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.