bcm.18_free_energy

Introduction

  • Topic: Free energy and protein folding

  • Objective: Understand how proteins adopt their shapes and the role of Gibbs free energy.

Learning Outcomes

  • Calculate if a reaction is spontaneous (exergonic) using Gibbs free energy change.

  • Connection between Gibbs free energy and equilibrium.

  • Calculate Gibbs free energy from enthalpy and entropy.

  • Use Nernst equation to find the equilibrium membrane potential for ionic species (Na+, K+).

  • Apply knowledge to resting and action potentials in mammalian neurons.

Gibbs Free Energy

Definition

  • Gibbs Free Energy (G): A measure of the maximum reversible work (useful chemical work) that can be obtained from a closed system at constant temperature and pressure.

  • Negative ΔG indicates a spontaneous process.

Equation

[ \Delta G = \Delta H - T \Delta S ]

  • ΔH = Enthalpy change

  • T = Temperature in Kelvin

  • ΔS = Entropy change

Spontaneity of Reactions

Dependence on Energy Change

  • Spontaneous reactions have a negative ΔG.

  • Factors affecting spontaneity:

    • Positive ΔH (endothermic) or negative ΔH (exothermic).

    • Positive or negative TΔS (increase or decrease in entropy).

Summary of Conditions

  • Exergonic: ΔG negative - spontaneous

  • Endergonic: ΔG positive - non-spontaneous

  • Equilibrium: ΔG = 0

Driver of Reactions

Influence of Enthalpy and Entropy

  • Some reactions are driven by enthalpy (ΔH).

  • Others are driven by entropy (ΔS), or both.

    • E.g. Fermentation of glucose, oxidation of ethanol, and decomposition of N2O5.

Protein Folding

Mechanism

  • Protein folding involves:

    • Bond enthalpy: Driven by interactions between protein and water.

      • Bonds include hydrogen bonds, Van der Waals forces.

    • Entropy gain: Water molecules become more free as protein folds (hydrophobic effect).

Energy Changes in Folding

  • Folding process illustrated by:

    • Protein unfolded + Water bound → Protein folded + Water free

  • Enthalpy (ΔH): Favorable process (often negative).

  • Entropy (ΔS): Contributes to spontaneity of folding.

Concentration and Reaction Dynamics

Relationship of Reactants and Products

  • Free energy (ΔG) depends on the relative concentrations of reactants and products in a reaction at equilibrium.

  • Key concepts:

    • Equilibrium constant (Keq): Indicates the state of the reaction at which ΔG = 0.

    • Calculating ΔG using concentrations of reactants and products to assess reaction directionality.

Nernst Equation and Membrane Potential

Concept

  • The Nernst equation relates concentration gradients of ions to membrane potential.

  • Formula: [ ext{E} = rac{RT}{zF} \ln \left( \frac{[ion]{outside}}{[ion]{inside}} \right) ]

  • Key for understanding ion movement across membranes and equilibrium potentials.

Application to Neurons

  • Comparison of resting and action potentials, particularly for K+ and Na+ ions.

  • Critical for understanding how neurons transmit signals.

Additional Resources

  • Suggested textbooks for further reading include:

    • Fisher & Arnold, "Chemistry for Biologists."

    • Voet & Voet, "Biochemistry."

    • Alberts et al., "Molecular Biology of the Cell."

Conclusion

  • Understanding free energy, protein folding, and ion dynamics is essential for comprehending biochemical processes and physiological functions in living organisms.