactors (metal ions)
• Coenzymes (organic molecules derived from vitamins)
• The region where the substrate binds and the reaction occurs is called the active site.

Role of Enzymes as Biocatalysts

Enzymes are often referred to as biocatalysts because they accelerate biochemical reactions under mild physiological conditions (normal temperature, pH, and pressure). Their catalytic efficiency is much higher than that of chemical catalysts.

Key Roles

1. Lowering Activation Energy
• Enzymes reduce the activation energy required for a reaction, allowing it to proceed faster at body temperature.
2. Enhancing Reaction Rate
• They can increase the rate of reactions by several million times compared to uncatalyzed reactions.
3. Specificity
• Enzymes are highly specific for their substrates, ensuring that only desired reactions occur in the cell.
4. Regulation of Metabolic Pathways
• Enzymes are regulated by factors such as temperature, pH, inhibitors, and activators, allowing control over metabolic processes.
5. Environmental Sustainability

• As natural catalysts, enzymes work under eco‑friendly conditions, making them valuable in industrial processes like food production, pharmaceuticals, and biotechnology.

Enzyme Classification

Role of IUBMB

• A systematic classification and nomenclature system was developed and maintained by the International Union of Biochemistry and Molecular Biology (IUBMB).
• The IUBMB established the Enzyme Commission (EC) to provide a standardized system of enzyme nomenclature and classification.
• The first official Enzyme Commission report was published in 1961, introducing six main classes of enzymes.
• In 2018, a seventh class (Translocases) was added, making the classification more comprehensive.

Enzyme Commission (EC) Number

• Each enzyme is assigned a unique EC number, representing its class, subclass, and specific reaction catalyzed.
• Example: EC 1.1.1.1 – Alcohol dehydrogenase

• 1 → Oxidoreductase (main class)
• 1 → Acts on CH–OH group of donors
• 1 → Uses NAD⁺ as acceptor
• 1 → Specific enzyme number

This system ensures uniformity, clarity, and precision in enzyme identification across scientific fields.

Classification of Enzymes

EC 1. Oxidoreductases

• Function: Catalyze oxidation–reduction (redox) reactions.
• Examples:
• Lactate dehydrogenase – Converts lactate → pyruvate
• Cytochrome oxidase – Involved in cellular respiration
• Catalase – Breaks down hydrogen peroxide into water and oxygen
• General Reaction:
  [ AH₂ + B → A + BH₂ ]

EC 2. Transferases

• Function: Transfer functional groups (methyl, amino, phosphate) from one molecule to another.
• Examples:
• Hexokinase – Transfers phosphate from ATP to glucose
• Transaminase – Transfers amino groups between amino acids and keto acids
• Methyltransferase – Transfers methyl groups
• General Reaction:
  [ A–X + B → A + B–X ]

EC 3. Hydrolases

• Function: Catalyze bond cleavage by adding water (hydrolysis).
• Examples:
• Amylase – Breaks starch into maltose
• Lipase – Hydrolyzes fats into fatty acids and glycerol
• Protease – Breaks peptide bonds in proteins
• General Reaction:
  [ AB + H₂O → A–H + B–OH ]

EC 4. Lyases

• Function: Add or remove groups to form double bonds without hydrolysis or oxidation.
• Examples:
• Fumarase – Converts fumarate → malate
• Decarboxylase – Removes CO₂ from organic acids
• Aldolase – Splits C–C bonds in carbohydrates
• General Reaction:
  [ A–B → A + B \quad \text{or} \quad A + B → A=B ]

EC 5. Isomerases

• Function: Catalyze structural rearrangements within a molecule.
• Examples:
• Phosphoglucose isomerase – Converts glucose‑6‑phosphate ↔ fructose‑6‑phosphate
• Racemase/Epimerase – Interconvert optical or positional isomers
• Mutase – Shifts functional groups within a molecule
• General Reaction:
  [ A → Iso‑A ]

EC 6. Ligases (Synthetases)

• Function: Join two molecules with ATP breakdown.
• Examples:
• DNA ligase – Joins DNA fragments during replication
• Glutamine synthetase – Forms glutamine from glutamate and ammonia
• Acetyl‑CoA carboxylase – Adds CO₂ to acetyl‑CoA
• General Reaction:
  [ A + B + ATP → A–B + ADP + Pi ]

EC 7. Translocases (Newly Added Class)

• Function: Catalyze movement of ions/molecules across membranes, coupled with ATP hydrolysis or electron transfer.
• Examples:
• ATP synthase (EC 7.1.2.2) – Synthesizes ATP while transporting protons
• Na⁺/K⁺‑ATPase (EC 7.2.2.13) – Pumps sodium and potassium ions
• Cytochrome c oxidase – Translocates protons during electron transport