Chp 9 - Catalysts and Enzymes

Key Sections:
1. Activation Energy Barriers
2. Enzymes Lower Activation Energy Barriers
3. Enzyme Function
4. Regulation of Enzyme Activity

Definition: Activation energy barriers prevent many spontaneous reactions from occurring by requiring a certain amount of energy to initiate.
Key Points:
- For a reaction to occur, chemical bonds need to be broken and/or formed.
- During this process, molecules reach unstable, high-energy states known as “transition states.”
- The energy required for reactants to transition from their normal state to the transition state is termed the “activation energy barrier.”
- If reactants lack sufficient energy to overcome this barrier, the reaction may proceed very slowly or not at all.

Definition of Enzymes: Enzymes are often proteins (some RNAs) that act as catalysts, speeding up reactions without being consumed.
Functionality:
- Enzymes lower the activation energy needed for reactants to reach their transition states, thereby increasing reaction rates.
- Importantly, enzymes do NOT affect elt;G (Gibbs free energy change) for the reaction.
- They cannot convert endergonic reactions into exergonic ones.
- They do not influence the equilibrium position (relative amounts of reactants and products).
- Enzymes merely speed up the rate of reactions without altering their overall energy profiles.
Active Site & Substrate:
- Enzymes bind to their substrates at the active site, forming an “enzyme-substrate complex.”
- Induced Fit:
- Describes how the binding of a substrate alters the enzyme's shape, allowing functional groups to catalyze the reaction more effectively.

Mechanisms to Lower Activation Energy:
1. Bring substrate molecules together in the correct orientation.
2. Stress critical chemical bonds in substrates, moving them towards transition state forms.
3. Provide a conducive environment (like optimal pH) for the reaction to occur.
4. Participate directly in the reaction; the enzyme must be restored to its original state post-reaction.
Enzyme Flexibility: Enzymes can catalyze both forward and reverse reactions without changing the net direction or relative equilibrium concentrations.

Enzymes depend on their three-dimensional structure, which can be influenced by:
- Temperature
- pH levels
Extremophiles: Organisms that thrive in extreme environments often produce enzymes effective at unconventional temperatures. These enzymes are of great utility in various applications, including biotechnology (e.g., heat-stable DNA polymerases).

Cofactors:
- Non-protein helpers (like some vitamins) that assist enzyme activity.
Regulation of Activities:
1. Competitive Inhibitors:
- Molecules that resemble the substrate and compete for the active site, thus reducing enzyme activity.
- Their effects can be mitigated by increasing substrate concentration.
1. Noncompetitive Inhibitors:
- Bind at a site other than the active site, altering the enzyme's shape and effectiveness.
- Their inhibition cannot be overcome by adding more substrate.
1. Allosteric Regulation:
- A regulatory molecule binds to a site other than the active site, affecting enzyme function.
- Allosteric activators enhance activity by stabilizing active forms, while inhibitors stabilize inactive forms.
ATP and ADP as Regulators:
- ATP acts as an inhibitor for catabolic pathways while activating anabolic ones, and vice versa for ADP.
Feedback Inhibition:
- A metabolic pathway is turned off by the binding of its product to an enzyme early in the pathway, preventing overproduction following unnecessary reactions.
- Examples include:
- ATP inhibiting enzymes in ATP-producing pathways.
- ADP inhibiting enzymes in ADP-producing pathways.
- Isoleucine inhibiting the production pathway from threonine.