Ch08_seminar

Energy, Enzymes, and Metabolism

8.1 Physical Principles Underlie Biological Energy Transformations

  • Metabolism: The sum total of all chemical reactions occurring in a biological system at a given time.

    • Involves energy changes.

    • Energy: The capacity to do work or change.

    • Example reaction: sucrose + H2O → glucose + fructose (reactants to products).

Forms of Energy

  • Potential Energy: Stored as chemical bonds, concentration gradient, charge imbalance.

  • Kinetic Energy: The energy of movement.

  • Energy can be converted from one form to another.

Table 8.1: Energy in Biology

  • Chemical-bond energy: Stored in bonds; released during hydrolysis of polymers.

  • Electrical energy: Separation of charges; drives ion movement via gradients.

  • Heat energy: Released by chemical reactions, impacting temperature.

  • Light energy: Captured by pigments in photosynthesis.

  • Mechanical energy: Used in muscle movements and cell activities.

Types of Metabolism

  • Anabolic Reactions: Complex molecules formed from simpler molecules; energy is required.

  • Catabolic Reactions: Complex molecules broken down into simpler ones; energy is released.

  • Anabolic and catabolic reactions are linked; energy released from catabolism drives anabolism.

Laws of Thermodynamics

  • First Law: Energy is neither created nor destroyed; total energy before and after conversion is the same.

  • Second Law: Part of energy becomes unavailable for work; energy transformation is not 100% efficient.

    • Increases disorder (entropy).

Entropy

  • Entropy: Measure of disorder; tends to increase over time in isolated systems.

  • Imposing order requires energy; living organisms depend on constant energy supply to maintain order.

Enthalpy and Free Energy

  • Total Energy: Enthalpy (H) is the system's internal energy plus pressure-volume work.

  • Free Energy (G): The usable energy for work in a system.

  • Change in free energy (ΔG): ΔG = ΔH – TΔS

    • ΔG negative: free energy released.

    • ΔG positive: free energy required.

    • Free energy must be available for reaction to occur.

Reaction Types

  • Exergonic Reactions: Release free energy; associated with catabolism.

  • Endergonic Reactions: Consume free energy; associated with anabolism.

Equilibrium

  • Reactions are reversible with an equilibrium point; at equilibrium, ΔG = 0.

  • Concentrations of reactants/products determine the favored direction of reaction.

  • Free energy relates to how far a reaction is from equilibrium.

8.2 ATP Plays a Key Role in Biochemical Energetics

ATP Overview

  • ATP (adenosine triphosphate) captures and transfers free energy.

  • The hydrolysis of ATP provides energy for endergonic reactions.

  • Characteristics of ATP leading to energy release:

    1. Negative charges in phosphate groups create energy stored in P~O bonds.

    2. Hydrolysis of phosphate groups is exergonic due to stabilization of ADP and Pi.

8.3 Enzymes Speed Up Biochemical Transformations

Enzyme Characteristics

  • Catalysts: Speed up reactions without altering the equilibrium.

  • Most enzymes are proteins with specific active sites for substrates.

  • Enzymes lower activation energy (Ea), allowing reactions to proceed faster.

Substrate Binding

  • Enzymes are specific to substrates; shape determines specificity.

  • The enzyme-substrate complex (ES) is held by various types of bonds.

Induced Fit Model

  • Enzymes change shape when binding substrates, fitting them more effectively.

Factors Affecting Reaction Rates

  1. Substrate Concentration: Reaction rate increases with more substrate until saturation.

  2. Temperature: High temperatures can denature enzymes; each enzyme has an optimal temperature.

  3. pH Levels: Enzyme activity is influenced by pH, affecting functional group ionization.

Regulation of Enzyme Activity

  • Enzyme Inhibitors: Slow down reaction rates by binding to enzymes.

    • Irreversible Inhibitors: Bind covalently, permanently inactivating the enzyme.

    • Reversible Inhibitors: Bind noncovalently, can be competitive or noncompetitive.

Allosteric Regulation

  • Allosteric Enzymes: Change shape upon substrate binding, affecting activity.

  • Feedback inhibition: Final product serves as an inhibitor to the first enzyme, shutting down the pathway.

Covalent Modifications

  • Enzyme activity can be controlled by reversible modifications (e.g., phosphorylation) that activate or deactivate the enzyme.

Nonprotein Partners of Enzymes

  • Enzymes may require prosthetic groups, inorganic cofactors, or coenzymes for activity.