Chapter 8: Energy and Enzymes – An Introduction to Metabolism

8.1 What Happens to Energy in Chemical Reactions?

Chemical reactions involve changes in energy, which can be classified as kinetic or potential. Understanding how energy is transferred and transformed is essential for studying metabolism.

• Kinetic Energy: The energy of motion. Example: movement of molecules.

• Potential Energy: Stored energy due to position or structure. Example: energy stored in chemical bonds.

• First Law of Thermodynamics: Energy can be transferred and transformed, but cannot be created or destroyed.

• Enthalpy (H): Total energy in a molecule, including potential energy in bonds and kinetic energy of movement.

• Exothermic Reaction: Releases heat; enthalpy decreases. Compare with endothermic.

• Endothermic Reaction: Absorbs heat; enthalpy increases.

• Entropy (S): Quantitative measure of disorder in a system. The second law of thermodynamics states that entropy of the universe increases in any closed system.

• Gibbs Free Energy (G): The energy available to do work. The change in Gibbs free energy () determines whether a reaction is spontaneous.

Standard Free-Energy Change Equation:

• Exergonic Reaction: ; spontaneous, releases energy.

• Endergonic Reaction: ; nonspontaneous, requires energy input.

Example: Hydrolysis of ATP is exergonic and powers many cellular processes.

8.2 Nonspontaneous Reactions May Be Driven Using Chemical Energy

Cells often couple exergonic and endergonic reactions to drive processes that require energy. Redox reactions and energetic coupling are key mechanisms.

• Energetic Coupling: Energy released from an exergonic reaction (e.g., ATP hydrolysis) is used to drive an endergonic reaction.

• Redox Reactions: Involve transfer of electrons between molecules. Oxidation is loss of electrons; reduction is gain of electrons.

• Oxidizing Agent: Accepts electrons and is reduced.

• Reducing Agent: Donates electrons and is oxidized.

• Electron Carriers: Molecules like NAD+, FAD, and NADP+ that transfer electrons in metabolic reactions.

• ATP (Adenosine Triphosphate): Main energy currency of the cell. Hydrolysis of ATP releases energy for cellular work.

Example: When a phosphate group from ATP is added to a substrate, it increases the substrate's potential energy, enabling endergonic reactions.

8.3 How Enzymes Work

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy. They are essential for metabolism.

• Activation Energy: Minimum energy required to start a chemical reaction.

• Transition State: High-energy, unstable state that reactants must reach for a reaction to proceed.

• Substrate: The reactant that an enzyme acts upon.

• Catalyst: Substance that increases reaction rate without being consumed.

• Active Site: Region on the enzyme where substrates bind and react.

• Induced Fit: The active site changes shape to better fit the substrate upon binding.

How Enzymes Catalyze Reactions:

1. Initiation: Substrates bind to the active site.

2. Transition State Facilitation: Enzyme stabilizes the transition state, lowering activation energy.

3. Termination: Products are released, and the enzyme returns to its original shape.

Example: Digestive enzymes break down food molecules efficiently by lowering activation energy.

8.4 What Factors Affect Enzyme Function?

Enzyme activity is influenced by environmental conditions and regulatory molecules. Enzymes may require helper molecules for optimal function.

• Environmental Factors: Temperature, pH, and modifications to enzyme structure affect activity.

• Regulatory Molecules: Can inhibit or activate enzymes by binding to the active site or elsewhere.

• Competitive Inhibition: Inhibitor competes with substrate for active site.

• Allosteric Regulation: Regulatory molecule binds at a site other than the active site, changing enzyme shape and activity.

• Enzyme "Helpers":

  • Cofactors: Inorganic ions (e.g., Zn2+, Mg2+, Fe2+) that assist enzyme function.

  • Coenzymes: Organic molecules (e.g., NADH, FADH2) required for enzyme activity.

  • Prosthetic Groups: Non-amino acid molecules permanently attached to enzymes, aiding function.

Example: Pepsin, a digestive enzyme, works best at low pH in the stomach.

8.5 Enzymes Can Work Together in Metabolic Pathways

Metabolic pathways consist of a series of enzyme-catalyzed reactions, where the product of one reaction serves as the substrate for the next. This organization allows for efficient regulation and control of cellular processes.

• Metabolic Pathway: Linked series of biochemical reactions, each catalyzed by a specific enzyme.

• Intermediates: Molecules that are both products and reactants within the pathway.

• Feedback Inhibition: End product of a pathway inhibits an earlier enzyme, preventing overproduction.

Example: Glycolysis is a metabolic pathway where glucose is broken down in a series of enzyme-catalyzed steps.

Table: Types of Enzyme Helpers

Additional info: These notes expand on the original content by providing definitions, examples, and a summary table for enzyme helpers, ensuring a self-contained study guide suitable for exam preparation.