Study Notes on Enzyme Catalysis, Kinetics, Mechanism, and Regulation

Enzyme Catalysis: Kinetics, Mechanism, and Regulation

Introduction

  • Understanding enzymatic rate enhancements

  • Basis of steady-state kinetics

  • Information gained through inhibition analysis

  • Basic mechanisms of enzyme action

Enzyme Overview

  • Enzymes: biological catalysts that increase reaction rates by lowering activation energy.

  • Substrate: reactant that binds to the enzyme to be chemically transformed.

Why Enzymes Over Inorganic Catalysts?

  • Enzymes are more specific than inorganic catalysts.

  • Example: Nitrogen fixation via nitrogenase is highly specific to particular types of nitrogen compounds.

  • Environmental conditions such as pH and temperature-specific enzyme evolution enhance catalytic efficiency.

  • Enzymes evolved specifically for regulatory functions, resulting in faster rates of reactions.

Naming Enzymes

  • Enzymes are typically named with the suffix “-ase.”

  • Common naming examples include:

    • Nitrogenase: involved in nitrogen transformation

    • First part of the name generally indicates the substrate or type of reaction.

International Classification of Enzymes (Table 6-3)

  1. Oxidoreductases: Transfer electrons (hydride ions or H atoms).

  2. Transferases: Group transfer reactions.

  3. Hydrolases: Hydrolysis involving functional groups and water.

  4. Lyases: Cleavage reactions that form double bonds or rings, or add to double bonds.

  5. Isomerases: Isomeric transformations within molecules.

  6. Ligases: Bond formation through condensation reactions, utilizing ATP or similar cofactors.

  7. Translocases: Movement or separation of molecules and ions across membranes.

Enzyme-Substrate Complex

  • The active site of an enzyme is where the substrate binds, creating an enzyme-substrate (ES) complex.

  • Interactions in the active site can include hydrogen bonding, van der Waals forces, and sometimes covalent bonds.

  • A dynamic reaction mechanism exists where substrates bind and products are released.

Free Energy and Reaction Rates

  • Relationship between the equilibrium constant and free energy change for uncatalyzed reactions defined mathematically:
    extΔG‡=RTextlnKext{ΔG}^‡ = RT ext{ln} K

  • Enzyme-catalyzed reactions involve a decrease in activation energy ( ext{ΔE}^{ ext{cat}}) but do not change ext{ΔG}.

Enzyme Catalysis and Mechanism

  • Enzymes lower activation energy, which accelerates reaction rates significantly.

  • Typical rate enhancements vary from 10^5 to 10^17 times faster than uncatalyzed reactions, with examples presented in Table 6-5.

Concepts in Catalysis

  • Binding Energy: The free energy of binding is a major component of catalysis; stabilizing the transition state can greatly increase reaction rates.

  • Induced Fit Model: Enzyme conformational change in response to substrate binding optimizes the active site for catalysis.

General Classes of Catalysis

  1. Acid/Base Catalysis: Can be specific (involving water) or general (involving weak acids/bases). Amino acids involved include Glu, Asp, and His.

  2. Covalent Catalysis: Forms transient covalent bonds between the enzyme and substrate.

  3. Metal Ion Catalysis: Involves metalloenzymes that utilize metal ions to facilitate substrate binding and redox reactions.

Example Catalysis: Chymotrypsin

  • Chymotrypsin as a serine protease that hydrolyzes peptide bonds, significantly speeding up reactions (10^9 times faster).

  • Key players in the catalytic site include:

    • Catalytic triad: Ser195, His57, Asp102 coordinated to stabilize the reaction.

Protease Subclasses

  1. Serine Proteases: Use serine for covalent catalysis.

  2. Cysteine Proteases: Utilize cysteine for the formation of the enzyme-substrate complex.

  3. Aspartyl Proteases: Facilitate direct water attack on the substrate.

  4. Metalloproteases: Require a metal ion at the active site.

Inhibition of Enzymes

  • Types of Inhibition:

    • Irreversible: Permanent binding to the enzyme, usually toxic.

    • Reversible: Binding is temporary, used for therapeutic effects. Includes:

    • Competitive

    • Uncompetitive

    • Noncompetitive

    • Mixed

Competitive Inhibition

  • The inhibitor competes with the substrate for the active site.

  • It increases the apparent Km but does not affect Vmax.

Uncompetitive Inhibition

  • The inhibitor binds to the ES complex, preventing product formation, affecting both Km and Vmax.

Mixed Inhibition

  • Inhibitor affects both the enzyme and the ES complex differently, thus influencing both Vmax and Km.

Kinetics: Michaelis-Menten Model

  • The model describes the rate of enzymatic reactions as a function of substrate concentration.

  • Derivation includes defining velocity equations under different reaction states and assumptions leading to the Michaelis-Menten equation:
    v=racV<em>max[S]K</em>m+[S]v = rac{V<em>{max}[S]}{K</em>m + [S]}

  • Important constants:

    • Vmax: Maximum velocity reached by the system at saturation.

    • Km: Substrate concentration at which the reaction velocity is half of Vmax.

Key Performance Metrics of Enzymes

  • Turnover Number (kcat): Represents the number of substrate molecules converted to product per enzyme molecule per unit time.

  • Specificity constant (kcat/Km) provides a measure for enzyme efficiency and how substrate affinity alters Vmax.