Free-Energy Change and Enzyme Function in Biological Reactions

Free-Energy Change for a Reaction

  • The second law of thermodynamics states a reaction can only occur spontaneously if it increases the universe's disorder (entropy).

  • Free energy (G) in chemistry refers to useful energy available to do work.

  • Free-energy change (ΔG):

    • Denoted as ΔG, indicates the change in free energy as a reaction progresses.

    • If ΔG is negative, the reaction is energetically favorable and can occur spontaneously, increasing disorder.

    • If ΔG is positive, the reaction is energetically unfavorable.

  • Energetically Favorable Reactions:

    • Create disorder, leading to a decrease in free energy. For example, if free energy of Y (10 kJ/mol) is greater than X (4 kJ/mol), then ΔG = 10 - 4 = -6 kJ/mol (favorable).

  • Energetically Unfavorable Reactions:

    • Create order and require coupling to an energetically favorable reaction to occur (e.g., peptide bond formation between amino acids).

Examples of Reactions

  • Energetically Favorable Reaction:

    • Dissolving salt (NaCl) in water, releasing energy and increasing disorder.

  • Energetically Unfavorable Reaction:

    • Formation of a peptide bond between amino acids cannot occur spontaneously; it requires a coupling to a favorable reaction.

  • Spontaneous reactions can be slow (e.g., glucose oxidation takes a long time without enzymes).

Role of Enzymes in Reactions

  • Enzymes act as catalysts, lowering the activation energy needed to initiate reactions.

  • Enzymes are substrate-specific, meaning they only catalyze specific reactions, allowing for precise control in metabolic pathways.

  • The binding of a substrate to an enzyme stabilizes the transition state, allowing the reaction to proceed more readily.

Energy Use in Cells

  • Nonliving things become disordered over time; living cells maintain and create order through chemical reactions that require energy inputs.

  • Cellular reactions must produce a greater increase in disorder in the environment than order within the cell to satisfy the second law of thermodynamics.

  • Cells achieve this by coupling energetically unfavorable reactions to energetically favorable ones.

Free Energy and Reaction Direction

  • Chemical reactions will proceed in the direction that minimizes free energy.

    • Equilibrium: At equilibrium, the concentrations of reactants and products remain constant; ΔG = 0.

    • Free energy and reaction spontaneity:

    • ΔG < 0: Reaction is spontaneous.

    • ΔG > 0: Reaction is non-spontaneous.

  • Equilibrium Constant (K):

    • Represents the ratio of product concentrations to reactant concentrations at equilibrium.

    • K is directly related to ΔG; larger K indicates a greater tendency for the reaction to proceed forward.

Activated Carriers in Metabolism

  • ATP is the most common activated carrier, involved in energy transfer across chemical reactions in cells.

    • ATP hydrolysis releases energy that can drive energetically unfavorable reactions.

    • NADH and NADPH are electron carriers, essential for metabolic reactions involving electron transfers.

  • Biosynthesis: Biological polymers require energy inputs due to energetically unfavorable condensation reactions, which are facilitated by activated carriers like ATP.

    • Example of ATP in Condensation: ATP can activate molecules in a pathway, leading to the formation of macromolecules through the release of energy from an accompanying reaction.

Metabolism refers to the collection of chemical reactions that occur within a living organism to maintain life. These reactions can be categorized into two main pathways:

  • Anabolic Reactions: These reactions build complex molecules from simpler ones and require energy input, facilitating growth and the maintenance of cellular structures.

  • Catabolic Reactions: These reactions break down complex molecules into simpler ones, releasing energy that can be harnessed for various cellular activities.
    Metabolism is crucial for energy production, resource utilization, and the synthesis of necessary biomolecules for cellular functions and life processes.

Summary of Key Concepts

  1. Energy input is essential for maintaining life amidst entropy.

  2. Enzymes facilitate reactions by lowering activation energy, enabling energy transfers essential for metabolism.

  3. Reactions must increase the universe's total disorder to be spontaneous.

  4. The relationship between free energy changes, reaction spontaneity, and equilibrium constants enables prediction of reaction pathways.

  5. Activated carriers, especially ATP, are crucial for driving biosynthetic processes and carrying energy in the cell.