Metabolism: Energy Changes and Electron Transfer

Unit II: The Importance of Energy Changes and Electron Transfer in Metabolism

• Energy Balance: Energy intake equals internal heat produced plus work plus storage. Heat and work are accounted for in mechanical work and anabolic processes (energy-requiring). Stored chemical energy drives endergonic reactions.

The Nature of Metabolism

• Definition: Metabolism is the sum total of all biochemical reactions in an organism.
• Catabolism: An oxidative process that releases energy.
• Anabolism: A reductive process that requires energy.
• Separate Pathways: Catabolism and anabolism are separate pathways, not simply reversed processes.
• Unique Thermodynamic Systems: Living things are unique thermodynamic systems.
  • Isolated system: No exchange of matter or energy.
  • Closed system: Energy exchange may occur.
  • Open system: Energy and/or matter exchange may occur.

Biochemical Reactions

• Five General Categories:
  • C-C bond making or breaking
  • Internal rearrangements, isomerizations, and eliminations
  • Free-radical reactions
  • Group transfers
  • Oxidation-reductions

Adenosine Triphosphate (ATP)

• Role: ATP plays a crucial role as cellular energy currency, linking catabolism and anabolism.
• Energy Donation: Energy donation typically involves the covalent participation of ATP. This process mostly converts ATP to ADP and Pi, in some cases to AMP and two Pi.

ATP Hydrolysis

• Charge Separation: Relief of electrostatic repulsion among the four negative charges on ATP.
• Hydrolysis Reaction: -ATP + H₂O → ADP + Pi
  -ΔG°' = -30.5 kJ/mol ;
  (-7.3 kcal/mol)
• Coupled Reactions: A thermodynamically unfavorable reaction can become favorable if coupled with hydrolysis of ATP.
• Two Step Process: ATP generally contributes to a reaction in a two-step process.

Nucleophilic Reactions of ATP

• Nucleophilic Attack: Any of the three P atoms (α, β, or γ) can serve as the electrophilic target for nucleophilic attack.
• Adenylylation: Nucleophilic attack at the α-position of the ATP displacing PP₁ and transferring adenylate (5'-AMP) as an adenylyl group.

Firefly Bioluminescence Cycle

• Description: The chemical reaction that produces light from fireflies.
• Involves a process: The cycle involves ATP, Luciferin, and a luciferase enzyme to produce light.

Flow of Electrons in the Biological System

• Role in Metabolism: The flow of electrons in oxidation-reduction reactions is a primary driving force for many biological processes.
• ATP Consumption: Shows different ATP consumption rates in resting and strenuous exertion.

Biological Redox Reactions

• Glucose Oxidation: Glucose oxidation involves the controlled transfer of electrons to O₂ producing energy in the form of ATP; 6 CO2 + 6 H₂0 + ATP
• More Efficient Fuel: Fats are more efficient fuel sources than carbohydrates because the carbon in fats is more reduced.
• Electron Carriers/Coenzymes: NAD⁺ and FAD⁺ are specialized coenzymes for carrying electrons in cellular redox reactions.

Coenzymes in Biological Redox Reactions

• Channels for Electron Transfer): Electrons released from redox reactions are transferred by nucleotide coenzymes.
• Water-Soluble Coenzymes: NAD, NADP, FMN, and FAD are water-soluble coenzymes. NAD and NADP move between enzymes, while FMN and FAD are tightly bound to enzymes.
• Recycling of Coenzymes: There is no net production or consumption of NAD⁺ or NADH; they function catalytically, recycling repeatedly.

Coupling of Production and Use of Energy

• ATP Synthesis: Cells constantly synthesize ATP in mitochondria.
• Energy Provision: ATP releases energy when the terminal phosphate is removed. The resultant ADP and free phosphates are recycled to form ATP.