How do enzymes work

Enzyme Functionality

Overview of Enzymes

• Enzymes are biological catalysts that accelerate chemical reactions in living organisms.

• They increase the rate of reactions without undergoing a net change after the reaction.

Hexokinase as an Example

• Hexokinase: A classic example of an enzyme.

  • Shape and location of the active site on hexokinase:

  • Located on the surface of the enzyme.

  • Creates a binding pocket for substrate interaction.

  • Demonstrates a hydrophobic core complex adjusted in three-dimensional space.

Active Site Characteristics

• Active Site: The region where substrate molecules bind, creating an environment favorable for catalytic activity.

  • Characteristics of the active site:

  • Surface-localized but shaped to form a pocket.

  • Partly hydrophobic within the pocket and more hydrophilic externally.

• Substrate Binding:

  • The entry of substrate into the active site induces a conformational change, aligning catalytic residues for optimal interaction.

Catalytic Properties and Mechanisms

• Enzymes as Catalysts:

  • Increase reaction rates while remaining unchanged in form after the reaction.

  • Activation Barrier:

  • Enzymes lower this barrier without affecting the reaction's equilibrium (denoted as $K_{eq}$) or free energy change (denoted as $riangle G$).

Energy Perspective

• Free Energy of Substrates and Products:

  • Example reaction: Substrate (S) to Product (P).

  • Activation Energy (E_a): Energy required to attain the transition state from the substrate.

  • The reaction proceeds through a transition state which represents the highest energy state in the process.

  • Transition State Energy: Denoted as $riangle G^{ ext{double dagger}}$; reduces overall activation energy when a catalyst is involved.

Effects of Enzymes on Reaction Rates

• When an enzyme is introduced:

  • The reaction pathway changes, leading to the formation of enzyme-bound substrates and products.

  • The activation energy is lowered, allowing for faster spontaneous reactions.

Mathematical Relationships

• Rate Constant Relation:

  • An inverse relationship exists between activation energy and reaction rate, typically expressed in the Arrhenius equation:
    $k = A e^{- rac{E_a}{RT}}$

  • Where:

  • $k$ = rate constant

  • $A$ = frequency factor

  • $E_a$ = activation energy

  • $R$ = gas constant

  • $T$ = temperature

Enzyme Example: Catalase

• Catalase Function:

  • Found in all living organisms exposed to oxygen; catalyzes decomposition of hydrogen peroxide into water and oxygen.

  • Protects against oxidative damage caused by reactive oxygen species.

• Activation Energy Comparison:

  • Uncatalyzed reaction activation energy: 75.2 kJ/mol.

  • Catalase activation energy significantly lower (factor of $10^8$ in reaction rate).

  • Inorganic catalyst (e.g., platinum): factor of $10^4$ increase in reaction rate.

Binding Models in Enzyme Activity

• Lock and Key Model:

  • Suggests that the enzyme's active site is complementary to the substrate, but does not account for enzyme flexibility.

• Induced Fit Model:

  • Proposes that the enzyme adjusts shape upon substrate binding, facilitating a more effective transition state formation.

  • Allows for more products to be formed as the substrate holds the active site in a strained position that favors catalysis.

Thermodynamic Considerations

• Specificity and Binding:

  • Enzymes prefer binding the transition state over the substrate, suggesting higher affinity.

• Binding Energy:

  • Release of free energy during weak interactions helps stabilize the transition state.

• Entropy Adjustments:

  • Enzymes decrease translational and rotational entropy of the substrate to promote reaction likelihood.

• Desolvation:

  • Involves replacement of water molecules interacting with the substrate, promoting closeness needed for the reaction.

Electrostatic Interactions

• Weak non-covalent interactions stabilize the transition state against energetic barriers, promoting catalysis.

Conceptual Check on Reaction Spontaneity

• A reaction can have a large negative $riangle G$, indicating spontaneity, but may still proceed slowly if the activation energy barrier is high.

  • Example: The conversion of sucrose to carbon dioxide in water is spontaneous yet slow due to a significant activation energy barrier.

Conclusion

• Enzymes play a critical role in biological processes by effectively lowering activation energies and stabilizing transition states, enabling rapid reactions necessary for life. Their flexibility and ability to engage in dynamic interactions with substrates set them apart from simple geometric models of interaction.