Key Concepts in Biochemistry: Energy Metabolism

15.1 Energy Requirements of Living Organisms

• Living organisms continuously need free energy for:
  • Mechanical Work: Muscle contraction and cellular movements.
  • Active Transport: Movement of molecules and ions against concentration gradients.
  • Synthesis of Biomolecules: Building macromolecules from simpler precursors.
• Sources of Free Energy:
  • Phototrophs: Convert sunlight into chemical energy (e.g., plants).
  • Chemotrophs: Obtain energy from the oxidation of carbon-based fuels (e.g., humans).

Energy Flow Principles in Living Systems

1. Metabolic Pathways: Fuels are degraded and large molecules constructed through linked reactions.
2. ATP as Energy Currency: ATP links energy-releasing pathways with energy-requiring pathways.
3. Carbon Fuel Oxidation: Oxidation of carbon fuels drives ATP formation
4. Common Intermediates: Many pathways share common types of reactions and intermediates.
5. Pathway Regulation: Metabolic pathways are highly regulated for efficiency and coordination.
6. Enzyme Complexes: Metabolic enzymes are organized into complexes to enhance efficiency and manage potentially harmful intermediates.

15.2 Interconnected Metabolic Reactions

• Metabolism: A series of chemical reactions transforming biomolecules (e.g., glucose) into useful products (e.g., carbon dioxide, energy).
• Intermediary Metabolism: Includes various defined pathways; interdependent pathways form a biochemical ecosystem.
• Communication among pathways is crucial, often regulated by allosteric enzymes.

15.3 Energy-Yielding vs. Energy-Consuming Reactions

• Metabolic reactions can be classified into:
  1. Catabolic Reactions: Convert fuel energy into usable forms (ATP, ion gradients).
  2. Anabolic Reactions: Require input of energy to synthesize complex molecules.
• Some pathways can function as both anabolic and catabolic, termed amphibolic pathways.
• Unique Reactions: Common reactions in biosynthetic vs. degradative pathways are distinct to allow metabolic control

Thermodynamics of Reactions

• Conditions for Pathway Construction:
  1. Specific individual reactions.
  2. Overall thermodynamic favorability (negative free energy change).
• A thermodynamically unfavorable reaction can be driven by coupling with a favorable reaction.
• Additivity of Free Energy Changes: The overall free energy change of a pathway equals the sum of its steps.

15.4 ATP: Universal Energy Currency

• ATP (Adenosine Triphosphate): Acts as a central energy currency in metabolism.
  • It is energy-rich due to phosphoanhydride linkages, which yield energy upon hydrolysis.
• The hydrolysis of ATP provides energy to drive various cellular processes (motion, biosynthesis).
• Other nucleoside triphosphates can also serve as energy sources.

Hydrolysis and Metabolic Reactions

• Hydrolysis of ATP often couples to endergonic reactions enabling their progression by shifting equilibrium through energy release.
• ATP hydrolysis has induced significant changes in reaction equilibrium.

15.5 Role of Oxidation of Carbon Fuels

• Oxidation of carbon fuels (like glucose) releases electrons used to regenerate ATP.
• It involves oxidation–reduction reactions where electrons flow from reduced carbon molecules to oxygen, releasing energy.
• The reduced state of carbons determines their fuel efficiency: more reduced carbons yield more energy upon oxidation.

15.6 Regulation of Metabolism

• Metabolic regulation can occur through:
  1. Control of enzyme amounts (synthesis vs. degradation).
  2. Regulation of catalytic activity (e.g., allosteric control, covalent modification).
  3. Control of substrate accessibility (e.g., insulin's role in glucose uptake).
• Energy charge (ratio of ATP to ADP) also regulates metabolic activities.

Conclusion

• Understanding the interconnected paths of metabolism, energy flows, and regulatory mechanisms provides insights into the functioning of biological organisms. This establishes an understanding of biochemical principles governing life processes.