Activation Energy and Enzyme Function (Lecture 14.2)

Definition: Activation energy ($E_A$) is the energy barrier between reactants and products in a chemical reaction. It must be overcome for a reaction to proceed.
Graphical Representation: The change in free energy is illustrated as riangle G, representing the difference in energy between reactants and products. The activation energy is visualized as a 'hill' that must be climbed to convert reactants to products.

Low Activation Energy: When the activation energy is low, thermal energy at room temperature can provide enough energy for reactants to reach the transition state.
High Activation Energy: If E_A is high, reactions occur very slowly at room temperature and typically require heating to proceed at a noticeable rate.

Role of Enzymes: Enzymes are macromolecules (primarily proteins) that act as catalysts, decreasing activation energy and speeding up reaction rates without being consumed.
Classification: Enzymes often have names ending in "-ase" and regulate metabolic pathways.
Example: Catalase decomposes hydrogen peroxide (a harmful byproduct) into water and oxygen, showcasing the enzyme's functionality in living organisms.

Enzyme-Substrate Interaction:
- A substrate binds to an enzyme at a specific site, forming an enzyme-substrate complex.
- This complex undergoes a conformational change, promoting the chemical reaction and reducing E_A.
- The products are then released, regenerating the enzyme for further reactions.

Substrate Specificity: Enzymes have specific active sites that allow them to interact with only particular substrates, facilitating controlled reaction rates in metabolic processes.
Examples of Enzyme Classes:
- Oxidoreductases, Transferases, Hydrolases, Isomerases, Ligases, Lyases.

Substrate Concentration: Reaction rates initially increase with substrate concentration until a saturation point is reached, at which point all enzyme active sites are occupied.
Temperature:
- Reaction rates increase with temperature until a peak is reached (optimal temperature), after which high temperatures may denature enzymes.
pH:
- Each enzyme has an optimal pH. Deviations from this pH can decrease enzyme activity.

Definition: Many enzymes require cofactors (often metal ions like zinc or iron) or coenzymes (organic molecules) for activity.
Example: Carboxypeptidase requires zinc.

Inhibition Types:
- Competitive Inhibition: Inhibitor resembles the substrate and competes for the active site.
- Noncompetitive Inhibition: Inhibitor binds elsewhere on the enzyme, altering its shape and effectiveness.
Examples:
- Competitive Inhibitors: Molecules competing for the active site.
- Noncompetitive Inhibitors: Molecules that change enzyme conformation.
- Toxins and poisons may be irreversible inhibitors (e.g., sarin gas inhibits acetylcholinesterase).

Allosteric Regulation: Binding of an effector molecule at a site other than the active site can enhance or inhibit enzyme action.
Gene Expression: Changing the number of enzyme copies via gene regulation can alter enzyme concentrations.
Compartmentalization: Locating enzymes and substrates in specific regions of the cell enhances efficiency and specificity of reactions.

Ethanol to Acetate Conversion:
- The presence of ethanol increases the expression of alcohol dehydrogenase, which catalyzes the conversion of ethanol to acetaldehyde, increasing the rate at which reactions can occur.
Point Mutations: Mutations in ALDH2 can slow the conversion of acetaldehyde to acetic acid, causing adverse reactions to ethanol.

Activation Energy: Essential barrier in chemical reactions.
Enzymes: Catalysts that lower E_A and improve metabolic efficiency.
Factors Influencing Enzyme Activity: Include substrate concentration, temperature, pH, and presence of inhibitors or cofactors.
Regulation of Enzyme Activity: Achieved through allosteric regulation, gene expression, and compartmentalization.

Understanding activation energy and enzyme function is crucial for studying metabolic pathways and biochemical reactions in living organisms. Enzymes are vital for maintaining efficient metabolic processes at physiological temperatures.