Chapter 4_ BIOCATALYSIS

Chapter 4: Biocatalysis

4.1 Properties of Enzyme and Mechanism of Actions

Structure of Enzymes

  • Definition: Biological catalysts produced by living cells that speed up chemical reactions by lowering activation energy.

  • Enzymes are globular proteins with tertiary or quaternary structures. They have:

    • Active Site: Specific region that binds to a substrate for catalysis.

    • Allosteric Site: Alternative site that alters enzyme activity when occupied.

  • Globular proteins are tightly folded polypeptides, unlike fibrous proteins which are elongated.

4.1 (b) Classes of Enzymes

  • Enzymes are classified based on the type of reaction catalyzed, ending in "ase" as per IUBMB classification. There are six major classes:

    • Hydrolases: Catalyze hydrolysis; e.g., Maltase, Amylase.

    • Isomerases: Catalyze rearrangement of atoms; e.g., Isomerase, Mutase.

    • Transferases: Transfer functional groups; e.g., Transaminase, Hexokinase.

    • Lyases: Add/remove functional groups; e.g., Decarboxylase.

    • Oxidoreductases: Catalyze redox reactions; e.g., Dehydrogenase.

    • Ligases: Form bonds using ATP; e.g., Synthetase.

4.1 (c) Enzymes Lowering Activation Energy

  • Activation Energy (EA): Minimum energy required to initiate a reaction.

  • Enzymes speed up reactions by lowering EA, enabling reactants to transition states with less energy required.

4.1 (d) Induced Fit Model

  • Proposed by Koshland (1959), it states that:

    1. The active site is flexible, and its conformation is not precisely complementary to the substrate.

    2. Substrate binding induces a slight change in the enzyme's active site for compatibility, forming an enzyme-substrate complex.

4.1 (e) Factors Affecting Enzymatic Reactions

  1. Substrate Concentration: Increases reaction rate until all active sites are saturated.

  2. pH: Each enzyme has an optimum pH; deviations can denature enzymes.

  3. Temperature: Rate increases with temperature until a threshold where enzymes denature.

  4. Enzyme Concentration: Higher concentrations increase reaction rates if substrates are not limiting.

4.2 Cofactor

  • Definition: Nonprotein molecules or ions essential for enzyme function. Can bind tightly or loosely to enzymes.

Types of Cofactors

  1. Metal Ion Activators: Inorganic ions, e.g., Mg2+, enhance enzyme activity.

  2. Coenzymes: Organic molecules, usually from vitamins, that assist enzymes, e.g., NAD+.

  3. Prosthetic Groups: Organic components that permanently bind to enzymes, e.g., haem group.

4.3 Inhibitors

Inhibitor Definition

  • Molecules that bind to enzymes and reduce reaction rates.

Types of Inhibitors

  1. Competitive Inhibitors: Compete for the active site. Can be overcome by increasing substrate concentration.

  2. Non-competitive Inhibitors: Bind to allosteric sites, altering enzyme conformation. Cannot be overcome by substrate increases.

Reversible vs. Irreversible Inhibition

  • Reversible: Temporary binding, normal function restored upon inhibitor removal.

  • Irreversible: Permanently bind to enzymes, resulting in functional loss (e.g., cyanide).

Comparison of Competitive and Non-Competitive Inhibitors

  • Similarities: Both bind temporarily and affect enzyme activity.

  • Differences:

    • Competitive inhibitors bind at the active site while non-competitive bind at an allosteric site.

    • Competitive inhibitors resemble substrates, whereas non-competitive do not.