ENZYMES 2024-2025

Enzymes

• Definition: Thermolabile biological catalysts that remain unchanged chemically during reactions.

• Nature: Enzymes are proteins and highly specific in their action. They can act both intracellularly and extracellularly.

Terminology of Enzymes

• Hydrolytic Enzymes: Named by adding "ase" to the substrate's name.

  • Examples: Sucrase, lipase, lactase.

• Other Enzymes: Named by substrate + mechanism of action + "ase".

  • Example: Succinate dehydrogenase.

Chemical Nature of Enzymes

• All enzymes are proteins, categorized into:

  1. Simple Protein Enzymes: Consist solely of protein (e.g., pepsin, maltase).

  2. Conjugated Protein Enzymes: Comprise a protein part and a non-protein part, also called a holoenzyme.

     • Apoenzyme: The protein part.

     • Non-protein part:

       • a. Prosthetic Group: Firmly attached.

       • b. Coenzyme: Loosely attached.

Coenzymes

• Often contain vitamin B:

  1. Co I: Nicotinamide adenine dinucleotide (NAD) - acts as hydrogen carrier.

  2. Co II: Nicotinamide adenine dinucleotide phosphate (NADP) - hydrogen carrier.

  3. FMN: Flavin mononucleotide - hydrogen carrier.

  4. FAD: Flavin adenine dinucleotide - hydrogen carrier.

Mechanism of Action

• Enzyme binds to its specific substrate to form an enzyme-substrate complex.

• Binding lowers the activation energy required to convert substrate to product.

• Intermediate complex decomposes, releasing the product while the enzyme remains unchanged.

• Each enzyme contains an active site for substrate binding.

Classification of Enzymes

• Enzymes are classified into six major classes based on the reactions they catalyze:

Class I: Oxidoreductases

• Catalyze oxidation-reduction reactions between two substrates.

• Examples:

  • Oxidases: Use oxygen as electron acceptor.

  • Dehydrogenases: Remove hydrogen.

  • Oxygenases: Incorporate oxygen into the substrate.

  • Peroxidases: Use H2O2 as electron acceptor.

Class II: Transferases

• Catalyze the transfer of a group from one substrate to another.

• Examples:

  • Methyltransferase

  • Aminotransferase: Transfer NH2 from amino acids to keto acids.

  • Kinases: Transfer phosphate from ATP to substrates.

  • Phosphorylases: Transfer inorganic phosphate (Pi).

Class III: Hydrolases

• Catalyze hydrolysis (addition of water to break a chemical bond).

• Examples:

  • Phosphatases: Remove phosphate.

  • Esterase: Cleave ester bonds in nucleic acids and lipids (e.g., nucleases, lipases).

  • Proteases: Cleave amid bonds in proteins (e.g., peptidases).

Class IV: Lyases

• Catalyze removal or addition of a group from a substrate.

  • Decarboxylases: Remove CO2.

Class V: Isomerases

• Catalyze the interconversion of one substrate to its isomer.

Class VI: Ligases

• Catalyze the joining of two substrates using energy.

Factors Affecting Enzyme Activity

1. Concentration of the Substrate: Increased substrate concentration usually increases velocity.

2. Concentration of the Enzyme: Increased enzyme concentration can increase reaction velocity, but only up to a limit.

3. Effect of Temperature: Each enzyme has an optimum temperature for maximum activity; extremes decrease activity.

4. Effect of pH: Each enzyme has an optimum pH, with decreased activity at extreme pH levels.

5. Activators: Inorganic ions that enhance enzyme activity (e.g., chloride ions activate amylase, magnesium activates kinases).

Enzyme Inhibition

Types of Inhibitors

• Non-Specific Inhibitors: Affect all enzymes or a variety of them by precipitating or denaturing proteins.

• Specific Inhibitors: Target one enzyme or a few; can be further classified as:

  1. Competitive Inhibitors: Compete with the substrate at the active site.

     • Examples:

       • Sulfonamide: Similar to para-aminobenzoic acid, used by microorganisms for folic acid synthesis.

       • Dicumarol: Similar to vitamin K, preventing prothrombin and factors VII, IX, and X synthesis.

  2. Non-Competitive Inhibitors:

     • Allosteric Modifiers: Bind at sites other than the active site.

     • Feedback Inhibition: The end product inhibits the first enzyme in the metabolic pathway.

     • Feedback Regulation: The end product inhibits the gene controlling the synthesis of the first enzyme.