Enzymes: Biological Catalysts

Introduction to Enzyme Function

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They are essential for metabolic processes and are highly specific for their substrates.

• Enzyme Classification: Enzymes are classified into six major classes based on the type of reaction they catalyze (e.g., oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases).

• Reaction Mechanism: Enzymes lower the activation energy required for reactions, increasing the rate without being consumed.

• Order of Reaction: The order is determined by the rate law and concentration dependence of reactants and products.

• Free Energy of Activation: The difference in free energy between substrate and transition state () determines reaction rate.

Enzyme Catalysis and Mechanisms

Enzymes employ various mechanisms to stabilize transition states and facilitate reactions.

• Transition State Stabilization: Enzymes bind substrates in a way that stabilizes the transition state, lowering the activation energy.

• Binding Energy: The energy derived from enzyme-substrate interactions contributes to catalysis.

• Acid-Base Catalysis: Enzyme side chains donate or accept protons to facilitate reactions.

• Covalent Catalysis: Enzyme forms a transient covalent bond with the substrate.

• Metal Ion Catalysis: Metal ions participate in catalysis by stabilizing charges or activating substrates.

• Proximity and Orientation Effects: Enzymes bring substrates together in the correct orientation to react.

Example: Serine proteases use a catalytic triad (Ser, His, Asp) for peptide bond hydrolysis.

Enzyme Kinetics

Enzyme kinetics studies the rates of enzyme-catalyzed reactions and how they change with varying substrate concentrations.

• Initial Rate (V0): The rate measured at the beginning of the reaction before substrate depletion.

• Michaelis-Menten Equation: Describes the relationship between substrate concentration and reaction rate:

• Vmax: Maximum velocity achieved by the system at saturating substrate concentration.

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

• Lineweaver-Burk Plot: Double reciprocal plot used to estimate Vmax and Km:

• Enzyme Efficiency: Measured by (catalytic constant over Michaelis constant).

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors, which decrease or block catalytic activity.

• Reversible Inhibition: Inhibitor binds non-covalently and can be removed.

• Types of Reversible Inhibition:

  • Competitive: Inhibitor competes with substrate for active site.

  • Uncompetitive: Inhibitor binds only to enzyme-substrate complex.

  • Noncompetitive (Mixed): Inhibitor binds to enzyme or enzyme-substrate complex.

• Irreversible Inhibition: Inhibitor covalently modifies the enzyme, permanently inactivating it.

Example: Penicillin irreversibly inhibits bacterial transpeptidase.

Enzyme Regulation and Allosteric Control

Enzymes are regulated to meet cellular needs and respond to environmental changes.

• Allosteric Regulation: Effectors bind at sites other than the active site, altering enzyme activity.

• Cooperativity: Substrate binding at one site affects binding at other sites (e.g., hemoglobin).

• Feedback Inhibition: End product of a pathway inhibits an earlier step.

Specialized Enzyme Mechanisms

Some enzymes use unique mechanisms for catalysis and specificity.

• Serine Proteases: Use a catalytic triad for peptide bond hydrolysis.

• Ribozymes: RNA molecules with catalytic activity.

• Isozymes: Different forms of an enzyme that catalyze the same reaction but differ in structure or regulation.

• Prosthetic Groups: Tightly bound cofactors required for enzyme activity.

Key Terms and Definitions

• Enzymatic Rate Enhancement:

• Reaction Coordinate: Diagram showing the energy changes during a reaction.

• Free Energy Barrier: Energy required for a substrate to reach the transition state.

• Transition State: High-energy state between reactants and products.

• Binding Energy: Energy from enzyme-substrate interactions.

• Acid-Base Catalysis: Proton transfer to facilitate reaction.

• Covalent Catalysis: Formation of transient covalent bond.

• Metal Ion Catalysis: Metal ions stabilize charges or activate substrates.

• Allosteric Effect: Regulation by binding at a site other than the active site.

• Prosthetic Group: Non-protein component required for enzyme function.

• Initial Rate (V0): Rate measured at the start of the reaction.

• Turnover Number (kcat): Number of substrate molecules converted per enzyme per unit time.

• Isoenzyme: Enzyme variant with similar function but different structure.

• Inhibitor: Molecule that decreases enzyme activity.

• Intermolecular Effect: Effect of one molecule on another in enzyme reactions.

Enzyme Kinetics Table

The following table summarizes key kinetic parameters and their significance:

Graphical Analysis in Enzyme Kinetics

• Plotting V0 vs [S] to estimate Vmax and Km

• Lineweaver-Burk plot ( vs ) for linear analysis

• Recognizing sigmoidal curves for allosteric enzymes

Summary of Enzyme Inhibition Types

Additional info:

• Students should be able to analyze enzyme mechanisms, predict products of protease digestion, and interpret kinetic data using graphical methods.

• Understanding the physiological relevance of enzyme kinetics and inhibition is essential for biochemistry.