Entropic and enthalpic contributions to annexin V-membrane binding: a comprehensive quantitative model

Area of Science:

• Biochemistry
• Biophysics
• Molecular Biology

Background:

• Annexin V exhibits high affinity for membranes, but the driving forces are not fully understood.
• Quantitative elucidation of annexin V-membrane interactions is crucial for understanding its biological roles.

Purpose of the Study:

• To quantitatively determine the thermodynamic and molecular factors governing annexin V's high-affinity membrane binding.
• To investigate the contributions of hydrophobic interactions, ionic forces, and protein-protein interactions to binding affinity.

Main Methods:

• Isothermal microcalorimetry and calcium titration to analyze binding thermodynamics.
• Mutagenesis studies to probe the role of surface residue hydrophobicity.
• Fluorescence resonance energy transfer (FRET) to assess protein multimerization.

Main Results:

• Binding is driven by both enthalpic and entropic forces, with a significant positive entropy change.
• Hydrophobic interactions, particularly dehydration of the protein surface, substantially contribute to binding affinity.
• Ionic interactions and specific head group interactions play minor roles; no annexin-annexin multimerization was observed.

Conclusions:

• Annexin V-membrane binding is primarily mediated by hydrophobic forces and favorable enthalpy/entropy changes.
• The high affinity is not dependent on specific phospholipid head groups or protein-protein interactions.
• Understanding these forces provides insights into annexin V's function in calcium-dependent membrane association.