Keywords

Biophysics, Enthalpy, Hydrophobic Effect, Ras, Conformational Selection, Membrane Insertion, Biophysical journal

Reference

DOI: 10.1529/biophysj.108.136481

Abstract

To elucidate the driving forces behind the hydrophobic effect that governs amphiphilic lipopeptide membrane insertion, this study conducts a thermodynamic analysis of an H-ras heptapeptide anchor (ANCH) in water and DMPC lipid bilayers.
Using molecular mechanics, continuum solvent models, and solute entropy estimation, they found that membrane insertion is enthalpy-driven, with a free energy of transfer of ∼−13 kcal/mol.
Contrary to classical entropy-driven hydrophobic effects, favorable ANCH-membrane interactions (especially van der Waals) drive insertion, overcoming unfavorable solvation and entropy terms.
Further, conformational selection, rather than purely induced fit, plays a key role in minimizing insertion energy costs.

Notes

1. General Summary

  • The study addresses why amphiphilic lipopeptides like ANCH insert into membranes — and finds that this process is driven by enthalpic forces, not merely entropy.
  • Van der Waals (vdW) interactions between ANCH lipid tails and membrane lipids are the main contributors to the favorable energy.
  • Conformational selection allows ANCH to sample membrane-competent conformers in water, reducing the cost of structural rearrangement upon insertion.
  • Total transfer free energy of insertion is about −13 kcal/mol, reflecting a balance of opposing entropic and enthalpic contributions.

2. Thermodynamic Dissection of Insertion

  • Favorable terms:
    • ANCH-membrane interaction energy (~−136 kcal/mol), primarily vdW (three-fourths of total), plus minor Coulombic contributions.
  • Opposing terms:
    • Solvation free energy (~+87 kcal/mol): cost of desolvating ANCH from water.
    • Conformational entropy loss (~+36 kcal/mol): cost of restricting ANCH’s flexible structure upon insertion.
  • Net free energy:
    • After accounting for all factors, the membrane partition is still favorable (~−13 kcal/mol).

3. Structural and Energetic Insights

  • vdW forces dominate: ANCH’s lipid tails (HD186, Palm181, Palm184) interact deeply with the membrane.
  • Conformational selection: Some ANCH conformers in water already resemble bound states, minimizing rearrangement costs.
  • Tail rearrangement:
    • HD186 shows the most significant extension (12 Å to 18 Å) — critical for membrane contact.
    • Restructuring incurs an entropy penalty (~9 kcal/mol), but preselection of extended conformations reduces this cost.
  • Compact-to-extended transition: In water, ANCH’s lipid tails are coiled (compact), while in the membrane, they unwind to interact with lipids.
  • Interaction breakdown:
    • ~75% of binding energy from vdW.
    • Remainder from electrostatic (Coulombic) interactions.
  • Reorganization costs:
    • ~59 kcal/mol needed for ANCH conformational rearrangement, largely due to entropy (~31 kcal/mol) and vdW adjustments.

4. Broader Implications: Revisiting the Hydrophobic Effect

  • Traditional view: Hydrophobic insertion is entropy-driven via release of ordered water molecules.
  • This study: Amphiphilic peptides like ANCH exhibit enthalpy-driven insertion, primarily via vdW contacts, challenging the classical view.
  • Entropy: Insertion reduces configurational freedom, resulting in unfavorable entropy, but enthalpic gain outweighs this cost.
  • Hydrophobic effect redefined: In lipid-modified proteins, enthalpic interactions (vdW, membrane packing) are more critical than solvent reorganization alone.

5. RD’s Notes and Learnings

  • RD appreciates the shift from viewing membrane insertion as purely entropy-driven to recognizing strong enthalpic contributions.
  • Conformational selection as a mechanism — important insight, paralleling ideas from protein-ligand binding and intrinsically disordered proteins (IDPs).
  • HD186 lipid tail emerges as key player — implications for engineering membrane-binding peptides.
  • Raises the point that entropy-enthalpy balance is context-dependent, varying with molecule type (e.g., K-Ras vs. H-Ras).
  • The methodology (combining molecular mechanics, continuum solvation, and entropy calculations) could be valuable for RD’s own membrane-related studies.
  • RD is inspired to revisit the hydrophobic effect in light of these insights — especially for amphiphilic and lipidated peptides.

Take-home Messages

  • Membrane insertion of amphiphilic lipopeptides like ANCH is primarily driven by enthalpy, dominated by vdW interactions, not just entropy.
  • Conformational selection plays a key role in minimizing energy costs during membrane partition.
  • Revises the classical hydrophobic effect, showing that vdW and membrane packing interactions drive insertion more than simple water exclusion.
  • Offers a framework for understanding membrane targeting by lipid-modified proteins like H-Ras and K-Ras.
  • Important for membrane protein engineering, drug delivery systems, and understanding lipid-modified protein dynamics.