A Rigorous Framework for Calculating Protein-Protein Binding Affinities in Membranes

Area of Science:

• Biophysics
• Computational Biology
• Structural Biology

Background:

• Integral transmembrane (TM) proteins mediate crucial cellular processes.
• Understanding TM protein association mechanisms requires accurate binding free energy calculations.
• The glycophorin A (GpA) homodimer is a model for TM protein interactions.

Purpose of the Study:

• To establish a robust methodological framework for calculating TM protein binding affinity in membrane environments.
• To compare the efficacy of different molecular dynamics approaches for GpA dimerization.
• To accurately determine the binding free energy of the GpA homodimer.

Main Methods:

• Molecular dynamics simulations were employed to study GpA dimerization.
• Two strategies were compared: unrestrained potential mean force (PMF) and a "geometrical route" with restraints.
• The geometrical route involved progressive separation of GpA α-helices with imposed restraints.

Main Results:

• The unrestrained PMF approach was found to be inadequate for GpA dimerization.
• The geometrical route yielded results in excellent agreement with experimental data.
• A dimerization free energy of -10.7 kcal/mol was obtained, closely matching experimental values.

Conclusions:

• The geometrical route provides an accurate and efficient method for calculating TM protein binding affinities in complex environments.
• Environmental forces play a significant role in initiating TM protein association before helical interactions stabilize dimers.
• A distinct intermediate state in GpA dimer formation was identified.